> For the complete documentation index, see [llms.txt](https://frinn.gitbook.io/custom-machinery-1.21/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://frinn.gitbook.io/custom-machinery-1.21/mod-integrations/kubejs/recipes/function.md).

# Function

A function requirement is a way to execute javascript code when a recipe is processing.

To do that 2 things are needed : 

* A method that will tell the recipe to send a KubeJS event when needed.
* An event listener where the JS code will be put.

Both the recipe method and the event listener are linked using a String id.

Note: This has nothing to do with MC functions (or commands), this requirement allow you to make your own code in JavaScript and pass it to be executed by the machine.

### In the recipe

```javascript
//Executed when the machine is idle and search a recipe to process.
.requireFunctionToStart("id")

//Executed the first tick of the crafting process.
.requireFunctionOnStart("id")

//Executed each tick of the crafting process.
.requireFunctionEachTick("id")

//Executed the last tick of the crafting process.
.requireFunctionOnEnd("id")
```

* The passed id MUST be a string.
* Several functions can share the same id, but they will all share the same event too.
* The function can be delayed, put `.delay(delay)` directly after any `.requireFunctionXXX()` call to make the function be executed at the specified delay.

### In a server script (outside the recipe event)

```javascript
CustomMachineryEvents.recipeFunction("id", event => {
  //Print the machine id in logs when the function is run.
  console.info(event.machine.id)

  //Immediately stop the function and allows the recipe to run.
  event.success()
  
  //Immediately stop the function and display an error in the machine.
  event.error("Error")
})
```

* If neither `event.success()` nor `event.error()` are called the recipe will be allowed to run.
* If `event.error()` is called without specifying an error message it will show the default one instead.
* The `event` object contains several useful methods, see more on [its dedicated wiki page](/custom-machinery-1.21/mod-integrations/kubejs/recipes/function/function-event.md).

### Example :

```javascript
ServerEvents.recipes(event => {
	event.recipes.custommachinery.custom_machine("custommachinery:power_crusher", 200)
	.requireFunctionEachTick("add_2_diamonds")
})

CustomMachineryEvents.recipeFunction("add_2_diamonds", event => {
	let remaining = event.machine.addItemToSlot("output1", "2x minecraft:diamond", true)

	if(remaining.count == 0) {
                event.machine.addItemToSlot("output1", "2x minecraft:diamond", false)
		event.success()
        }
	event.error("Can't add 2 diamonds in output slot")
})
```

### Migrating from previous function requirement

Previously : 

```javascript
ServerEvents.recipes(event => {
	event.recipes.custommachinery.custom_machine("id", time)
	.requireFunctionXXX(ctx => {
		//Do code here
	})
})
```

Now : 

```javascript
ServerEvents.recipes(event => {
	event.recipes.custommachinery.custom_machine("id", time)
	.requireFunctionXXX("id")
})

CustomMachineryEvents.recipeFunction("id", event => {
  	//Do code here
  	//Use event.success() and event.error("error")
  	//instead of return ctx.success() and return ctx.error("error")
})
```