This wiki is still very much work in progress! Feel free to suggest changes in the #off-topic Discord channel.



What does this mod do?

This mod lets you create scripts in JavaScript language to manage your server, add new blocks and items, change recipes, add custom handlers for quest mods and more!

How to use it?

Run the game with mod installed once. It should generate kubejs folder in your minecraft directory with example scripts and README.txt. Read that!

I don't know JavaScript

There's examples and pre-made scripts here. And you can always ask in discord support channel for help with scripts, but be specific.

Can I reload scripts?

Yes, use /reload to reload server_scripts/F3 + T to reload client_scripts/ and /kubejs reload startup_scripts to reload startup_scripts/. If you don't care about reloading recipes but are testing some world interaction event, you can run /kubejs reload server_scripts. Note: Not everything is reloadable. Some things require you to restart game, some only world, some work on fly. Reloading startup scripts is not recommended, but if you only have event listeners, it shouldn't be a problem.

What mod recipes does it support / is mod X supported?

If the mod uses datapack recipes, then it's supported by default. Some more complicated mods require addon mods, but in theory, still would work with datapack recipes. See Recipes section for more info.

What features does this mod have?

The feature list would go here if I actually wrote it. But basically, editing and creating recipes, tags, items, blocks, fluids, worldgen. Listening to chat, block placement, etc. events. Just look at the event list on Wiki.

How does this mod work?

It uses a fork of Rhino, a JavaScript engine by Mozilla to convert JS code to Java classes at runtime. KubeJS wraps minecraft classes and adds utilities to simplify that a lot and remove need for mappings. Architectury lets nearly the same source code be compiled for both Forge and Fabric making porting extremely easy.

Ok, but what if it.. doesn't work?

You can report issues here.

I have more questions/suggestions!

If wiki didn't have the answer for what you were looking for, you can join the Discord server and ask for help on #support channel!



Website: https://kubejs.com/

Source and issue tracker: https://github.com/KubeJS-Mods/KubeJS

Download: https://www.curseforge.com/minecraft/mc-mods/kubejs

Anything below 1.16 is no longer supported!


Migrating to KubeJS 6

This page is still being worked on, so if some info is missing, please check back later!

What's changed in new KubeJS 6 (1.19.2+)?


onEvent('string.event.id', event => {}) syntax has been replaced by
SomeEventGroup.someEventName(event => {}).

// Before

onEvent('block.right_click', event => {
  if(event.block.id === 'minecraft:dirt') {

// After

BlockEvents.rightClicked(event => {
  if(event.block.id === 'minecraft:dirt') {

Not only that, but new events also support extra parameter for IDs and other things! You can now chose to make each id have it's own event handler:

// Before

onEvent('block.right_click', event => {
  if(event.block.id === 'minecraft:dirt') {
  } else if(event.block.id === 'minecraft:stone') {

// After

BlockEvents.rightClicked('minecraft:dirt', event => {

BlockEvents.rightClicked('minecraft:stone', event => {

Some events require ID, such as registry and tag events:

// Before

onEvent('item.registry', event => {

// After

StartupEvents.registry('item', event => {
// Before

onEvent('tags.items', event => {

// After

ServerEvents.tags('item', event => {

Using parameter is actually faster on CPU than checking some event.id == 'id'.

You can find full list of new events here.


onForgeEvent('package.ClassName', event => {}) has been replaced by
ForgeEvents.onEvent('package.ClassName', event => {}).

// Before

onForgeEvent('net.minecraftforge.event.level.BlockEvent$PortalSpawnEvent', event => {

// After

ForgeEvents.onEvent('net.minecraftforge.event.level.BlockEvent$PortalSpawnEvent', event => {

New! It now supports generic events:

ForgeEvents.onGenericEvent('net.minecraftforge.event.AttachCapabilitiesEvent', 'net.minecraft.world.entity.Entity', event => {

Server settings

settings.log... properties have been removed from server scripts, and instead, moved to local/kubejsdev.properties file. By default it won't be shipped with back, but you can change saveDevPropertiesInConfig to true to instead save the file in kubejs/config/dev.properties.


java('package.ClassName') has been replaced by

// Before

const CactusBlock = java('net.minecraft.world.level.block.CactusBlock')

// After

const CactusBlock = Java.loadClass('net.minecraft.world.level.block.CactusBlock')

There might be some more reflective helper methods later in Java util class, such as listing all fields and methods in a class, etc.

Bye Bye Wrapper classes

None of the vanilla classes are wrapper anymore - EntityJS, LevelJS, ItemStackJS, IngredientJS and others are gone. This may introduce some bugs, but in general, should make it significantly easier to interact with minecraft and other mods. Almost all old methods are still supported by coremodding vanilla. This should also significantly boost performance, as it doens't need to constantly wrap and unwrap classes.

Other questions

If you have any other questions, feel free to ask them on my discord.


Events that get fired during game to control recipes, world, etc.


List of all events

This is a list of all events. It's possible that not all events are listed here, but this list will be updated regularly.

Click on event ID to open it's class and see information, fields and methods.

Type descriptions:

ID Cancellable Type Note
init No Startup
postinit No Startup
loaded No Startup
command.registry No Server
command.run Yes Server
client.init No Client
client.debug_info.left No Client
client.debug_info.right No Client
client.logged_in No Client
client.logged_out No Client
client.tick No Client
server.load No Server
server.unload No Server
server.tick No Server
server.datapack.first No Server
server.datapack.last No Server
recipes No Server
recipes.after_load No Server Does not work 1.18+
level.load No Server Replace level with world in 1.16
level.unload No Server Replace level with world in 1.16
level.tick No Server Replace level with world in 1.16
level.explosion.pre Yes Server Replace level with world in 1.16
level.explosion.post No Server Replace level with world in 1.16
player.logged_in No Server
player.logged_out No Server
player.tick No Server
player.data_from_server. Yes Client
player.data_from_client. Yes Server
player.chat Yes Server
player.advancement No Server
player.inventory.opened No Server
player.inventory.closed No Server
player.inventory.changed No Server
player.chest.opened No Server
player.chest.closed No Server
entity.death Yes Server
entity.attack Yes Server
entity.drops Yes Server
entity.check_spawn Yes Server
entity.spawned Yes Server
block.registry No Startup
block.missing_mappings No Server
block.tags No Server
block.right_click Yes Server
block.left_click Yes Server
block.place Yes Server
block.break Yes Server
block.drops No Server
item.registry No Startup
item.missing_mappings No Server
item.tags No Server
item.right_click Yes Server
item.right_click_empty No Server
item.left_click No Server
item.entity_interact Yes Server
item.modification No Startup
item.pickup Yes Server
item.tooltip No Client
item.toss Yes Server
item.crafted No Server
item.smelted No Server
fluid.registry No Startup
fluid.tags No Server
entity_type.tags No Server
worldgen.add No Startup
worldgen.remove No Startup


Custom Items

This is a startup_scripts/ event

// Listen to item registry event
onEvent('item.registry', event => {
  // The texture for this item has to be placed in kubejs/assets/kubejs/textures/item/test_item.png
  // If you want a custom item model, you can create one in Blockbench and put it in kubejs/assets/kubejs/models/item/test_item.json
  // You can chain builder methods as much as you like
  // You can specify item type as 2nd argument in create(), some types have different available methods
  event.create('custom_sword', 'sword').tier('diamond').attackDamageBaseline(10.0)

Valid item types:

Other methods item builder supports: [you can chain these methods after create()]

Methods available if you use 'sword', 'pickaxe', 'axe', 'shovel' or 'hoe' type:

Valid tool tiers:

Methods available if you use 'helmet', 'chestplate', 'leggings' or 'boots' type:

Valid armor tiers:

Valid group/creative tab IDs:

Creating custom tool and armor tiers

All values are optional and by default are based on iron tier

onEvent('item.registry.tool_tiers', event => {
  event.add('tier_id', tier => {
    tier.uses = 250
    tier.speed = 6.0
    tier.attackDamageBonus = 2.0
    tier.level = 2
    tier.enchantmentValue = 14
    tier.repairIngredient = '#forge:ingots/iron'
onEvent('item.registry.armor_tiers', event => {
  // Slot indicies are [FEET, LEGS, BODY, HEAD]
  event.add('tier_id', tier => {
    tier.durabilityMultiplier = 15 // Each slot will be multiplied with [13, 15, 16, 11]
    tier.slotProtections = [2, 5, 6, 2]
    tier.enchantmentValue = 9
    tier.equipSound = 'minecraft:item.armor.equip_iron'
    tier.repairIngredient = '#forge:ingots/iron'
    tier.toughness = 0.0 // diamond has 2.0, netherite 3.0
    tier.knockbackResistance = 0.0
Custom Foods

These methods are each optional, and you may include as many or as few as you like.

onEvent('item.registry', event => {
	event.create('magic_steak').food(food => {
    		.saturation(6)//This value does not directly translate to saturation points gained
      		//The real value can be assumed to be:
      		//min(hunger * saturation * 2 + saturation, foodAmountAfterEating)
      		.effect('speed', 600, 0, 1)
      		.alwaysEdible()//Like golden apples
      		.fastToEat()//Like dried kelp
      		.meat()//Dogs are willing to eat it
      		.eaten(ctx => {//runs code upon consumption
        		ctx.player.tell('Yummy Yummy!')
          		//If you want to modify this code then you need to restart the game.
          		//However, if you make this code call a global startup function
          		//and place the function *outside* of an 'onEvent'
          		//then you may use the command:
          		//  /kubejs reload startup_scripts
          		//to reload the function instantly.




This event is the most basic event class, parent of all other events.

Parent class


Can be cancelled


Variables and Functions

Name Return Type Info
cancel() void Cancels event. If the event can't be cancelled, it won't do anything.

Custom Blocks

This is a startup script.

onEvent('block.registry', event => {
       .displayName('Test Block') // No longer required in 1.18.2+
       .tagBlock('minecraft:mineable/shovel') // Make it mine faster using a shovel in 1.18.2+
  	   .tagBlock('minecraft:requries_iron_tool') // Make it require an iron or higher level tool on 1.18.2+
  	   .requiresTool(true) // Make it require a tool to drop ay loot
  // Block with custom type (see below for list of types)
  event.create('test_block_slab', 'slab').material('glass').hardness(0.5)

The texture for this block has to be placed in kubejs/assets/kubejs/textures/block/test_block.png.
If you want a custom block model, you can create one in Blockbench and put it in kubejs/assets/kubejs/models/block/test_block.json.

List of available materials - to change break/walk sounds and to change some properties.

Materials (1.18.2)



Other methods block builder supports: 

Event callbacks:



Detector Block Types:

The detector block type can be used to run code when the block is powered with redstone signal.

Startup script code:

onEvent('block.registry', event => {

Server script code:

onEvent('block.detector.myDetector.unpowered', event => { // you can also use powered and changed instead of upowered


This event needs cleanup! Using it is not recommended.


This event is fired when a command is executed on server side.

Parent class


Can be cancelled


Variables and Functions

Name Type Info
parseResults ParseResults<CommandSource> Command params 
exception Exception Error, set if something went wrong



This event is fired when a tag collection is loaded, to modify it with script. You can add and remove tags for items, blocks, fluids and entity types.

This goes into server scripts.

Tags are per item/block/fluid/entity type and as such cannot be added based on things like NBT data!

Parent class


Can be cancelled


Variables and Functions

Name Type Info
type String Tag collection type.
get(String tag) TagWrapper Returns specific tag container which you can use to add or remove objects to. tag parameter can be something like 'forge:ingots/copper'. If tag doesn't exist, it will create a new one.
add(String tag, String[]/Regex ids) TagWrapper Shortcut method for event.get(tag).add(ids).
remove(String tag, String[]/Regex ids) TagWrapper Shortcut method for event.get(tag).remove(ids).
removeAll(String tag) TagWrapper Shortcut method for event.get(tag).removeAll().
removeAllTagsFrom(String[] ids) void Removes all tags from object

TagWrapper class

Variables and Functions

Name Type Info
add(String[]/Regex ids) TagWrapper (itself) Adds an object to this tag. If string starts with # then it will add all objects from the second tag. It can be either single string, regex (/regex/flags) or array of either.
remove(String[]/Regex ids) TagWrapper (itself) Removes an object from tag, works the same as add().
removeAll() TagWrapper (itself) Removes all entries from tag.
getObjectIds() Collection<ResourceLocation> Returns a list of all entries in a tag. Will resolve any sub-tags.


// Listen to item tag event
onEvent('tags.items', event => {
  // Get the #forge:cobblestone tag collection and add Diamond Ore to it
  event.add('forge:cobblestone', 'minecraft:diamond_ore')
  // Get the #forge:cobblestone tag collection and remove Mossy Cobblestone from it
  event.remove('forge:cobblestone', 'minecraft:mossy_cobblestone')
  // Get #forge:ingots/copper tag and remove all entries from it
  // Required for FTB Quests to check item NBT
  event.add('itemfilters:check_nbt', 'some_item:that_has_nbt_types')
  // You can create new tags the same way you add to existing, just give it a name
  event.add('forge:completely_new_tag', 'minecraft:clay_ball')
  // Removes all tags from this entry
  // Add all items from the forge:stone tag to the c:stone tag, unless the id contains diorite
  const stones = event.get('forge:stone').getObjectIds()
  const blacklist = Ingredient.of(/.*diorite.*/)
  stones.forEach(stone => {
    if (!blacklist.test(stone)) {
      event.add('c:stone', stone)

Recipes use item tags, not block or fluid tags, even if items representing those are blocks. Like minecraft:cobblestone even if it's a block, it will still be an item tag for recipes.

tags.blocks and tags.fluids are for adding tags to block and fluid types, they work the same way. You can find existing block and fluid tags if you look at a block with F3 mode enabled, on side. These are mostly only used for technical reasons, and like mentioned above, if its for recipes/inventory, you will want to use tags.items even for blocks.


Loot Table Modification

onEvent('block.loot_tables', event => {
  event.addSimpleBlock('minecraft:dirt', 'minecraft:red_sand')
onEvent('block.loot_tables', event => {
  event.addSimpleBlock('minecraft:dirt') // To drop itself (fix broken blocks)
  event.addSimpleBlock(/minecraft:.*_ore/, 'minecraft:red_sand') // To drop a different item
onEvent('block.loot_tables', event => {
  event.addBlock('minecraft:dirt', table => { // Build loot table manually
    table.addPool(pool => {
      pool.rolls = 1 // fixed
      // pool.rolls = [4, 6] // or {min: 4, max: 6} // uniform
      // pool.rolls = {n: 4, p: 0.3} // binominal
      pool.addItem('minecraft:dirt', 40) // 40 = weight
      pool.addItem('minecraft:dirt', 40, [4, 8]) // [4-8] = count modifier, uses same syntax as rolls
      // pool.addCondition({json condition, see vanilla wiki})
      // pool.addEntry({json entry, see vanilla wiki for non-items})

Example from Factorial: (adds 1-3 leaves dropped from all Leaves blocks, 4-8 logs from all log and wood blocks and 4-8 stone from Stone, Cobblestone, Andesite, Diorite and Granite)

onEvent('block.loot_tables', event => {
	event.addBlock(/minecraft:.*_leaves/, table => {
		table.addPool(pool => {
			pool.addItem('factorial:leaf', 1, [1, 3])

	event.addBlock(/minecraft:.*_(log|wood)/, table => {
		table.addPool(pool => {
			pool.addItem('factorial:wood', 1, [4, 8])

	], table => {
		table.addPool(pool => {
			pool.rolls = [4, 8] // Roll the pool instead of individual items
			pool.addItem('factorial:stone', 1)

You can also modify existing loot tables to add items to them:

onEvent('block.loot_tables', event => {
  // all dirt blocks have a 50% chance to drop an enchanted diamond sword named "test"
  event.modifyBlock(/^minecraft:.*dirt/, table => {
    table.addPool(pool => {
      pool.addItem('minecraft:diamond_sword').randomChance(0.5).enchantWithLevels(1, true).name(Text.of('Test').blue())

Other loot table types work too:

onEvent('entity.loot_tables', event => {
  // Override zombie loot table that will drop 5 of either carrot (25% chance) or apple (75% chance)
  event.addEntity('minecraft:zombie', table => {
    table.addPool(pool => {
      pool.rolls = 5
      pool.addItem('minecraft:carrot', 1)
      pool.addItem('minecraft:apple', 3)
  event.modifyEntity('minecraft:pig', table => {
    table.addPool(pool => {
      // Modify pig loot table to *also* drop dirt on top of its regular drops

Supported table types:

Event ID Override method name Modify method name
generic.loot_tables addGeneric modify
block.loot_tables addBlock modifyBlock
entity.loot_tables addEntity modifyEntity
gift.loot_tables addGift modify
fishing.loot_tables addFishing modify
chest.loot_tables addChest modify





The most basic script to add a single recipe:

onEvent('recipes', event => {
  event.shaped('3x minecraft:stone', [
    'S S',
  ], {
    S: 'minecraft:sponge',
    A: 'minecraft:apple'

The most basic script to remove a recipe:

onEvent('recipes', event => {
  event.remove({output: 'minecraft:stick'})

Example recipe script:

// kubejs/server_scripts/example.js
// This is just an example script to show off multiple types of recipes and removal methods
// Supports /reload

// Listen to server recipe event
onEvent('recipes', event => {
  // Remove broken recipes from vanilla and other mods
  // This is on by default, so you don't need this line
  //event.removeBrokenRecipes = true

  event.remove({}) // Removes all recipes (nuke option, usually not recommended)
  event.remove({output: 'minecraft:stone_pickaxe'}) // Removes all recipes where output is stone pickaxe
  event.remove({output: '#minecraft:wool'}) // Removes all recipes where output is Wool tag
  event.remove({input: '#forge:dusts/redstone'}) // Removes all recipes where input is Redstone Dust tag
  event.remove({mod: 'quartzchests'}) // Remove all recipes from Quartz Chests mod
  event.remove({type: 'minecraft:campfire_cooking'}) // Remove all campfire cooking recipes
  event.remove({id: 'minecraft:glowstone'}) // Removes recipe by ID. in this case, data/minecraft/recipes/glowstone.json
  event.remove({output: 'minecraft:cooked_chicken', type: 'minecraft:campfire_cooking'}) // You can combine filters, to create ANDk logic
  // You can use 'mod:id' syntax for 1 sized items. For 2+ you need to use '2x mod:id' or Item.of('mod:id', count) syntax. If you want NBT or chance, 2nd is required

  // Add shaped recipe for 3 Stone from 8 Sponge in chest shape
  // (Shortcut for event.recipes.minecraft.crafting_shaped)
  // If you want to use Extended Crafting, replace event.shapeless with event.recipes.extendedcrafting.shapeless_table
  event.shaped('3x minecraft:stone', [
    'S S',
  ], {
    S: 'minecraft:sponge',
    A: 'minecraft:apple'

  // Add shapeless recipe for 4 Cobblestone from 1 Stone and 1 Glowstone
  // (Shortcut for event.recipes.minecraft.crafting_shapeless)
  // If you want to use Extended Crafting, replace event.shapeless with event.recipes.extendedcrafting.shaped_table
  event.shapeless('4x minecraft:cobblestone', ['minecraft:stone', '#forge:dusts/glowstone'])

  // Add Stonecutter recipe for Golden Apple to 4 Apples
  event.stonecutting('4x minecraft:apple', 'minecraft:golden_apple')
  // Add Stonecutter recipe for Golden Apple to 2 Carrots
  event.stonecutting('2x minecraft:carrot', 'minecraft:golden_apple')

  // Add Furnace recipe for Golden Apple to 3 Carrots
  // (Shortcut for event.recipes.minecraft.smelting)
  event.smelting('2x minecraft:carrot', 'minecraft:golden_apple')
  // Similar recipe to above but this time it has a custom, static ID - normally IDs are auto-generated and will change. Useful for Patchouli
  event.smelting('minecraft:golden_apple', 'minecraft:carrot').id('mymodpack:my_recipe_id')

  // Add similar recipes for Blast Furnace, Smoker and Campfire
  event.blasting('3x minecraft:apple', 'minecraft:golden_apple')
  event.smoking('5x minecraft:apple', 'minecraft:golden_apple')
  event.campfireCooking('8x minecraft:apple', 'minecraft:golden_apple')
  // You can also add .xp(1.0) at end of any smelting recipe to change given XP
  // Add a smithing recipe that combines 2 items into one (in this case apple and gold ingot into golden apple)
  event.smithing('minecraft:golden_apple', 'minecraft:apple', 'minecraft:gold_ingot')

  // Create a function and use that to make things shorter. You can combine multiple actions
  let multiSmelt = (output, input, includeBlasting) => {
    event.smelting(output, input)
    if (includeBlasting) {
      event.blasting(output, input)
  multiSmelt('minecraft:blue_dye', '#forge:gems/lapis', true)
  multiSmelt('minecraft:black_dye', 'minecraft:ink_sac', true)
  multiSmelt('minecraft:white_dye', 'minecraft:bone_meal', false)

  // If you use custom({json}) it will be using vanilla Json/datapack syntax. Must include "type": "mod:recipe_id"!
  // You can add recipe to any recipe handler that uses vanilla recipe system or isn't supported by KubeJS
  // You can copy-paste the json directly, but you can also make more javascript-y by removing quotation marks from keys
  // You can replace {item: 'x', count: 4} in result fields with Item.of('x', 4).toResultJson()
  // You can replace {item: 'x'} / {tag: 'x'} with Ingredient.of('x').toJson() or Ingredient.of('#x').toJson()
  // In this case, add Create's crushing recipe, Oak Sapling to Apple + 50% Carrot
  // Important! Create has integration already, so you don't need to use this. This is just an example for datapack recipes!
    type: 'create:crushing',
    ingredients: [
    results: [
    processingTime: 100
  // Example of using items with NBT in a recipe
  event.shaped('minecraft:book', [
  ], {
    C: '#forge:cobblestone',
    // Item.of('id', {key: value}), it's recommended to use /kubejs hand
    L: Item.of('minecraft:enchanted_book', {StoredEnchantments:[{lvl:1,id:"minecraft:sweeping"}]}),
    // Same principle, but if its an enchantment, there's a helper method
    W: Item.of('minecraft:enchanted_book').enchant('minecraft:respiration', 2),
    G: '#forge:glass'
  // In all shapeless crafting recipes, replace any planks with Gold Nugget in input items
  event.replaceInput({type: 'minecraft:crafting_shapeless'}, '#minecraft:planks', 'minecraft:gold_nugget')
  // In all recipes, replace Stick with Oak Sapling in output items 
  event.replaceOutput({}, 'minecraft:stick', 'minecraft:oak_sapling')
  // By default KubeJS will mirror and shrink recipes, which makes things like UU-Matter crafting (from ic2) harder to do as you have less shapes.
  // You can use noMirror() and noShrink() to stop this behaviour.
  event.shaped('9x minecraft:emerald', [
    ' D ',
    'D  ',
    '   '
  ], {
    D: 'minecraft:diamond'

Possible settings you can change for recipes. It's recommended that you put this in it's own server scripts file, like settings.js

// priority: 5

// Enable recipe logging, off by default
settings.logAddedRecipes = true
settings.logRemovedRecipes = true
// Enable skipped recipe logging, off by default
settings.logSkippedRecipes = true
// Enable erroring recipe logging, on by default, recommended to be kept to true
settings.logErroringRecipes = false

As mentioned before, you can add any recipe from any mod with JSON syntax (see event.custom({})) but these mods are supported as addons with special syntax:

Ingredient Actions

Poorly documented things below!

You can transform ingredients in shaped and shapeless recipes by adding these functions at end of it:

IngredientFilter can be either


onEvent('recipes', event => {
  	event.shapeless('9x minecraft:melon_slice', [ // Craft 9 watermelon slices
		Item.of('minecraft:diamond_sword').ignoreNBT(), // Diamond sword that ignores damage
		'minecraft:melon' // Watermelon block
	]).damageIngredient(Item.of('minecraft:diamond_sword').ignoreNBT()) // Damage the sword (also has to ignore damage or only 0 damage will work)
    // Craft example block from 2 diamond swords and 2 dirt. After crafting first diamond sword is damaged (index 0) and 2nd sword is kept without changes.
	event.shaped('kubejs:example_block', [
		'SD ',
		'D S'
	], {
		S: Item.of('minecraft:diamond_sword').ignoreNBT(),
		D: 'minecraft:dirt'

    // Craft example block from 2 diamond swords and 2 stones. After crafting, diamond sword is replaced with stone sword
	event.shapeless('kubejs:example_block', [
	]).replaceIngredient('minecraft:diamond_sword', 'minecraft:stone_sword')

    // Craft clay from sand, bone meal, dirt and water bottle. After crafting, glass bottle is left in place of water bottle
	event.shapeless('minecraft:clay', [
		Item.of('minecraft:potion', {Potion: "minecraft:water"})
	]).replaceIngredient({item: Item.of('minecraft:potion', {Potion: "minecraft:water"})}, 'minecraft:glass_bottle')
  	// Register a customIngredientAction, and recipe that uses it
  	// This one takes the nbt from an enchanted book and applies it to a tool in the crafting table, for no cost.
  	// Thanks to Prunoideae for providing it!
  	Ingredient.registerCustomIngredientAction("apply_enchantment", (itemstack, index, inventory) => {
        let enchantment = inventory.get(inventory.find(Item.of("minecraft:enchanted_book").ignoreNBT())).nbt;
        if (itemstack.nbt == null)
            itemstack.nbt = {}
        itemstack.nbt = itemstack.nbt.merge({ Enchantments: enchantment.get("StoredEnchantments") })
        return itemstack;
 	event.shapeless("minecraft:book", ["#forge:tools", Item.of("minecraft:enchanted_book").ignoreNBT()])
        .customIngredientAction("#forge:tools", "apply_enchantment")

Item Modification

item.modification event is a startup script event that allows you to change properties of existing items

onEvent('item.modification', event => {
  event.modify('minecraft:ender_pearl', item => {
    item.maxStackSize = 64
    item.fireResistant = true

All available properties:


WorldgenAddEventJS (1.16)

This event isn't complete yet and can only do basic things. Adding dimension-specific features also isn't possible yet, but is planned.

Example script: (kubejs/startup_scripts/worldgen.js)

onEvent('worldgen.add', event => {
  event.addLake(lake => { // Create new lake feature
    lake.block = 'minecraft:diamond_block' // Block ID (Use [] syntax for properties)
    lake.chance = 3 // Spawns every ~3 chunks

  event.addOre(ore => {
    ore.block = 'minecraft:glowstone' // Block ID (Use [] syntax for properties)
    ore.spawnsIn.blacklist = false // Inverts spawn whitelist
    ore.spawnsIn.values = [ // List of valid block IDs or tags that the ore can spawn in
      '#minecraft:base_stone_overworld' // Default behavior - ores spawn in all stone types
    ore.biomes.blacklist = true // Inverts biome whitelist
    ore.biomes.values = [ // Biomes this ore can spawn in
      'minecraft:plains', // Biome ID
      '#nether' // OR #category, see list of categories below
    ore.clusterMinSize = 5 // Min blocks per cluster (currently ignored, will be implemented later, it's always 1)
    ore.clusterMaxSize = 9 // Max blocks per cluster
    ore.clusterCount = 30 // Clusters per chunk
    ore.minHeight = 0 // Min Y ore spawns in
    ore.maxHeight = 64 // Max Y ore spawns in
    ore.squared = true // Adds random value to X and Z between 0 and 16. Recommended to be true
    // ore.chance = 4 // Spawns the ore every ~4 chunks. You usually combine this with clusterCount = 1 for rare ores
  event.addSpawn(spawn => { // Create new entity spawn
    spawn.category = 'creature' // Category, can be one of 'creature', 'monster', 'ambient', 'water_creature' or 'water_ambient'
    spawn.entity = 'minecraft:pig' // Entity ID
    spawn.weight = 10 // Weight
    spawn.minCount = 4 // Min entities per group
    spawn.maxCount = 4 // Max entities per group

All values are optional. All feature types have biomes field like addOre example

Valid biome categories ('#category'):

You can also use ('$type' (case doesn't matter)) on Forge's BiomeDictionary:

This is the order vanilla worldgen happens:

  1. raw_generation
  2. lakes
  3. local_modifications
  4. underground_structures
  5. surface_structures
  6. strongholds
  7. underground_ores
  8. underground_decoration
  9. vegetal_decoration
  10. top_layer_modification

It's possible you may not be able to generate some things in their layer, like ores in dirt, because dirt hasn't spawned yet. So you may have to change the layer by calling ore.worldgenLayer = 'top_layer_modification'. But this is not recommended.

If you want to remove things, see this event.


Block Modification

block.modification event is a startup script event that allows you to change properties of existing blocks

onEvent('block.modification', event => {
  event.modify('minecraft:stone', block => {
    block.destroySpeed = 0.1
    block.hasCollision = false

All available properties:


JEI Integration

All JEI events are client sided and so go in the client_scripts folder


onEvent('jei.subtypes', event => {
  event.useNBTKey('example:item', 'type')

Hide Items & Fluids

onEvent('jei.hide.items', event => {

onEvent('jei.hide.fluids', event => {

Add Items & Fluids

onEvent('jei.add.items', event => {
  event.add(Item.of('example:item', {test: 123}))

onEvent('jei.add.fluids', event => {

Add Information

onEvent('jei.information', event => {
  event.add('example:ingredient', ['Line 1', 'Line 2'])

Hide categories

onEvent('jei.remove.categories', event => {
  console.log(event.getCategoryIds()) //log a list of all category ids to logs/kubejs/client.txt

WorldgenRemoveEventJS (1.16)

For more information on biomes field, see worldgen.add event page.

onEvent('worldgen.remove', event => {
  event.removeOres(ores => {
    ores.blocks = [ 'minecraft:coal_ore', 'minecraft:iron_ore' ] // Removes coal and iron ore
    ores.biomes.values = [ 'minecraft:plains' ] // Removes it only from plains biomes
  event.removeSpawnsByID(spawns => {
    spawns.entities.values = [
  event.removeSpawnsByCategory(spawns => {
    spawns.biomes.values = [
    spawns.categories.values = [

If something isn't removing, you may try to remove it "manually" by first printing all features (this will spam your console a lot, I suggest reading logs/kubejs/startup.txt) and then removing them by ID where possible.

onEvent('worldgen.remove', event => {
  // May be one of the decoration types/levels described in worldgen.add docs
  // But ores are *most likely* to be generated in this one
onEvent('worldgen.remove', event => {
  event.removeFeatureById('underground_ores', 'mekanism:ore_copper')

REI Integration

Note: REI integration only works on Fabric in 1.16. In 1.18+, it works on both Forge and Fabric!

All REI events are client sided and so go in the client_scripts folder

Hide Items

onEvent('rei.hide.items', event => {

Add Items

onEvent('rei.add.items', event => {
  event.add(item.of('example:item').nbt({test: 123}))

Add Information

onEvent('rei.information', event => {
  event.add('example:ingredient', 'Title', ['Line 1', 'Line 2'])

Yeet categories

onEvent('rei.remove.categories', event => {
  console.log(event.getCategoryIds()) //log a list of all category ids to logs/kubejs/client.txt
  //event.remove works too, but yeeting is so much more fun 😉

Grouping / Collapsible Entries (1.18.2+)

onEvent("rei.group", (event) => {
    // This event allows you to add custom entry groups to REI, which can be used to clean up the entry list significantly.
    // As a simple example, we can add a "Swords" group which will contain all (vanilla) swords
    // Note that each group will need an id (ResourceLocation) and a display name (Component / String)
    event.groupItems("kubejs:rei_groups/swords", "Swords", [

    // An easy use case for grouping stuff together could be using tags:
    // In this case, we want all the Hanging Signs and Sign Posts from Supplementaries to be grouped together
    event.groupItemsByTag("supplementaries:rei_groups/hanging_signs", "Hanging Signs", "supplementaries:hanging_signs");
    event.groupItemsByTag("supplementaries:rei_groups/sign_posts", "Sign Posts", "supplementaries:sign_posts");

    // Another example: We want all of these items to be grouped together ignoring NBT,
    // so you don't have a bajillion potions and enchanted books cluttering up REI anymore
    const useNbt = ["potion", "enchanted_book", "splash_potion", "tipped_arrow", "lingering_potion"];

    useNbt.forEach((id) => {
        const item = Item.of(id);
        const { namespace, path } = Utils.id(item.id);
        event.groupSameItem(`kubejs:rei_groups/${namespace}/${path}`, item.getName(), item);

    // Items can also be grouped using anything that can be expressed as an IngredientJS,
    // including for example regular expressions or lists of ingredients
    event.groupItems("kubejs:rei_groups/spawn_eggs", "Spawn Eggs", [

    // you can even use custom predicates for grouping, like so:
    event.groupItemsIf("kubejs:rei_groups/looting_stuff", "Stuff with Looting I", (item) =>
        // this would group together all items that have the Looting I enchantment on them
        item.hasEnchantment("minecraft:looting", 1)

    // you can also group fluids in much the same way as you can group items, for instance:
    event.groupFluidsByTag("kubejs:rei_groups/fluid_tagged_as_water", `"Water" (yeah right lmao)`, "minecraft:water");


A client event that allows adding tooltips to any item!

onEvent('item.tooltip', tooltip => {
  // Add tooltip to all of these items
  tooltip.add(['quark:backpack', 'quark:magnet', 'quark:crate'], 'Added by Quark Oddities')
  // You can also use any ingredient except #tag (due to tags loading much later than client scripts)
  tooltip.add(/refinedstorage:red_/, 'Can be any color')
  // Multiple lines with an array []. You can also escape ' by using other type of quotation marks
  tooltip.add('thermal:latex_bucket', ["Not equivalent to Industrial Foregoing's Latex", 'Line 2 text would go here'])
  // Use some data from the client to decorate this tooltip. name returns a component so we just append that.
  tooltip.add('minecraft:skeleton_skull', Text.of('This used to be ').append(Client.player.name).append("'s head"))
  tooltip.addAdvanced('thermal:latex_bucket', (item, advanced, text) => {
    text.add(0, Text.of('Hello')) // Adds text in first line, pushing the items name down a line. If you want the line below the item name, the index must be 1
  tooltip.addAdvanced('minecraft:beacon', (item, advanced, text) => {
    // shift, alt and ctrl are all keys you can check!
    if (!tooltip.shift) {
      text.add(1, [Text.of('Hold ').gold(), Text.of('Shift ').yellow(), Text.of('to see more info.').gold()])
    } else {
      text.add(1, Text.green('Gives positive effects to players in a range').bold(true))
      text.add(2, Text.red('Requires a base built out of precious metals or gems to function!'))
      text.add(3, [Text.white('Iron, '), Text.aqua('Diamonds, '), Text.gold('Gold '), Text.white('or even '), Text.green('Emeralds '), Text.white('are valid base blocks!')])
  // Neat utility to display NBT in the tooltip
  tooltip.addAdvanced(Ingredient.all, (item, advanced, text) => {
    if (tooltip.alt && item.nbt) {
      text.add(Text.of('NBT: ').append(Text.prettyPrintNbt(item.nbt)))
  // Show the name of the player who owns the skull in a skulls tooltip
  tooltip.addAdvanced('minecraft:player_head', (item, advanced, text) => {
    let playername = item.nbt?.SkullOwner?.Name
    if (playername) {
      text.add(Text.red(`The head of ${playername}`))

Worldgen Events

These following examples will only work on 1.18+! If you need examples for 1.16, you can look here if you want to add new features to world generation and here if you want to remove features from it.

General Notes

Biome Filters:

Biome filters work similarly to recipe filters, and can be used to create complex and exact filters to fine tune exactly where your features may and may not spawn in the world. They are used for the biomes field of a feature and may look something like this:

onEvent('worldgen.add', event => {
	event.addOre((ore) => {
      // let's look at all of the "simple" filters first
      ore.biomes = "minecraft:plains" 		// only spawn in exactly this biome
      ore.biomes = /^minecraft:.*/			// spawn in all biomes that match the given pattern (here: anything that starts with minecraft:)
      ore.biomes = "#minecraft:is_forest" 	// [1.19+] spawn in all biomes tagged as "minecraft:is_forest"
      ore.biomes = "^nether" 				// [1.18 only!] spawn in all biomes in the "nether" category (see Biome Categories)
      ore.biomes = "$hot"					// [Forge 1.18 only!] spawn in all biomes that have the "hot" biome type (see Biome Dictionary)
      // filters can be arbitrarily combined using AND, OR and NOT logic
      ore.biomes = {}						// empty AND filter, always true
      ore.biomes = []						// empty OR filter, also always true
      ore.biomes = {
        not: "minecraft:ocean"				// spawn in all biomes that are NOT "minecaraft:ocean"
      // since AND filters are expressed as maps and expect string keys,
      // all of the "primitive" filters can also be expressed as such
      ore.biomes = {					// see above for an explanation of these filters
        id: "minecraft:plains",
        id: /^minecraft:.*/,			// regex (also technically an id filter)
        tag: "#minecraft:is_forest",
        category: "^nether",
        biome_type: "$hot",
      // note all of the above syntax may be mixed and matched individually
      // for example, this will spawn the feature in any biome that is
      // either plains, or a hot biome that is not in the nether or savanna categories
      ore.biomes = [
          biome_type: "$hot",
          not: [
            	category: "savanna"


Rule Tests and Targets:

In 1.18, Minecraft worldgen has changed to a "target"-based replacement system, meaning you can specify specific blocks to be replaced with specific other blocks within the same feature configuration. (This is used to replace stone with the normal ore and deepslate with the deepslate ore variant, for example).

Each target gets a "rule test" as input (something that checks if a given block state should be replaced or not) and produces a specific output block state. On the KubeJS script side, both of these concepts are expressed as the same class: BlockStatePredicate.

Syntax-wise, BlockStatePredicate is pretty similar to biome filters as they too can be combined using AND or OR filters (which is why we will not be repeating that step here), and can be used to match one of three different things fundamentally:

  1. Blocks (these are simply parsed as strings, so for example "minecraft:stone" to match Stone)
  2. Block States (these are parsed as the block id followed by an array of properties, so you would use something like "minecraft:furnace[lit=true]" to match only furnace blocks that are active (lit). You can use F3 to figure out a block's properties, as well as possible values through using the debug stick.
  3. Block Tags (as you might expect, these are parsed in the "familiar" tag syntax, so you could for example use "#minecraft:base_stone_overworld" to match all types of stone that can be found generating in the ground in the overworld. Note that these are block tags, not item tags, so they may (and probably will) be different! (F3 is your friend!)

You can also use regular expressions with block filters, so /^mekanism:.*_ore$/ would match any block from Mekanism whose id ends with "_ore". Keep in mind this will not match block state properties!

When a RuleTest is required instead of a BlockStatePredicate, you can also supply that rule test directly in the form of a JavaScript object (it will then be parsed the same as vanilla would parse JSON or NBT objects). This can be useful if you want rule tests that have a random chance to match, for example!

More examples on how targets work can be found in the example script down below.

Height Providers:

Another system that may appear a bit confusing at first is the system of "height providers", which are used to determine at what Y level a given ore should spawn and with what frequency. Used in tandem with this feature are the so-called "vertical anchors", which may be used to get the height of something relative to a specific anchor point (for example the top or bottom of the world)

In KubeJS, this system has been simplified a bit to make it easier to use for script developers: To use the two most common types of ore placement, uniform (the feature has the same chance to spawn anywhere in between the two anchors) and triangle (the feature is more likely to spawn in the center of the two anchors than it is to spawn further outwards), you can use the methods uniformHeight and traingleHeight in AddOreProperties, respectively. Vertical anchors have also been simplified, as you can use the aboveBottom / belowTop helper methods in AddOreProperties, or, in newer KubeJS versions, the builtin class wrapper for VerticalAnchor (Note that the former has been deprecated in favour of the latter), as well as if you want to specify absolute heights as simple numbers, instead.

Once again, see the example script for more information!

(1.18 only!) Biome Categories:

Biome categories are a vanilla system that can be used to roughly sort biomes into predefined categories, which are noted below. Note that other mods may add more categories through extending the enum, however since there is no way for us to know this we will only provide you with the vanilla IDs here:

(1.18 and Forge only!) Biome Dictionary:

Much like Vanilla biome categories, Forge uses a "Biome Dictionary" to sort biomes based on their properties. Note that this system is designed to be extended by mods, so there is no way for us to give a complete list of all categories to you, however some of the ones you might commonly find yourself using are listed here:

In 1.19, both of these systems have been removed with no replacement in favour of biome tags!

Example script: (kubejs/startup_scripts/worldgen.js)

onEvent('worldgen.add', event => {
	// use the anchors helper from the event (NOTE: this requires newer versions of KubeJS)
	// if you are using an older version of KubeJS, you can use the methods in the ore properties class
	const {anchors} = event

	event.addOre((ore) => {
		ore.id = "kubejs:glowstone_test_lmao" // (optional, but recommended) custom id for the feature
		ore.biomes = {
			not: "^savanna" // biome filter, see above (technically also optional)

		// examples on how to use targets
		ore.addTarget('#minecraft:stone_ore_replaceables', 'minecraft:glowstone') // replace anything in the vanilla stone_ore_replaceables tag with Glowstone
		ore.addTarget('minecraft:deepslate', 'minecraft:nether_wart_block')       // replace Deepslate with Nether Wart Blocks
			'minecraft:gravel',   // replace gravel...
			/minecraft:(.*)_dirt/ // or any kind of dirt (including coarse, rooted, etc.)...
		], 'minecraft:tnt')       // with TNT (trust me, it'll be funny)

		ore.count([15, 50])                      // generate between 15 and 50 veins (chosen at random), you could use a single number here for a fixed amount of blocks
				.squared()                       // randomly spreads the ores out across the chunk, instead of generating them in a column
				.triangleHeight(				 // generate the ore with a triangular distribution, this means it will be more likely to be placed closer to the center of the anchors
						anchors.aboveBottom(32), // the lower bound should be 32 blocks above the bottom of the world, so in this case, Y = -32 since minY = -64
						anchors.absolute(96)	 // the upper bound, meanwhile is set to be just exactly at Y = 96
				)								 // in total, the ore can be found between Y levels -32 and 96, and will be most likely at Y = (96 + -32) / 2 = 32
        // more, optional parameters (default values are shown here)
        ore.size = 9                            // max. vein size
        ore.noSurface = 0.5                     // chance to discard if the ore would be exposed to air
        ore.worldgenLayer = "underground_ores"  // what generation step the ores should be generated in (see below)
      	ore.chance = 0							// if != 0 and count is unset, the ore has a 1/n chance to generate per chunk

	// oh yeah, and did I mention lakes exist, too?
	// (for now at least, Vanilla is likely to remove them in the future)
	event.addLake((lake) => {
		lake.id = "kubejs:funny_lake" // BlockStatePredicate
		lake.chance = 4
		lake.fluid = "minecraft:lava"
		lake.barrier = "minecraft:diamond_block"

onEvent('worldgen.remove', event => {
	//console.debugEnabled = true;

	// print all features for a given biome filter
	event.printFeatures('', 'minecraft:plains')

	event.removeOres((props) => {
		// much like ADDING ores, this supports filtering by worldgen layer...
		props.worldgenLayer = 'underground_ores'
		// ...and by biome
		props.biomes = [{
			category: "icy",
		}, {
			category: "savanna",
		}, {
			category: "mesa",

		// BlockStatePredicate to remove iron and copper ores from the given biomes
		// Note tags may NOT be used here, unfortunately...
		props.blocks = ["minecraft:iron_ore", "minecraft:copper_ore"]

	// remove features by their id (first argument is a generation step)
	event.removeFeatureById('underground_ores', ['minecraft:ore_coal_upper', 'minecraft:ore_coal_lower'])

Generation Steps

  1. raw_generation
  2. lakes
  3. local_modifications
  4. underground_structures
  5. surface_structures
  6. strongholds
  7. underground_ores
  8. underground_decoration
  9. vegetal_decoration
  10. top_layer_modification

It's possible you may not be able to generate some things in their layer, like ores in dirt, because dirt hasn't spawned yet. So you may have to change the layer to one of the above generation steps by calling ore.worldgenLayer = 'top_layer_modification'. This is, however, not recommended.


Chat Event

This script is peak of human evolution. Whenever someone says "Creeper" in chat, it replies with "Aw man".

onEvent('player.chat', (event) => {
  // Check if message equals creeper, ignoring case
  if (event.message.trim().equalsIgnoreCase('creeper')) {
    // Schedule task in 1 tick, because if you reply immidiently, it will print before player's message
    event.server.scheduleInTicks(1, event.server, (callback) => {
      // Tell everyone Aw man, colored green. Callback data is the server
      callback.data.tell(Text.green('Aw man'))

Another example, cancelling the chat event. No need to schedule anything now, because player's message wont be printed,

onEvent('player.chat', (event) => {
  // Check if message equals creeper, ignoring case
  if (event.message.startsWith('!some_command')) {

Custom Fluids

Supported by Forge on all versions, and Fabric on 1.18.2+

// Startup script
onEvent('fluid.registry', event => {
  // These first examples are 1.16.5 and 1.18.2 syntax
  // Basic "thick" (looks like lava) fluid with red tint
    .displayName('Thick Fluid')
  // Basic "thin" (looks like water) fluid with cyan tint, has no bucket and is not placeable
    .displayName('Thin Fluid')
  	.noBucket() // both these methods are 1.18.2+ only
  // Fluid with custom textures
  	.displayName('Strawberry Cream')
  // For 1.18.1 the syntax is slightly different
  event.create('thick_fluid', fluid => {
       fluid.textureThick(0xFF0000) // the texture method names are different in 1.18.1 and below, textureXyz instead of xyzTexture
       fluid.displayName('Thick Fluid')

In 1.18.1, 1.17 and 1.16 the texture method names are swapped, so textureStill and textureThin instead of stillTexture and thinTexture


Available fields and methods and examples on how to use them



Parent class of all Java objects. 


None (and itself at the same time, don't question it)

Variables and Functions

Name Type Info
toString() String Tag collection type.
equals(Object other) boolean Checks equality with another object.
hashCode() int Hash code of this object. It is used to optimize maps and other things, should never be used for object equality.
class Class Object's type/class.


Class of string objects, such as "abc" (and in JS 'abc' works as well) 



Variables and Functions

Name Type Info
empty boolean Returns if string is empty a.k.a string === ''
toLowerCase() String Returns a copy of this string, but with all characters in upper case
toUpperCase() String Returns a copy of this string, but with all characters in lower case
equalsIgnoseCase(String other) boolean Hash code of this object. It is used to optimize maps and other things, should never be used for object equality.
length() int Number of characters
charAt(int index) char Single character at index

Primitive Types


Primitive types are objects that don't have a real class and don't inherit methods from Object.

All primitive types

Type Java class Info
void Void No type
byte Byte 8 bit decimal number.
short Short 16 bit decimal number.
int Integer 32 bit decimal number, most common decimal type.
long Long 64 bit decimal number.
float Float 32 bit floating point number.
double Double 64 bit floating point number.
char Character Single character in String such as 'a' or '-'.
boolean Boolean Only true and false values. Can be checked in if function without comparing to true, as if (x) or if (!x) instead of if (x == true) or if (x == false).


Constants, classes and functions


Components, KubeJS and you!

In 1.18.2 and beyond KubeJS uses Components in a lot of places. It returns them for entity names, item names and accepts them for everything from tooltips to sending messages to players.

All examples use event.player.tell from the player.chat event to output their example, but they will with anywhere that accepts a Component!

Making your own Components starts from the ComponentWrapper class, invokable with just Component or Text from anywhere. The examples all use Component but Text works just the same.

ComponentWrapper methods:


Return Type Info
of(Object o) MutableComponent Returns a component based on what was input. Accepts strings, primitives like numbers, snbt/nbt format of Components and a couple others.
clickEventOf(Object o) ClickEvent Returns a ClickEvent based on what was input. See examples below
prettyPrintNbt(Tag tag) Component Returns a component with a prettified version of the input NBT.
join(MutableComponent seperator, Iterable<? extends Component> texts) MutableComponent Returns the result of looping through texts and joining them, separating each one with seperator.
string(String text) MutableComponent Returns a basic unformatted TextComponent with just the input text
translate(String key) MutableComponent Returns a basic unformatted TranslatableComponent with the input key.
translate(String key, Object... objects) MutableComponent Returns an unformatted TranslatableComponent with objects as the replacements for %s in the translation output. 
keybind(String keybind) MutableComponent Returns a basic unformatted KeybindComponent with the specified keybind.
<color>(Object text) MutableComponent

Returns a basic Component with the specified color for text coloring. Valid colors are in the list below. Do not include the <> brackets.

A list of colors accepted in various places:

Basic examples:

onEvent('player.chat', event => {
  // Tell the player a normal message
  event.player.tell(Component.string('Hello world'))
  // Now in black
  event.player.tell(Component.black('Welcome to the dark side, we have cookies!'))
  // Tell them the diamond item, in whatever language they have set
  // Now tell them whatever key they have crouching set to
  // And finally show them the nbt data of the item they are holding


These are methods you can call on any MutableComponent. This includes ComponentKJS, which is a KubeJS extension for vanilla's components and is injected into vanillas code on runtime. All methods from ComponentKJS are included, but only relevant ones from vanilla are included.


Return Type Info
iterator() Iterator<Component> Returns an Iterator for the components contained in this component, useful for when multiple have been joined or appended. From ComponentKJS.
self() MutableComponent Returns the component you ran it on. From ComponentKJS.
toJson() JsonElement Returns the Json representation of this Component. From ComponentKJS.
<color>() MutableComponent Modifies the Component with the specified color applied as formatting, and returns itself. Do not include the <> brackets. From ComponentKJS.
color(Color c) MutableComponent Modifies the Component to have the input Color, and returns itself. (Color is a Rhino color). From ComponentKJS.
noColor() MutableComponent Modifies the Component to have no color, and returns itself. From ComponentKJS.






MutableComponent Modifies the Component to have said formatting and returns itself. From ComponentKJS.

bold(@Nullable Boolean value)

italic(@Nullable Boolean value)

underlined(@Nullable Boolean value)

strikethrough(@Nullable Boolean value)

obfuscated(@Nullable Boolean value)

MutableComponent Modifies the Component to have said formatting and returns itself. From ComponentKJS.
insertion(@Nullable String s) MutableComponent Makes the Component insert the specified string into the players chat box when shift clicked (does not send it) and returns itself. From ComponentKJS.
font(@Nullable ResourceLocation s) MutableComponent Changes the Components font to the specified font and returns itself. For more information on adding fonts see the Minecraft Wiki's Resource packs page. From ComponentKJS.
click(@Nullable ClickEvent s) MutableComponent Sets this components ClickEvent to the specified ClickEvent. From ComponentKJS.
hover(@Nullable Component s) MutableComponent Sets the hover tooltip for this Component to the input Component. From ComponentKJS.
setStyle(Style style) MutableComponent Sets the style to the input Style (net.minecraft.network.chat.Style) and returns itself. Not recommended for use, use the specific methods added by CompontentKJS instead.
append(String string) MutableComponent Appends the input string as a basic TextComponent to this Component then returns itself.
append(Component component) MutableComponent Appends the input Component to this Component then returns itself.
withStyle(Style style) MutableComponent Merges the input style with the current style, preffering properties from the new style if a conflict exists.
getStyle() Style Returns this Components current Style.
getContents() MutableComponent Returns this Components contents. Will return the text for TextComponents, the pattern for SelectorComponents and an empty string for all other Components.
getSiblings() List<Component> Returns a list of all Components which have been append()ed to this Component 
plainCopy() BaseComponent Returns a basic copy of this, preserving only the contents and not the style or siblings.
copy() MutableComponent Returns a full copy of this Component, preserving style and siblings
getString() String Returns this components text as a String. Will return a blank string for any non-text component

More complex examples:

// First a prefix, like a rank. This won't be changing so we can just declare it up here.
const prefix = Component.darkRed('[Admin]').underlined()

onEvent('player.chat', event => {
  // First cancel the event because we are going to be sending the message ourselves
  // The main Component we will be adding stuff to. It is just a copy of the prefix component for now
  let component = prefix.copy() // If we didn't copy it all the modifications we made to it would be applied to the original as well!
  // Make a component of the players name and then surround with < > and make it white again. Then append it our main copmponent.
  // A component will inherit any styiling it doesnt have from whatever it has been .append()ed to, so you need to apply formatting	rather liberally some times!
  let playerName = Component.string(event.getUsername())
  // Doing it this way means we only have to apply the white formatting and no underline once to the name
  let nameComponent = Component.white(' <').underlined(false).append(playerName).append('> ')
  // Finnally add the message (obfuscated, of course) and send it!
  // We make sure to set its color and underline though, otherwise it will end up inheriting the red and underline from the prefix!


Item and Ingredient

When making recipes you can specify items in many ways, the most common is just to use 'namspace:id', like 'minecraft:diamond', however you can also use Item#of and Ingredient#of for advanced additions, such as NBT or count.

Note that Item and Ingredient are not the same! They may work similarly but there are differences. Item can only ever represent a single item type whereas Ingredient can represent multiple item types (and multiple instances of the same item type with different properties such as NBT data). For most cases Ingredient should be preferred over Item.


Its Java class name is ItemWrapper but it is bound to Item in JS.


Return Type Info
of(ItemStackJS in) ItemStackJS

Returns an ItemStackJS based on what was input. 

Note that this relies mostly on Rhinos type wrapping to function, see paragraph below about ItemStackJS#of for more info

of(ItemStackJS in, int count) ItemStackJS

See above. count will override any other count set from the first parameter.

of(ItemStackJS in, CompoundTag tag) ItemStackJS

See above. NBT is merged, with the input NBT taking priority over existing NBT.

of(ItemStackJS in, int count, CompoundTag nbt) ItemStackJS

Combines the functionality of the above two.

withNBT(ItemStackJS in, CompoundTag nbt) ItemStackJS

Same as the corresponding #of.

withChance(ItemStackJS in, double chance) ItemStackJS

Same as #of, chance will override currently set chance.

getList() ListJS

Returns a list of ItemStackJS, one per registered item.

getTypeList() ListJS

Returns a list of String, one per registered item.

getEmpty() ItemStackJS

Returns ItemSTackJS.EMPTY

clearListCache() void

Clears the caches used for #getList and #getTypeList 

fireworks(Map<String, Object> properties) FireworkJS

Returns a FireworkJS based on the input map of propeties. See FireworkJS#of on the FireworkJS page for more information <TODO: Make and link FireworkJS page>

getItem(ResourceLocation id) Item

Returns the instance of the Item class associated with the item id passed in.

@Nullable findGroup(String id) CreativeModTab

Returns the Creative tab associated with the id passed in, returns null if none found.

exists(ResourceLocation id) boolean

Returns if the item id passed in exists or not.

isItem(@Nullable Object o) boolean

Just does an instanceof ItemStackJS check on the object passed in.

Item#of relies on Rhinos type wrapping to function, which calls ItemStackJS#of. This tries its best to turn the input into an ItemStackJS. If no match is found ItemStackJS.EMPTY is returned. Valid inputs:


Its Java class name is IngredientWrapper but it is bound to Ingredient in JS. All static methods.


Return Type Info
getNone() IngredientJS

Returns ItemStack.EMPTY

getAll() IngredientJS

Returns an IngredientJS of every single item in game. All of them.

of(Object object) IngredientJS

Works exactly the same as Item#of except it recognises Ingredient and forge json ingredient syntax.

of(Object object, int count) IngredientJS

Same as above. The count passed in will override any from the first parameter.

custom(Predicate<ItemStackJS> predicate) IngredientJS

Takes the arrow function or anonymous function passed in and makes an IngredientJS with that as IngredientJS#test.

Return true from the function if the ItemStackJS passed should match as an ingredient. 

custom(IngredientJS in, Predicate<ItemStackJS> predicate) IngredientJS

Same as above except it must match the IngredientJS passed in as the first parameter before the custom function is called.

customNBT(IngredientJS in, Predicate<CompoundTag> predicate) IngredientJS

Same as above except the Predicate is passed the items NBT instead of the full ItemStackJS. Useful for advanced NBT matching. 

matchAny(Object objects) IngredientJS

Adds the passed in object to an ingredient. If it is a list then it adds all items in the list. All objects are passed through #of before adding.

registerCustomIngredientAction(String id, CustomIngredientActionCallback callback) void

Registers a custom ingredient action. See the recipe page for more information.

isIngredient(@Nullable Object o) boolean

Just does an instanceof IngredientJS check on the object passed in.

Remember that Item and Ingredient are not equivalent!


<TODO: examples>


A wrapper class for vanilla's ItemStack. All methods listed here are instance methods, all useful static methods are wrapped in ItemWrapper. Implements IngredientJS and overrides most of its default methods.


Return Type Info
getItem() Item Returns the instance of the Item class associated with this ItemStackJS.
getItemStack() ItemStack

Returns the vanilla ItemStack that this wraps.

getId() String

Returns the item id associated with this ItemStackJS in the form mod_name:item_name

getTags() Colletion<ResourceLocation>

Returns all item tags the item has. (NOT NBT tags).

hasTag(ResourceLocation tag) boolean

Returns if the item has the input tag or not.

copy() ItemStackJS

Returns a copy of this ItemStackJS.

setCount(int count) void

Sets the count on this ItemStackJS.

getCount() int

Gets the count.

withCount() ItemStackJS

Returns a copy of this ItemStackJS with a different count.

isEmpty() boolean

Returns if this is an empty item or not.

isInvalidRecipeIngredient() boolean

Returns if this is a valid recipe ingredient.

isBlock() boolean

Returns if this item is a BlockItem, that is it can be placed and form a block.

@Nullable getNbt() CompoundTag

Gets this items NBT data.

setNbt(@Nullable CompoundTag tag) void

Sets this items NBT data

hasNBT() boolean

Returns if this item has NBT data.

getNbtString() String

Returns this items NBT data as a string. If you want to display it to the player see Text#prettyPrintNbt.

removeNBT() ItemStackJS

Returns a copy with no NBT data.

withNBT(CompoundTag nbt) ItemStackJS

Returns a copy with the specified NBT data. Any tags from the original NBT are kept if not overwritten by the NBT passed in.

hasChance() boolean

Returns if the ItemStackJS has a chance.

removeChance() void

Removes the chance from this ItemStackJS.

setChance(double c) void

Sets the chance for this ItemStackJS.

getChance() double

Returns the chance.

withChance(double c) ItemStackJS

Returns a copy with the chance passed in, unless the chance passed in is the same as the current chance, in which case it returns this.

getName() Components

Returns this items name. Probably a Translateable Component unless its been overridden by something else (ie method below).

withName(@Nullable Component displayName) ItemStackJS

Returns a copy with a different display name set. 

toString() String

Returns a string representing this ItemStackJS. The same method used for the /kubejs hand command.

test(ItemStackJS other) boolean

Returns if this ItemStackJS equals another one. Tests for item type and NBT data.

testVanilla(ItemStack other) boolean

Returns if this ItemStackJS equals the passed in ItemStack. Tests for item type and NBT data.

testVanillaItem(Item item)



Returns if the Item passed in is the same as this ItemStackJS's Item. Basically checks they are the same item type.

getStacks() Set<ItemStackJS>

Returns this ItemStackJS as the only entry in a Set.

getVanillaItems() Set<Item>

Returns this ItemStackJS associated Item as the only entry in a Set.

getFirst() ItemStackJS

Retuns a copy of this ItemStackJS

hashCode() int

Returns a hash code of the Item and NBT data.

equals(Object o) boolean

Returns if this is equal to the input object. 

strongEquals(Object o) boolean

Returns if this is equal to the input object. Checks count as well.

getEnchantments() MapJS

Returns a MapJS of this itemStackJS enchament id's to their level.

hasEnchantment(Enchantment enchantment, int level) boolean

Returns if this ItemStackJS is enchanted with a minimum of the passed in enchantment level.

enchant(MapJS enchantments) ItemStackJS

Enchants a copy of this ItemStackJS with the MapJS passed in (it should be a map of enchantment ids to levels), then returns the copy.

enchant(Enchantment enchantment, int level) ItemStackJS

Enchants a copy of this item with the passed in Enchantment at the specified level, then returns the copy.

getMod() String

Returns the mod id of the mod this item is from.

ignoreNBT() IngredientJS

Returns a new IgnoreNBTIngredientJS of this item.

weakNBT() IngredientJS

Returns a new WeakNBTIngredientJS of this item.

areItemsEqual(ItemStackJS other) boolean

Returns if this item type is equal to the item type of the passed in ItemStackJS

areItemsEqual(ItemStack other) boolean

Returns if this item type is equal to the item type of the passed in ItemStack

isNBTEqual(ItemStackJS other) boolean

Returns if the NBT of this ItemStackJS is equal to the NBT of the ItemStackJS passed in.

isNBTEqual(ItemStack other) boolean

Returns if the NBT of this ItemStackJS is equal to the NBT of the ItemStack passed in.

getHarvestSpeed(@Nullable BlockContainerJS block) float

Returns the mining speed of this ItemStackJS if used to mine the passed in BlockContainerJS

getHarvestSpeed() float

Returns this items default mining speed





Returns a Json representation of this ItemStackJS. They all appear to work almost identically.

toNBT() CompoundTag

Returns an NBT representation of this ItemStackJS, the same sort that vanilla uses to store items in blocks.

onChanged(@Nullable Tag o) void

Sets the items NBT data to the tag passed in, only if it is a CompoundTag or null.

getItemGroup() String

Returns the name of the creative tab this item belongs in. An empty string if it does not exist in the creative tabs (like a jigsaw block).

getItemIds() Set<String> Returns a set with this items id as the only entry.
getFluidStack() FluidStackJS Returns null, by default. Overriden by some superclasses to return the FluidStackJS that this item represents.
getTypeData() CompoundTag Unknown purpose.
<TODO: Examples>




Example scripts for various things you can do with KubeJS


FTB Quests Integration

onEvent('ftbquests.custom_task.75381f79', event => {
  log.info('Custom task!')
  event.checkTimer = 20
  event.check = (task, player) => {
    if (player.world.daytime && player.world.raining) {

onEvent('ftbquests.custom_reward.e4f76908', event => {
  log.info('Custom reward!')

// specific object completion
onEvent('ftbquests.completed.d4f36905', event => {
  if (event.player) {
    event.notifiedPlayers.tell(Text.of(`${event.player.name} completed... something!`).green())

// generic 'quest' object completion. Note: There isnt actually a way to get reliable title on server side, so dont use event.object.title
onEvent('ftbquests.completed', event => {
  if (event.player && event.object.objectType.id === 'quest') {
    event.notifiedPlayers.tell(Text.of(`${event.player.name} completed a quest!`).blue())

// object with tag 'ding' completion
onEvent('ftbquests.completed.ding', event => {

onEvent('entity.death', event => {
  && event.source.actual
  && event.source.actual.player
  && event.source.actual.mainHandItem.id === 'minecraft:wooden_sword'
  && event.entity.type === 'minecraft:zombie') {
    event.source.actual.data.ftbquests.addProgress('12345678', 1)

Reflection / Java access

Very limited reflection is possible, but is not recommended. Use it in cases when KubeJS doesnt support something.

In 1.18.2+ internal Minecraft classes are remapped to MojMaps at runtime, so you don't have to use obfuscated names if accessing internal Minecraft fields and methods.


An example of adding a block with a custom material, built using reflection to get the MaterialJS class, then make a new instance of that with amethyst sounds and material properties from internal Minecraft classes.

// Startup script, 1.18.2
const MaterialJS = java("dev.latvian.mods.kubejs.block.MaterialJS")
const Material = java('net.minecraft.world.level.material.Material')
const SoundType = java('net.minecraft.world.level.block.SoundType')

amethystMaterial = new MaterialJS('amethyst', Material.AMETHYST, SoundType.AMETHYST) // f_164531_ and f_154654_ are the respective obfuscated names of these fields, for older versions

//This builder uses 1.18.2 syntax, it will not work in 1.16 or 1.18.1
onEvent('block.registry', event => {
	event.create('amethyst_slab', 'slab')
		.material(amethystMaterial)// Use the new MaterialJS instance we created as the material
		.texture('texture', 'minecraft:block/amethyst_block')

This does come at a cost, it takes 1-2 seconds to load this script, instead of the normal milliseconds. You should import your classes at the top of the script, instead of in an event, especially if the event gets triggered more than once.


Painter API


Painter API allows you to draw things on the screen, both from server and directly from client. This can allow you to create widgets from server side or effects on screen or in world from client side.

Currently it doesn't support any input, but in future, in-game menus or even GUIs similar to Source engine ones will be supported.

Paintable objects are created from NBT/Json objects and all have an id. If id isn't provided, a random one will be generated. Objects x and z are absolute positions based on screen, but you can align elements in one of the corners of screen. You can bulk add multiple objects in one json object. All properties are optional, but obviously some you should almost always override like size and position for rectangles.

paint({...}) is based on upsert principle - if object doesn't exist it will create it (if the object also contains valid type), otherwise, update existing:

You can bulk update/create multiple things in same object:

You can remove object with remove: true, bulk remove multiple objects or remove all objects:

These methods have command alternatives:

If the object is re-occuring, it's recommended to create objects at login with all of its static properties and visible: false, then update it later to unhide it. Painter objects will be cleared when players leave world/server, if its persistent, then it must be re-added at login every time.

Currently available objects

Underlined objects are not something you can create


(available for all objects)



Available Unit variables
Available Unit constants


onEvent('player.logged_in', event => {
		example_rectangle: {
			type: 'rectangle',
			x: 10,
			y: 10,
			w: 50,
			h: 20,
			color: '#00FF00',
			draw: 'always'
		last_message: {
			type: 'text',
			text: 'No last message',
			scale: 1.5,
			x: -4,
			y: -4,
			alignX: 'right',
			alignY: 'bottom',
			draw: 'always'

onEvent('player.chat', event => {
	// Updates example_rectangle x value and last_message text value to last message + contents from event
	event.player.paint({example_rectangle: {x: '(sin((time() * 1.1)) * (($screenW - 32) / 2))', w: 32, h: 32, alignX: 'center', texture: 'kubejs:textures/item/diamond_ore.png'}})
	event.player.paint({last_message: {text: `Last message: ${event.message}`}})
	// Bulk update, this is the same code as 2 lines above, you can use whichever you like better
	// event.player.paint({example_rectangle: {x: 120}, last_message: {text: `Last message: ${event.message}`}})
	event.player.paint({lava: {type: 'atlas_texture', texture: 'minecraft:block/lava_flow'}})



This page describes all functions and operations available for units


Most basic unit is plain number, such as '1' or '4.5'.

You can use variables with $ like '$example'.

Each function requires name parenthesis and comma separated arguments e.g. 'min(PI, $example)'.

You can combine as many as you want, e.g. 'min(PI, 10 + $example)'.

You can do pretty complex infix, e.g. 'atan2($mouseY, $mouseX) - HALF_PI - HALF_PI / 2'.


Network Packets

This script shows how to use network packets:

// Listen to a player event, in this case item right-click
// This goes in either server or client script, depending on which side you want to send the data packet to
onEvent('item.right_click', event => {
  // Check if item was right-clicked on client or server side
  if (event.server) {
    // Send data {test: 123} to channel "test_channel_1". Channel ID can be any string, but it's recommended to keep it to snake_case [a-z_0-9].
    // Receiving side will be client (because its sent from server).
    event.player.sendData('test_channel_1', { test: 123 })
  } else {
    // It's not required to use a different channel ID, but it's recommended.
    // Receiving side will be server (because its sent from client).
    event.player.sendData('test_channel_2', { test: 456 })

// Listen to event that gets fired when network packet is received from server.
// This goes in a client script
onEvent('player.data_from_server.test_channel_1', event => {
  log.info(event.data.test) // Prints 123

// Listen to event that gets fired when network packet is received from client.
// This goes in a server script
onEvent('player.data_from_client.test_channel_2', event => {
  log.info(event.data.test) // Prints 456

Starting Items

This server script adds items on first time player joins, checking stages. GameStages mod is not required

// Listen to player login event
onEvent('player.logged_in', event => {
  // Check if player doesn't have "starting_items" stage yet
  if (!event.player.stages.has('starting_items')) {
    // Add the stage
    // Give some items to player
    event.player.give(Item.of('minecraft:stone_pickaxe', "{Damage: 10}"))
    event.player.give('30x minecraft:apple')

FTB Utilities Rank Promotions

With this script you can have FTB Utilities roles that change over time.

Is for 1.12 only. Requires FTB Utilities.

events.listen('player.tick', function (event) {
  // This check happens every 20 ticks, a.k.a every second
  if (event.player.server && event.player.ticksExisted % 20 === 0) {
    var rank = event.player.data.ftbutilities.rank
    events.post('test_event', {testValue: rank.id})
    var newRank = ftbutilities.getRank(rank.getPermission('promotion.next'))

    if (newRank) {
      var timePlayed = event.player.stats.get('stat.playOneMinute') / 20 // Seconds player has been on server
      var timeRequired = newRank.getPermissionValue('promotion.timer').getInt()

      if (timeRequired > 0 && timePlayed >= timeRequired && rank.addParent(newRank)) {
        if (!events.postCancellable('ftbutilities.rank.promoted.' + newRank.id, {'player': event.player, 'rank': newRank})) {
          event.player.tell('You have been promoted to ' + newRank.getPermission('promotion.name') + '!')

// When player gets promoted to 'trusted' rank, give them gold ingot (uncomment the line)
events.listen('ftbutilities.rank.promoted.trusted', function (event) {
  // event.data.player.give('minecraft:gold_ingot')

3 example roles in ranks.txt:

power: 1
default_player_rank: true
promotion.name: Player
promotion.next: newcomer
promotion.timer: 5
command.ftbutilities.rtp: false
command.ftbutilities.home: false

power: 5
promotion.name: Newcomer
promotion.next: regular
promotion.timer: 15
ftbutilities.chat.name_format: <&aNewcomer &r{name}>
command.ftbutilities.rtp: true

power: 10
promotion.name: Regular
promotion.next: trusted
promotion.timer: 30
ftbutilities.chat.name_format: <&9Regular &r{name}>
command.ftbutilities.home: true

After 5 seconds of play time, player will be promoted to newcomer.
After 15 seconds (or 10 since previous role) they will be promoted to regular.
After 30 seconds (or 15 since previous role) they will be promoted to trusted, etc.


Clearlag 1.12

This script removes all items from world every 30 minutes. Only works in 1.12.

// Create item whitelist filter that won't be deleted with clearlag
var whitelist = Ingredient.matchAny([
  'minecraft:diamond', // Adds diamond to whitelist
  '@tinkersconstruct', // Adds all items from tinkerscontruct to whitelist

// Create variable for last clearlag result
var lastClearLagResult = Utils.newList()
// Create variable for total number of items
var lastTotalClearLagResult = Utils.newCountingMap()

// Create new function that clears lag
var clearLag = server => {
  // Get a list of all entities on server with filter that only returns items
  var itemList = server.getEntities('@e[type=item]')
  // Create new local map for item counters
  var lastResult = Utils.newCountingMap()
  // Clear old result lists
  // Iterate over each entity in itemList and add item counters
  itemList.forEach(entity => {
    if (!whitelist.test(entity.item)) {
      // Get the name of item
      var key = entity.item.name
      // Add to entity count
      lastResult.add(key, 1)
      // Add to total item count
      lastTotalClearLagResult.add(key, entity.item.count)
      // Kill the item entity

  // Update and sort last result list

  // Tell everyone how many items will be removed
    ' Removed ',
    ' items. ',
    Text.yellow('Click here').click('command:/clearlagresults'),
    ' for results.'

// Listen for server load event
events.listen('server.load', event => {
  // Log message in console
  event.server.tell([ Text.lightPurple('[ClearLag]'), ' Timer started, clearing lag in 30 minutes!' ])
  // Schedule new task in 30 minutes
  event.server.schedule(MINUTE * 30, event.server, callback => {
    // Tell everyone on server that items will be removed
    callback.data.tell([ Text.lightPurple('[ClearLag]'), ' Removing all items on ground in one minute!' ])
    // Schedule a subtask that will clear items in one minute
    callback.data.schedule(MINUTE, callback.data, callback2 => {
    // Re-schedule this task for another 30 minutes (endless loop)

// Doesnt work in 1.16+!
// Register commands
events.listen('command.registry', event => {
  // Register new OP command /clearlag, that instantly runs clearlag
    .execute(function (sender, args) {

  // Register new non-OP command /clearlagresults, that displays stats of all removed items from previous /clearlag
    .execute((sender, args) => {
      sender.tell([ Text.lightPurple('[ClearLag]'), ' Last clearlag results:' ])

      lastClearLagResult.forEach(entry => {
        var total = lastTotalClearLagResult.get(entry.key)

        if (entry.value == total) {
          sender.tell([ Text.gold(entry.key), ': ', Text.red(entry.value) ])
        } else {
          sender.tell([ Text.gold(entry.key), ': ', Text.red(entry.value), ' entities, ', Text.red(total), ' total' ])

Scheduled Server Events

At server load, you can schedule anything to happen at later time. Within callback handler you can also call callback.reschedule() to repeat this event after initial timer or callback.reschedule(newTime) to change it.

Whatever you pass as 2nd argument will be returned in callback as data.

The example script restarts server after 2 hours but notifies players 5 minutes before that.

onEvent('server.load', function (event) {
  event.server.schedule(115 * MINUTE, event.server, function (callback) {
    callback.data.tell('Server restarting in 5 minutes!')
  event.server.schedule(120 * MINUTE, event.server, function (callback) {

Running Commands


Sometimes, you might want to run a command (such as /tell @a Hi!), in your code.

Most always, there is better method, but sometimes, you just don't want to learn more complicated topics, and just run a command.

Basic Usage

The most basic usage would be to call runCommand() from a server class.

Utils.server.runCommand(`tell @a Hi!`)

If this command returns a message (usually an error) that is normally placed chat, it will be logged. This is not desired outside of debugging situations.

 So instead you can use the following to not log these messages.

Utils.server.runCommandSilent(`tell @a Hi!`)

If the server is not loaded at the time this is ran, then the code will not work.

Although you can use player.runCommandSilent(), it is not recommend as the command runs with the players permission level.

Using the execute command

Commands are ran in the default dimension (the overworld usually) at 0, 0, 0

To get around this, you can use the execute command:

//This example makes a bedrock box around creepers when they spawn
onEvent('entity.spawned', event => {
	if (event.entity.type != "minecraft:creeper") return // the following code only runs when creepers are spawned
	event.server.runCommandSilent(`execute in ${event.entity.level.dimension} positioned ${event.entity.x} ${event.entity.y} ${event.entity.z} run fill ~-1 ~-1 ~-1 ~1 ~2 ~1 bedrock hollow`)



Spawning Entities



Spawning entities consists of 3 steps:

Making a variable to store the entity


level is just a placeholder, in your code it needs to be defined, for many events you can use event.level in place of level and it will work

You can create a entity from a block instead of level, and this is often preferred to learn that, scroll to that section afterward

let myEntity = level.createEntity("cow")
Breaking down the example

Modifying the properties

myEntity.x = 0
myEntity.y = 69
myEntity.z = 0
myEntity.motionY = 0.1
myEntity.noGravity = true
Breaking Down the Example

Spawning the entity


With understanding from the previous sections you should be able to figure out what this does.

It get myEntity, then calls the method .spawn().

This spawn() method creates the entity in the world.

Note: myEntity is still a variable! So you may not use let myEntity again within the scope! However this variable is still linked to the entity so calling myEntity.motionY = 0.1 will still set the vertical motion of the entity. (This can be a useful thing, but bad if you are unaware)

Creating the entity from a block

You can also call createEntity from a block! This is handy if you want to spawn the entity in the position of a block.

let myEntity = block.createEntity("cow")

Again, block is just a place holder, you will need to change it to something else like maybe event.block for your code to work!

This does not spawn the entity in the center of the block, it just sets the entity's coordinates to that of the block, thus being misaligned

This code offsets the entity to be in the center of the block.

let myEntity = block.createEntity("cow")

Setting NBT

You can set the NBT to whatever you want! It's recommend using mergeFullNBT to do this.


myEntity.fullNBT.VillagerData = {} will not work, because .fullNBT is a beaned method, not a property! The only thing that the beaned method lets do is to be able to use let nbt = myEntity.fullNBT to set a variable to NBT to be read or use myEntity.fullNBT = {} to set all of it at once.

Note it is fullNBT not nbt, because kubejs uses nbt for a different purpose. A bit confusing, but it is what it is.

Item Entities

Creating an item entity is identical to any other entity, except you get a couple more methods.

let itemEntity = block.createEntity("item")
itemEntity.item = Item.of("encahanted_book").enchant("thorns",2)
itemEntity.item.count = 1
itemEntity.pickupDelay = 600
itemEntity.noGravity = true
itemEntity.motionY = 0.08

In this example


Spawns an endermite when braking dirt with a 5% chance

onEvent("block.break", event => {
	if (event.block.id != "dirt" || Math.random()  > 0.05) return
  	//only if its dirt and only has 5% chance
  	let myEndermite = event.block.createEntity("endermite")
  	myEndermite.x += 0.5
  	myEndermite.y += 0.5
  	myEndermite.z += 0.5

Overrides enchanting table behavior when clicking on it with an item in you hand. Instead will make the item float up a while, then fall back down.

onEvent('block.right_click', event => {
    if (event.block.id !='minecraft:enchanting_table') return
    if (event.item.count == 0) return
    let item = event.item.copy()
    //if did not use .copy() the item would still be referencing the one in the hand, so setting the count to 1 would set the count in the hand to 1
    item.count = 1
    let itemEntity = event.block.createEntity('item')
    itemEntity.y+=0.8 // on the top of the encahnting table, not in it
    itemEntity.item = item
    itemEntity.item.count = 1
    itemEntity.pickupDelay = 100
    itemEntity.noGravity = true
    itemEntity.motionY = 0.08
  	function callback (i) {
    	//changes the scope of itemEntity (otherwise if used 2 times in a row within 5 seconds, problems would occur)
    	event.server.scheduleInTicks(100, callback => { // this code runs 5 seconds later
    		i.noGravity = false


Examples and how-tos of other things KubeJS can do!


Changing Window Title and Icon

Yes, you can do that with KubeJS too.

To change title, all you have to do is change title in kubejs/config/client.properties.

To change icon, you create a kubejs/config/packicon.png image in standard Minecraft texture size preferably (64x64, 128x128, 256x256, that kind of size).

The image has to be saved as 32-bit PNG, not Auto-detect/24-bit, otherwise you will get a JVM crash!

Here's how to do that in PaintNET:

Example result:

Currently incompatible with Fancy Menu!


Loading Assets and Data

You can also use KubeJS to load assets from resource packs and data from datapacks! While this isn't the only method, its one of the easiest. Other options are <TODO: make and link server datapack load page and client generate assets event page>

The data folder is loaded identically to the data folder in a datapack. If you already have a datapack just copy the folder(s) from inside the datapacks data folder to KubeJS' data folder.

The assets folder is loaded identically to the assets folder in a resourcepack. If you already have a resourcepack just copy the folder(s) from inside the resourcepacks assets folder to KubeJS' assets folder.


Default Options

You can ship default options from options.txt with KubeJS. This includes keybindings, video settings, enabled resource packs, controls like autojump and toggle sprint and wierd things like advanced tooltips.

Why use this instead of just shipping options.txt? If you ship options.txt then the users options will get overridden every time they update your modpack, where-as KubeJS only sets the options once, on the first time the modpack boots.


To use it simply make a file called defaultoptions.txt in the kubejs/config folder. Then copy any lines you want to set by default over from the normal options.txt file. You can also just copy the entire file if you want to include everything.

A full list of what options the options.txt file can contain is available on the Minecraft Wiki: https://minecraft.fandom.com/wiki/Options.txt


Scripts using various KubeJS addons for recipes.



You can also always look at existing modpack using KubeJS UI to see how they do it

onEvent('ui.main_menu', event => {
  event.replace(ui => {
    ui.tilingBackground('kubejsui:textures/example_background.png', 256)
    ui.button(b => {
      b.name = 'Test'
      b.x = 10
      b.y = 10
      b.action = 'minecraft:singleplayer'
    ui.button(b => {
      b.name = 'Test but in bottom right corner'
      b.x = ui.width - b.width - 10
      b.y = ui.height - b.height - 10
      b.action = 'https://feed-the-beast.com/'
    ui.label(l => {
      l.name = Text.yellow('FTB Stranded')
      l.x = 2
      l.y = ui.height - 12
      l.action = 'https://feed-the-beast.com/'

    ui.image(i => {
      i.x = (ui.width - 40) / 2
      i.y = (ui.height - 30) / 2
      i.width = 40
      i.height = 30
      i.action = 'https://feed-the-beast.com/'
    ui.label(l => {
      l.name = Text.aqua('Large label')
      l.x = 100
      l.y = ui.height - 20
      l.height = 15
      l.shadow = true

KubeJS Thermal


You can use KubeJS Thermal to add recipes to a lot of the machines from the Thermal Series.

Tip: you can use Ctrl/Cmd + F to search this page for the machine you are looking for.

onEvent('recipes', event => {
    // Redstone Furnace
    // Turn four coal into one diamond
    event.recipes.thermal.furnace('minecraft:diamond', '4x minecraft:coal')
    // Dried kelp to leather, with a high energy cost
    event.recipes.thermal.furnace('minecraft:leather', 'minecraft:dried_kelp').energy(20000)
    // Sawmill
    // Input one oak leaf and have a 5% chance of an apple, and 10% of a sapling
    event.recipes.thermal.sawmill([Item.of('minecraft:apple').withChance(0.05), Item.of('minecraft:oak_sapling').withChance(0.1)], 'minecraft:oak_leaves')
    // Turn an acacia slab into 4 buttons
    event.recipes.thermal.sawmill('4x minecraft:acacia_button', 'minecraft:acacia_slab')
    // Pulverizer
    // Turn any leaf block into 4 sticks with a 50% chance of a fifth. Has a low energy cost.
    event.recipes.thermal.pulverizer(Item.of('minecraft:stick').withChance(4.5), '#minecraft:leaves').energy(100)
    // Pulverise a flint into an iron nugget with a 10% chance of a second
    event.recipes.thermal.pulverizer(Item.of('minecraft:iron_nugget').withChance(1.1), 'minecraft:flint')
    // Induction Smelter
    // Turn one coal block into 4 diamonds with a 50% chance of a fifth
    event.recipes.thermal.smelter(['4x minecraft:diamond', Item.of('minecraft:diamond').withChance(0.5)], 'minecraft:coal_block')
    // Turn an iron ingot and a copper ingot into a gold ingot and require 10,000 FE
    event.recipes.thermal.smelter('minecraft:gold_ingot', ['minecraft:iron_ingot', 'minecraft:copper_ingot']).energy(10000)
    // Centrifugal Separator
    // Centrifuge one sapling into 50% chance of a stick and 300mb of water
    event.recipes.thermal.centrifuge([Item.of('minecraft:stick').withChance(0.5), Fluid.of('minecraft:water', 300)], '#minecraft:saplings')
    // Turn 2 sweet berries into red dye
    event.recipes.thermal.centrifuge('minecraft:red_dye', '2x minecraft:sweet_berries')
    // Multiservo Press
    // Press seven bonemeal into a bone.
    event.recipes.thermal.press('minecraft:bone', '7x minecraft:bone_meal')
    // Press an iron dust into an iron nugget using the coin die. To use an item as a die they must have the thermal:crafting/dies tag!
    event.recipes.thermal.press('minecraft:iron_nugget', ['#forge:dusts/iron', 'thermal:press_coin_die'])
    // Magma Crucible
    // Turn a sapling into 400mb of water
    event.recipes.thermal.crucible(Fluid.of('minecraft:water', 400), '#minecraft:saplings').energy(100)
    // Melt ores into lava
    event.recipes.thermal.crucible(Fluid.of('minecraft:lava', 500), '#forge:ores')
    // Blast Chiller
    // Chill an arrow into an arrow of slowness
    event.recipes.thermal.chiller(Item.of('minecraft:tipped_arrow', '{Potion:"minecraft:slowness"}'), [Fluid.of('minecraft:water', 100), 'minecraft:arrow'])
    // Chill lava into raw iron using the ball cast. For an item to count as a cast it needs to have the thermal:crafting/casts tag!
    event.recipes.thermal.chiller('minecraft:raw_iron', [Fluid.of('minecraft:lava', 1000), 'thermal:chiller_ball_cast'])
    // Fractionating Still
    // Refine Creosete oil into Tree oil and latex, with a chance of producing rubber
    event.recipes.thermal.refinery([Item.of('thermal:rubber').withChance(0.8), Fluid.of('thermal:tree_oil', 100), Fluid.of('thermal:latex', 50)], Fluid.of('thermal:creosote', 200))
    // Refine tree oil into a small amount of refined fuel with a high energy cost
    event.recipes.thermal.refinery(Fluid.of('thermal:refined_fuel', 50), Fluid.of('thermal:tree_oil', 100)).energy(20000)
    // Unbrew an awkward potion. This uses the cofh core potion fluid with some nbt.
    event.recipes.thermal.refinery([Fluid.of('minecraft:water', 1000), 'minecraft:nether_wart'], Fluid.of('cofh_core:potion', 1000, '{Potion:"minecraft:awkward"}'))
    // Alchemical Imbuer
    // Combine a redstone dust and 200mb of lava to make 200mb of destabilized redstone
    event.recipes.thermal.brewer(Fluid.of('thermal:redstone', 200), [Fluid.of('minecraft:lava', 200), 'minecraft:redstone'])
    // Brew an uncraftable potion (potion with no nbt) with 64 bedrock and an awkward potion. Oh, and an insane energy cost
    event.recipes.thermal.brewer(Fluid.of('cofh_core:potion', 1000), [Fluid.of('cofh_core:potion', 1000, '{Potion:"minecraft:awkward"}'), '64x minecraft:bedrock'])
    // Fluid Encapsulator
    // Fill a sponge with water. Why? Well why not?
    event.recipes.thermal.bottler('minecraft:wet_sponge', [Fluid.of('minecraft:water', 10000), 'minecraft:sponge'])
    // Turn any gear into a machine frame by filling it with destabilized redstone. Nice and low energy cost too
    event.recipes.thermal.bottler('thermal:machine_frame', ['#forge:gears', Fluid.of('thermal:redstone', 500)]).energy(500)

KubeJS Create

Create integration for KubeJS. This mod allows you to add and properly edit recipes of Create mod in KubeJS scripts. All supported recipe types and examples are below. See Recipes page for more info.

Simple Recipe Types

Bulk Smoking and Bulk Blasting recipes are auto generated from vanilla smelting, smoking, and blasting recipes.


event.recipes.create.mixing(output[], input[])
event.recipes.createMixing(output[], input[])

Output can be an item, fluid, or an array of multiple.

Input can be an ingredient, fluid, or an array of multiple.

onEvent('recipes', event => {
		'2x bone_meal',
		Item.of('5x bone_meal').withChance(0.5)
	], 'bone_block')


	event.recipes.createFilling('create:blaze_cake', [
		Fluid.of('minecraft:lava', 250)

		Fluid.of('create:honey', 250)
	], 'minecraft:honey_bottle')

Mechanical Crafter


event.recipes.create.mechanicalCrafting(output, pattern[], {patternKey: input})
event.recipes.createMechanicalCrafting(output, pattern[], {patternKey: input})

This recipe type is the same as regular crafting table shaped recipe, however the pattern can be up to 9x9, instead of 3x3.

onEvent('recipes', event => {
	event.recipes.createMechanicalCrafting('minecraft:piston', [
	], {
		C: '#forge:cobblestone',
		P: '#minecraft:planks',
		R: '#forge:dusts/redstone',
		I: '#forge:ingots/iron'

Sequenced Assembly


event.recipes.create.sequencedAssembly(output[], input, sequence[]).transitionalItem(transitionalItem).loops(loops)
event.recipes.createSequencedAssembly(output[], input, sequence[]).transitionalItem(transitionalItem).loops(loops)

Output is an item or an array of items.

If it is an array:

Input is an ingredient.

Transitional Item is any item* and is used during the intermediate stages of the assembly.

Sequence is an array of recipes.

Loops is the number of time that the recipes repeats. Calling .loops() is optional, and defaults to one.

onEvent('recipes', event => {
	event.recipes.createSequencedAssembly([ // start the recipe
		Item.of('create:precision_mechanism').withChance(130.0), // this is the item that will appear in JEI as the result
		Item.of('create:golden_sheet').withChance(8.0), // the rest of these items will part of the scrap
		Item.of('2x gold_nugget').withChance(2.0),
	],'create:golden_sheet',[ // 'create:golden_sheet' is the input
		// the transitional item set by "transitionalItem('create:incomplete_large_cogwheel')" is the item used during the intermediate stages of the assembly
 	 	// like a normal recipe function, is used as a sequence step in this array. Input and output have the transitional item
	]).transitionalItem('create:incomplete_precision_mechanism').loops(5) // set the transitional item and the loops (amount of repetitions)

	// for this code to work, kubejs:incomplete_spore_blossom need to be added to the game
	let inter = 'kubejs:incomplete_spore_blossom' // making a varrible to store the transition item makes the code more readable
			Item.of('spore_blossom').withChance(16.0), // this is the item that will appear in JEI as the result
			Item.of('flowering_azalea_leaves').withChance(16.0), // the rest of these items will part of the scrap
		],'flowering_azalea_leaves', [ // 'flowering_azalea_leaves' is the input
			// the transitional item is a varrible, that is "kubejs:incomplete_spore_blossom", and is used during the intermediate stages of the assembly
			event.recipes.createPressing(inter, inter),
			// like a normal recipe function, is used as a sequence step in this array. Input and output have the transitional item
			event.recipes.createDeploying(inter, [inter, MC('hanging_roots')]),
			event.recipes.createFilling(inter, [inter, Fluid.of(MC('water'),420)]),
			event.recipes.createDeploying(inter, [inter, MC('moss_carpet')]),
  			event.recipes.createCutting(inter, inter)
		]).transitionalItem(inter).loops(2) // set the transitional item and the loops (amount of repetitions)

Transitional Items

As mentioned earlier, any item can be a transition item. However, this is not completely recommended.

If you wish to make your own transitional item, its best if you make the type create:sequenced_assembly.

1.16 syntax
onEvent('item.registry', event => {
	event.create('incomplete_spore_blossom').displayName('Incomplete Spore Blossom').type('create:sequenced_assembly')
1.18 syntax
onEvent('item.registry', event => {

Mysterious Conversion

Mysterious Conversion recipes are client side only, so the only way to add them currently is using reflection.


Goes inside of client scripts and not in an event.

//makes the varribles used
let MysteriousItemConversionCategory = java('com.simibubi.create.compat.jei.category.MysteriousItemConversionCategory')
let ConversionRecipe = java('com.simibubi.create.compat.jei.ConversionRecipe')

//adds in the recipes
MysteriousItemConversionCategory.RECIPES.add(ConversionRecipe.create('minecraft:apple', 'minecraft:carrot'))

MysteriousItemConversionCategory.RECIPES.add(ConversionRecipe.create('minecraft:golden_apple', 'minecraft:golden_carrot'))

Preventing Recipe Auto-Generation

If you don't want a smelting, blasting, smoking, crafting, or stone-cutting to get an auto-generated counter part, then include manual_only at the end of the recipe id.

onEvent('recipes', event => {

Other types of prevention, can be done in the create config (the goggles button leads you there).

If it is not in the config, then you can not change it.


3rd Party addons

3rd party add-ons: (Not including mods with optional dependencies of KubeJS)

Name: Description Links Loader Versions
Ponder for KubeJS Make custom Create Ponder scenes with KubeJS. Wiki CurseForge Discord Github Forge



LootJS A mod for packdevs to easily modify the loot system with KubeJS. Wiki CurseForge Modrinth Discord Github Forge & Fabric 1.18.2
MoreJS A mod for packdevs to extend KubeJS with more events and utilities. Wiki CurseForge Modrinth Discord Github Forge & Fabric 1.18.2
ProbeJS A typing generator mod to generate KubeJS typings. Enabling Intellisense for your KubeJS environments! Wiki CurseForge Github Forge & Fabric 1.18.2
KubeJS ComputerCraft Adds support for KubeJS to add ComputerCraft peripherals to any block. CurseForge Github Forge & Fabric 1.18.2
KubeJS Borealis Adds a form of "documentation" to the mod KubeJS using the mod Borealis Example CurseForge Github Forge



KubeJS TwitchIntegration Cool twitch integration Events Examples CurseForge Github Forge


KubeJS: RTJC A proof of concept add-on that allows you to compile and run Java code at runtime. Description CurseForge Github Forge


Kubejs Debug Adapter A Debug Adapter Protocol implementation for KubeJS scripts. Modrinth Github Forge


Getting Started

A step by step guide for learning the basics of KubeJS

Getting Started

Introduction and Instillation


Install the mod and its two dependencies Architectury and Rhino.
Make you use the most resent version of each mods for your version.
If you are using 1.16 fabric then use this instead.

When you first install KubeJS, you will need to launch Minecraft with the mods (and the game not crashing) to generate the some folders and files.

The kubejs folder

Finding it

Everything you do in KubeJS in located in the kubejs folder in your instance.

From now on this will be referenced as the kubejs folder.

The contents of it

You can find type-specific logs in logs/kubejs/ directory

Other Useful Tools

Code is just a language that computers can understand. However, the grammar of the language, called syntax for code, is very precise. When you code has a syntactical error, the computer does not know what to do and will probably do something that you do not desire.

With KubeJS we will be writing a lot of code, so it important to avoid these errors. Luckily, there are tools called code editors, that can help us write code correctly.

We recommend installing Visual Studio Code as it is light-ish and has great built in JS support. Now when you edit you java script files, it will not only warn you when you make most syntactical errors, but also help you prevent them in the first place.

Getting Started

Your First Script

Writing Your First Script

If you have launched the game at least once before you will find kubejs/server_scripts/example_server_script.js It looks like this:

// priority: 0

settings.logAddedRecipes = true
settings.logRemovedRecipes = true
settings.logSkippedRecipes = false
settings.logErroringRecipes = true

console.info('Hello, World! (You will see this line every time server resources reload)')

onEvent('recipes', event => {
	// Change recipes here

onEvent('item.tags', event => {
	// Get the #forge:cobblestone tag collection and add Diamond Ore to it
	// event.get('forge:cobblestone').add('minecraft:diamond_ore')

	// Get the #forge:cobblestone tag collection and remove Mossy Cobblestone from it
	// event.get('forge:cobblestone').remove('minecraft:mossy_cobblestone')

Lets break it down:

Finally Writing Code For Real

Lets start off by adding a recipe to craft flint from three gravel.

To do so, insert this code right after the recipes event.

event.shapeless("flint", ["gravel", "gravel", "gravel"])

It should look like this:

// priority: 0

settings.logAddedRecipes = true
settings.logRemovedRecipes = true
settings.logSkippedRecipes = false
settings.logErroringRecipes = true

console.info('Hello, World! (You will see this line every time server resources reload)')

onEvent('recipes', event => {
	// Change recipes here
	event.shapeless("flint", ["gravel", "gravel", "gravel"])

onEvent('item.tags', event => {
	// Get the #forge:cobblestone tag collection and add Diamond Ore to it
	// event.get('forge:cobblestone').add('minecraft:diamond_ore')

	// Get the #forge:cobblestone tag collection and remove Mossy Cobblestone from it
	// event.get('forge:cobblestone').remove('minecraft:mossy_cobblestone')

Now lets test it!

Run the command /reload in game, then try crafting three gravel together in any order.

But how does it work?

There you go! You can make custom shapeless recipes!

If you want to make other types of recipes, learn about it here, and if you have an addon that adds more recipe types, loot at its mod page, or here.

Getting Started

Basics Custom Mechanics

By now you have created a custom recipe, or maybe multiple, or even manipulated tags, or created custom items or blocks.

But you want to do more then that, you want to add a custom mechanic, for example milking a goat.

The first step is to break down your idea into smaller pieces, until each piece is something you can code.
One thing to note, is that most all things are caused by some trigger. Such as an entity dieing, or a block being placed. These are detected by events.

Detecting Events

This is just like when we made recipes, but that time the event was triggered not by a players action, but by the game doing internal operations, that being getting to the time that is for registering recipes.

As a refresher, here is detecting the recipes event:

onEvent('recipes', event => {

To change the event detected, we need to change what is in the 's. But to what? Luckily there is a list of all event page in this wiki!

Searching the ID column, we can scroll down and find that there is an event named item.entity_interact which happens to be the one that we want for milking the goat.

Look at the type column and it will tell you which folder, you will need to put you code into.

Now we just put that in there, and we can now run code when a player right clicks an entity.

onEvent('item.entity_interact', event => {

To test we can use Utils.server.tell() to detect when the event occurs.

The are many situations that console.log(), would be better, which put the result in to instance/logs/kubejs/server.txt.

onEvent('item.entity_interact', event => {
	Utils.server.tell("Entity Interation Detected!")

Now to test you can try right clicking an entity and see you will see a message appears in the chat.

But this occurs to it entities, and want to only affect what happens to goats.
To do this, we need to know information about the context of the event.

Calling Methods of an Event

Up to this point you may have been wondering what the purpose of the event => { is.

You can recall that for the custom recipe, used it to call the method that added the recipe.

onEvent('recipes', event => {
	event.shapeless('flint', ['gravel', 'gravel', 'gravel'])

For each event that we detect the variable event will have different methods. The item.entity_interact event has methods:

How are you supposed to know this? Using ProbeJS! There is a whole wiki page about this addon!

So in our code we can write:

onEvent('item.entity_interation', event => {

.getEntity() gets the player, while .getTarget() gets the entity

What does this do?
Because the .getEntity() method does not do anything, but it returns the entity.

To see this we can put it into the chat.

onEvent('item.entity_interation', event => {
	Utils.server.tell( event.getTarget() )

Now when you interact with an entity you can see what some details about it!

What is put in the chat is not the actual value is, as only Strings can be displayed. All other types (such as EntityJS) have a toString() method that is called which extracts some information and returns a string that is then displayed instead.

Because of this, you might think what we need to do is run event.getEntity().toString() to get the entity type.

But this is wrong. You should not be using .toString() as there is almost always a better way. In this case its using the method .getType() of entity that returns a string of the type of the entity.

onEvent('item.entity_interation', event => {
	Utils.server.tell( event.getTarget().getType() )

This code is good, but it can be better because of a feature called BEANS.

This feature is very simple:

So in our case the code can be shortened to:

onEvent('item.entity_interation', event => {
	Utils.server.tell( event.target.type )

Alright, this is all good, but we want to make the code do stuff, not just tell tell us about the entities type. Notably we want to run code if an the type is a certain value.

We do this by using a control structure called: if!

If Statements

The basic syntax is as following:
if (condition) {result}
The condition is a boolean, which holds a value: true or false.
And if the boolean equates to true, then the code in result runs, otherwise it does not.

Here is an example:

onEvent('item.entity_interact', event => {
  	Utils.server.tell( event.target.type )
  	if (true) {
    if (false) {

When you interact with an entity in the chat you will be told the True, but not the False.

Lets make this useful, we need to use a condition to run the code based on the entity type.

Testing equality:

"foo" == "foobar" // this is false
"foo" == "foo" // this is true

"foo" = "foobar" // a single '=' does assignment (we will get to this later) NOT equality

So our code can look like:

onEvent('item.entity_interact', event => {
  	if (event.target.type == "minecraft:goat") {
    	Utils.server.tell("Is a Goat")

Now interacting with a goat will provide the message Is a Goat when interacting with a goat!

Now any code that we want to run when a goat is interacted with, we will place inside of this if statement.

This works, but it can be better.

Something that as you write more code will become increasingly important is code readability. In this case it can be improved with what is know as guard statements. In this case it will look like:

Guard Statements
onEvent('item.entity_interact', event => {
  	if (event.target.type != "minecraft:goat") return
    Utils.server.tell("Is a Goat")

This might look more confusing at first but is really quite simple.

Firstly, I am using != instead of ==, which is the same as, except it returns the opposite,  so true if they are unequal, and false if they do equal.

Secondly, id you do not include {} then the if will only apply to the next line immediately after, and everything after is considered to be out of the if.

Thirdly, return in this context will end the execution of the code.
So if the entity type is not a goat, then execution will not get passed line 2.

Learn more about ifs here.

The next step is to take a bucket, but before we can do that, we need to ensure the player is holding a bucket.

Getting the item in the players hand

 We can use the method .getItem() so event.getItem() which can be beaned to event.item.

Now we get the type of item we can use .getId() so event.item.getId() so event.item.id.

We could use another if, but I want to show you a different option, the OR boolean operator:

true || true // is true
true || false // is true
false || true // is true
false || false // is false

false || false || true || false // is false
false || false || false || false // is false

so we can put this in or code to be:

onEvent('item.entity_interact', event => {
  	if (event.target.type != "minecraft:goat" || event.item.id != "minecraft:bucket") return
    Utils.server.tell("Is a Goat and is Holding a Bucket")

This should say in the chat Is a Goat and is Holding a Bucket if you right click a goat with a bucket

Now to take the item, we will manipulate the count of it. We can get the count of the item, subtract one from it, then set the count to the result.

We can write the code:

onEvent('item.entity_interact', event => {
  	if (event.target.type != "minecraft:goat" || event.item.id != "minecraft:bucket") return
    Utils.server.tell("Is a Goat and is Holding a Bucket")
  	event.item.setCount( event.item.getCount() - 1)

Now we bean it to:

onEvent('item.entity_interact', event => {
  	if (event.target.type != "minecraft:goat" || event.item.id != "minecraft:bucket") return
    Utils.server.tell("Is a Goat and is Holding a Bucket")
  	event.item.count = event.item.count - 1

But there is a better way to write this using something know as syntactical sugar. This is just a fancy term for using symbols in a special order that lets you write a piece of code with less total characters to do a different thing with under the hood.

In the example above we used the basic assignment operator =.
But there are other assignment operators! Such as the subtraction assignment operator -=.

Here is it in the code:

onEvent('item.entity_interact', event => {
  	if (event.target.type != "minecraft:goat" || event.item.id != "minecraft:bucket") return
    Utils.server.tell("Is a Goat and is Holding a Bucket")
  	event.item.count -= 1

Instead of getting the value, then subtracting one, it can now be thought of as simply reducing the value by 1.

There are other assignment operators, such as one for addition, +=, multiplication, *=, division, /=, modulo, %=, logical or ||=, logical and, &&==, bitwise xor, ^=, bitwise and, &=, bitwise or, |=, left bitshift, <<=, right bitshift, >>>=, signed right bitshift >>=, and of course minus, -=

But wait, there's more! For adding or subtracting by 1, you can make the code even smaller, appending ++ or -- to then end.

onEvent('item.entity_interact', event => {
  	if (event.target.type != "minecraft:goat" || event.item.id != "minecraft:bucket") return
    Utils.server.tell("Is a Goat and is Holding a Bucket")

Epic, we made it smaller! None of that was required, but it looks a lot nicer.

Giving the Player Items

We can go quick cause we know all the steps for all that is left.

event.getPlayer() for player but event.player because of beans. Player has a method called .give(ItemStack) to give an item so event.player.give(ItemStack). And in our case ItemStackJS is 'milk_bucket'. So our final code:

onEvent('item.entity_interact', event => {
  	if (event.target.type != "minecraft:goat" || event.item.id != "minecraft:bucket") return
    Utils.server.tell("Is a Goat and is Holding a Bucket")

Now we can remove the debugging line Utils.server.tell("Is a Goat and is Holding a Bucket").

onEvent('item.entity_interact', event => {
  	if (event.target.type != "minecraft:goat" || event.item.id != "minecraft:bucket") return

When holding a stack of buckets and right clicking a goat, a bucket will be consumed and you gain a milk bucket.

It seem good. Right? All done. Wrong!

When programming, you always have to be careful about edge-cases. These are situations that are typically at extremes on situations. For example you write some code to function differently if you have 5 or more levels, but when you have 5 levels exactly, some logic differently causing an expected result.

Our code currently mishandles an edge case. The edge case is when the player has one bucket in their hand.

When holding one bucket in your hand, not in the first slot, and with nothing else in the first slot. When right-clicking a goat the milk bucket does not stay in your hand as is intuitive, but instead get placed in the first slot.

To resolve this bug, we could add an if to check if the count is one, then change the logic, but this is not required because their is method that does everything for us. The method .giveInHand() is identical to the .give() except it first attempts to put the item in the players hand if it is empty.

Putting this in our code looks like:

onEvent('item.entity_interact', event => {
  	if (event.target.type != "minecraft:goat" || event.item.id != "minecraft:bucket") return

Now it seems like we are done! But compare with milking a cow, its just not as satisfying.

Adding Sound

Although adding feedback in to you creations, usually in the form of sound effects, and particles, does not change the effect or your creation, it has a major effect on how engaging, polished, and interesting your creation appears.

Luckily playing sound is really easy with KubeJS, because many different classes have a .playSound() method.
We want the sound to originate from the goat being milked so we can use event.target to get the goat, then just call .playSound().

.playSound() takes some parameters:
Either the id of the sound, or the id, the volume, and the pitch.
Lets keep thing simple by only using the id.

Although you can register new sounds with KubeJS, it would be easier to use the existing cow milking sound. The id of this sound is entity.cow.milk.

Putting this into the code looks like:

onEvent('item.entity_interact', event => {
  	if (event.target.type != "minecraft:goat" || event.item.id != "minecraft:bucket") return

Now when milking a goat, you hear the milking sound.

There we go! We are done!


Now that we implemented a feature together you will be able to make some of your own basic custom features too!

Don't be too intimidated by how long it took us, we went through every single detail, but you already know those so it will take you a fraction of the time it took to make this.

Here is a step by step list of how you can make your own mechanic:

  1. Determine what triggers the mechanic.
    1. This is the event.
    2. In this example we did item.entity_interact.
    3. A list of all events is here.
  2. Narrow down when the code of you event runs with guard statements.
    1. Use an if and return.
    2. In our case it is detecting the entity as a goat and the item as a bucket.
    3. Use ProbeJS or the second wiki or the source code to get the information you need.
  3. Break down what you want to do as code you can write.
    1. In our case instead of the idea of filling a bucket with milk, the code takes one of the item and give the player a bucket of milk.
    2. Use ProbeJS or the second wiki or the source code to get the information you need.
  4. Double check edge cases.
    1. You should be always testing you code with most every change you make.
    2. You need to be extra careful with edge cases, when making changes too.
    3. In our case we replaced player.give() with player.giveInHand().
  5. Add Polish.
    1. This includes fixing minor bugs on edge cases.
    2. This also involves making sure the player gets feedback such as sound or particles.
    3. In our case this is the milking sound.
Other Helpful Things to Know

Although we did not get to it with the example, here are some simple things that would be helpful to know:

Getting Started

Using ProbeJS

ProbeJS is an add-on that is built exclusively to help you program.

What it does:

It generates documentation files from digging around in he game code itself. So you get all the methods, not only from KubeJS, but also from base Minecraft and ones added by your modloader and ones from all the other mods in you install. Not only can you view these docs, but they are also formatted in a way that an sufficiently advanced code editor, like VSCode, can understand. So you will now get more relevant code suggestions too.


Find ProbeJS on the 3rd Party addons list and download the relevant version to you.
Once you've installed it and relaunched your game, run the command /probejs dump.
Now you will need to wait a little while, but after some time, you should see a message altering you that the dump is complete.

You are now done with ProbeJS and can uninstall it if you choose, or you could keep it around to use occasionally to update your docs. But try to remember to uninstall before release

What just happened?

You can now look and see that there is a new folder located at instance/kubejs/probe and inside of here there are a more folders and files. These are your docs.

Setting up VS Code

  1. In VS Code select file > open folder
  2. This opens up a file explore window, select the KubeJS folder (instance/kubejs) and choose select folder.

Your done!


Properties and Methods of a Class

To know the methods of a class just type in the class name, like Item or BlockProperties, then type a . now you will see a list of the public methods and properties.

ProbeJS will not display the beaned assessors and mutators methods. So if you see a .getFoo() remeber that you can use .foo and .setFoo(bar) then you can use .foo = bar.

Searching by Keyword

If you are in VS Code press the explorer button in the top-ish left to open up the explorer pane.

Now navigate to probe > generated > globals.d.ts.
Press Ctrl + F and a little search window should pop up in your editor.
Now type in you key word and look through all the matches.


If you append class  to the front and   to the end then you will look for classes so like Item has 8635 result for me, but if I type class Item then the one I want!

In the events.documented.d.ts file you will find most all the events with descriptions of what they do.

In events.d.ts you will find even more events but less information about them.

In constants.d.ts you can see different pieces that you can use where ever.


If you want to find the methods of an event, say item.pickup find it in one of the files (In this case events.documented.d.ts) and here is the line describing it:

declare function onEvent(name: "item.pickup", handler: (event: Internal.ItemPickupEventJS) => void);

Look closely and find Internal.ItemPickupEventJS. Since it says Internal, we will look in the the globals.d.ts file, but if it says Registry then we use registries.d.ts.

Now we will go to the generated file and search ItemPickupEventJS.
Then we find:

    * Fired when an item is about to be picked up by the player.
    * @javaClass dev.latvian.mods.kubejs.item.ItemPickupEventJS
    class ItemPickupEventJS extends Internal.PlayerEventJS {
        getItem(): Internal.ItemStackJS;
        getEntity(): Internal.EntityJS;
        getItemEntity(): Internal.EntityJS;
        canCancel(): boolean;
        get item(): Internal.ItemStackJS;
        get itemEntity(): Internal.EntityJS;
        get entity(): Internal.EntityJS;
        * Internal constructor, this means that it's not valid unless you use `java()`.
        constructor(player: Internal.Player, entity: Internal.ItemEntity, stack: Internal.ItemStack);

This means that we can use the methods .getItem() .getEntity() .getItemEntity() .canCancel() .item .itemEntity and .entity.


But if we did potion.registry then we get Registry.Potion which brings us to:

    class Potion extends Internal.RegistryObjectBuilderTypes$RegistryEventJS<any> {
        create(id: string, type: "basic"): Internal.PotionBuilder;
        create(id: string): Internal.PotionBuilder;

So we can use event.create("cactus_juice") but that does not do much so we need to follow one step further and go to the potion builder, which you see is Internal.PotionBuilder. Now we search PotionBuilder in globals.d.ts then we see:

    * @javaClass dev.latvian.mods.kubejs.misc.PotionBuilder
    class PotionBuilder extends Internal.BuilderBase<Internal.Potion> {
        getRegistryType(): Internal.RegistryObjectBuilderTypes<Internal.Potion>;
        effect(effect: Internal.MobEffect_, duration: number, amplifier: number, ambient: boolean, visible: boolean): this;
        effect(effect: Internal.MobEffect_, duration: number, amplifier: number, ambient: boolean, visible: boolean, showIcon: boolean): this;
        effect(effect: Internal.MobEffect_, duration: number, amplifier: number, ambient: boolean, visible: boolean, showIcon: boolean, hiddenEffect: Internal.MobEffectInstance_): this;
        effect(effect: Internal.MobEffect_, duration: number): this;
        effect(effect: Internal.MobEffect_, duration: number, amplifier: number): this;
        effect(effect: Internal.MobEffect_): this;
        addEffect(effect: Internal.MobEffectInstance_): this;
        createObject(): Internal.Potion;
        get registryType(): Internal.RegistryObjectBuilderTypes<Internal.Potion>;
        * Internal constructor, this means that it's not valid unless you use `java()`.
        constructor(i: ResourceLocation);

Now we see the methods that we can call after this.

So in our code we could write:

onEvent('potion.registry', event => {