WebGL: Interacting with browser scripting

When building content for the web, you might need to communicate with other elements on your web page. Or you might want to implement functionality using Web APIs which Unity doesn’t currently expose by default. In both cases, you need to directly interface with the browser’s JavaScript engine. Unity WebGLA JavaScript API that renders 2D and 3D graphics in a web browser. The Unity WebGL build option allows Unity to publish content as JavaScript programs which use HTML5 technologies and the WebGL rendering API to run Unity content in a web browser. More info
See in Glossary provides different methods to do this.

Calling JavaScript functions from Unity scripts

The recommended way of using browser JavaScript in your project is to add your JavaScript sources to your project, and then call those functions directly from your script code. To do so, place files with JavaScript code using the .jslib extension under a “Plugins” subfolder in your Assets folder. The plugin file needs to have a syntax like this:

mergeInto(LibraryManager.library, {

  Hello: function () {
    window.alert("Hello, world!");
  },

  HelloString: function (str) {
    window.alert(Pointer_stringify(str));
  },

  PrintFloatArray: function (array, size) {
    for(var i = 0; i < size; i++)
    console.log(HEAPF32[(array >> 2) + i]);
  },

  AddNumbers: function (x, y) {
    return x + y;
  },

  StringReturnValueFunction: function () {
    var returnStr = "bla";
    var bufferSize = lengthBytesUTF8(returnStr) + 1;
    var buffer = _malloc(bufferSize);
    stringToUTF8(returnStr, buffer, bufferSize);
    return buffer;
  },

  BindWebGLTexture: function (texture) {
    GLctx.bindTexture(GLctx.TEXTURE_2D, GL.textures[texture]);
  },

});

Then you can call these functions from your C# scriptsA piece of code that allows you to create your own Components, trigger game events, modify Component properties over time and respond to user input in any way you like. More info
See in Glossary like this:

using UnityEngine;
using System.Runtime.InteropServices;

public class NewBehaviourScript : MonoBehaviour {

    [DllImport("__Internal")]
    private static extern void Hello();

    [DllImport("__Internal")]
    private static extern void HelloString(string str);

    [DllImport("__Internal")]
    private static extern void PrintFloatArray(float[] array, int size);

    [DllImport("__Internal")]
    private static extern int AddNumbers(int x, int y);

    [DllImport("__Internal")]
    private static extern string StringReturnValueFunction();

    [DllImport("__Internal")]
    private static extern void BindWebGLTexture(int texture);

    void Start() {
        Hello();
        
        HelloString("This is a string.");
        
        float[] myArray = new float[10];
        PrintFloatArray(myArray, myArray.Length);
        
        int result = AddNumbers(5, 7);
        Debug.Log(result);
        
        Debug.Log(StringReturnValueFunction());
        
        var texture = new Texture2D(0, 0, TextureFormat.ARGB32, false);
        BindWebGLTexture(texture.GetNativeTexturePtr());
    }
}

You can pass simple numeric types to JavaScript in function parameters without doing any conversion. You can pass other data types as a pointer in the emscripten heap which is just a big array in JavaScript.
For strings, you can use the UTF8ToString helper function to convert to a JavaScript string.
To return a string value, call _malloc to allocate some memory and the stringToUTF8 helper function to write a JavaScript string to it. If the string is a return value, then the IL2CPPA Unity-developed scripting back-end which you can use as an alternative to Mono when building projects for some platforms. More info
See in Glossary runtime automatically frees up the memory for you.
For arrays of primitive types, emscripten provides different ArrayBufferViews into its heap for different sizes of integer, unsigned integer or floating point representations of memory: HEAP8, HEAPU8, HEAP16, HEAPU16, HEAP32, HEAPU32, HEAPF32, HEAPF64.
To access a texture in WebGL, emscripten provides the GL.textures array which maps native texture IDs from Unity to WebGL texture objects. You can call WebGL functions on emscripten’s WebGL context, GLctx.

For more information on how to interact with JavaScript, refer to the emscripten documentation.

In addition, note that in the Unity installation folder there are several plugins that you can use as reference, in PlaybackEngines/WebGLSupport/BuildTools/lib and PlaybackEngines/WebGLSupport/BuildTools/Emscripten/src/library*.

Calling Unity scripts functions from JavaScript

Sometimes you need to send some data or notification to the Unity script from the browser’s JavaScript. The recommended way of doing it is to call methods on GameObjectsThe fundamental object in Unity scenes, which can represent characters, props, scenery, cameras, waypoints, and more. A GameObject’s functionality is defined by the Components attached to it. More info
See in Glossary in your content. If you are making the call from a JavaScript plugin, embedded in your project, you can use the following code:

unityInstance.SendMessage(objectName, methodName, value);

Where objectName is the name of an object in your scene; methodName is the name of a method in the script, currently attached to that object; value can be a string, a number, or can be empty. For example:

unityInstance.SendMessage('MyGameObject', 'MyFunction');
unityInstance.SendMessage('MyGameObject', 'MyFunction', 5);

unityInstance.SendMessage('MyGameObject', 'MyFunction', 'MyString');

If you would like to make a call from the global scope of the embedding page, see the Code Visibility section below.

Calling C functions from Unity scripts

Unity compiles your sources into JavaScript from C/C++ code using emscripten, so you can also write plugins in C/C++ code, and call these functions from C#. So, instead of the jslib file in the example above, you could have a C/C++ file in your project - it will automatically get compiled with your scripts, and you can call functions from it, just like in the JavaScript example above.

If you are using C++ (.cpp) to implement the plugin then you must ensure the functions are declared with C linkage to avoid name mangling issues:

#include <stdio.h>

extern "C" void Hello ()
{
    printf("Hello, world!\n");
}

extern "C" int AddNumbers (int x, int y)
{
    return x + y;
}

Code visibility

The recommended approach is to execute all the build code in its own scope. This allows you to embed your content on an arbitrary page without causing conflicts with the embedding page code, and lets you embed more than one build on the same page.

If you have all your JavaScript code in the form of .jslib plugins inside your project, then this JavaScript code will run inside the same scope as the compiled build and your code should work pretty much the same way as in previous versions of Unity (for example, the following objects and functions should be directly visible from the JavaScript plugin code: Module, SendMessage, HEAP8, ccall etc.).

However, if you are planning to call the internal JavaScript functions from the global scope of the embedding page, you must use the unityInstance variable in your WebGL Template index.html. Do this after the Unity engine instantiation succeeds, for example:

  var myGameInstance = null;
  script.onload = () => {
    createUnityInstance(canvas, config, (progress) => {...}).then((unityInstance) => {
      myGameInstance = unityInstance;
      …

Then you can send a message to the build using myGameInstance.SendMessage(), or access the build Module object using myGameInstance.Module.

unity.Instance replaced with createUnityInstance in 2020.1
Fixed error in code example.
WebGL instance renamed from gameInstance to unityInstance in 2019.1

Memory in Unity WebGL

WebGL performance considerations