This is not meant to be a tutorial on writing ASM. If you need a tutorial on that, you shouldn’t be reading this.

This is mostly about how to update coremods from 1.12.2 to 1.15.2

This is just a draft of the tutorial, more details will be added soon™

To have a javascript coremod you need

A mod
A coremods.json file in META-INF containing a list of all your transformers
Javascript (.js) transformer files (listed in coremods.json)

`META-INF/coremods.json` (Example)

{
	"Transformer 1 Name": "path/to/transformer-1.js",
	"Transformer 2 Name": "path/to/transformer-2.js"
}

Javascript transformers (Example)

Your transformer script must have a function called initializeCoreMod which returns a JSON object containing all your transformer objects.
Each transformer can target either a FIELD, METHOD or CLASS.
ModLauncher runs transformers in the order FIELD, then METHOD then CLASS for each class it loads.

Field transformers

Format:

"Transformer Name": {
	"target": {
		"type": "METHOD",
		"class": "cannonical.name.of.Class",
		"fieldName": "srgFieldName"
	},
	"transformer": function(fieldNode) {
		// Apply transformations
		return fieldNode;
	}
}

Method transformers

Format:

"Transformer Name": {
	"target": {
		"type": "METHOD",
		"class": "cannonical.name.of.Class",
		"methodName": "srgMethodName",
		"methodDesc": "srgMethodDesc"
	},
	"transformer": function(methodNode) {
		// Apply transformations
		return methodNode;
	}
}

Class transformers

Format: Single Class:

"Transformer Name": {
	"target": {
		"type": "CLASS",
		"name": "cannonical.name.of.Class"
	},
	"transformer": function(classNode) {
		// Apply transformations
		return classNode;
	}
}

Multiple Classes:

"Transformer Name": {
	"target": {
		"type": "CLASS",
		"names": function(listOfClasses) {
			return ["cannonical.name.of.Class1", "cannonical.name.of.Class2"];
		}
	},
	"transformer": function(classNode) {
		// Apply transformations
		return classNode;
	}
}

Transformer setup

Using Java classes

var Opcodes = Java.type("org.objectweb.asm.Opcodes");
var ASMAPI = Java.type("net.minecraftforge.coremod.api.ASMAPI");

See allowed classes at net.minecraftforge.coremod.CoreModEngine#ALLOWED_CLASSES

ASMAPI

var mappedFieldName = ASMAPI.mapField("srgFieldName");
var mappedMethodName = ASMAPI.mapMethod("srgMethodName");

Examples:

Transforming

Loop Instructions (Instructions are an InsnList)

var arrayLength = instructions.size();
for (var i = 0; i < arrayLength; ++i) {
	var instruction = instructions.get(i);
	// Do stuff with instruction
}

for (var it = instructions.iterator(); it.hasNext();) {
	var instruction = it.next();
	// Do stuff with instruction
}

Loop Methods (Methods are a List<MethodNode>)

for (var i in methods) {
	var method = methods[i];
	// Do stuff with method
}

methods.forEach(function(method) {
	// Do stuff with method
});

Debugging

If the coremod has syntax errors or something check the very top of your log (before anything like Scanning Mod File: appears). This is at the very top and is usually to high up for IntelliJ’s console to catch it so use the log in /run/logs/.
If the coremod failed to transform properly search for <eval> and/or XFORM in the log. This will be wherever the class gets initialised so probably towards the end.

Full Transformer

Example

function initializeCoreMod() {
	FIELD_INSN = Java.type("org.objectweb.asm.tree.AbstractInsnNode").FIELD_INSN;
	nameTessellatorINSTANCE = Java.type("net.minecraftforge.coremod.api.ASMAPI").mapField("field_78398_a");
	return {
		"Tessellator.getInstance() Transformer": {
			"target": {
				"target": {
					"type": "METHOD",
					"class": "net.minecraft.client.renderer.Tessellator",
					"methodName": "func_178181_a",
					"methodDesc": "()Lnet/minecraft/client/renderer/Tessellator;"
				},
				"transformer": function(methodNode) {
					// Changes "return Tessellator.INSTANCE" in "Tessellator.getInstance()" to "return NewTessellator.INSTANCE"
					for (var it = methodNode.instructions.iterator(); it.hasNext();) {
						var instruction = it.next();
						if (instruction.getType() == FIELD_INSN && instruction.name.equals(nameTessellatorINSTANCE)) {
							instruction.owner = "io/github/cadiboo/examplemod/client/hooks/NewTessellator";
							instruction.name = "INSTANCE";
							instruction.desc = "Lio/github/cadiboo/examplemod/client/hooks/NewTessellator;";
							break;
						}
					}
					return classNode;
				}
			}
		}
	}
}