Interaction 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 expose by default. In both cases, you need to directly interface with the browser’s JavaScript engine. Unity Web 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 in the Assets folder > Plugins sub-folder. The plug-in file needs to have a syntax like this:

mergeInto(LibraryManager.library, {

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

  HelloString: function (str) {
    window.alert(UTF8ToString(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 as follows:

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(1, 1, TextureFormat.ARGB32, false);
        BindWebGLTexture(texture.GetNativeTexturePtr());
    }
}

More Details:

• 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 WebGLA JavaScript API that renders 2D and 3D graphics in a web browser. The Unity Web 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, 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.

There are several plug-insA set of code created outside of Unity that creates functionality in Unity. There are two kinds of plug-ins you can use in Unity: Managed plug-ins (managed .NET assemblies created with tools like Visual Studio) and Native plug-ins (platform-specific native code libraries). More info
See in Glossary in the Unity installation folder that you can use as reference: PlaybackEngines/WebGLSupport/BuildTools/lib and PlaybackEngines/WebGLSupport/BuildTools/Emscripten/src/library*.

Code visibility

The recommended approach is to execute all the build code in its own scope. This lets you 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 plug-ins inside your project, then this JavaScript code will run inside the same scope as the compiled build and your code should work 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 Web 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.

Calling Unity scripts functions from JavaScript

The recommended way of sending data or notification to the Unity script from the browser’s JavaScript 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 plug-in, embedded in your project, you can use the following code:

MyGameInstance.SendMessage(objectName, methodName, value);

Where objectName is the name of an object in your sceneA Scene contains the environments and menus of your game. Think of each unique Scene file as a unique level. In each Scene, you place your environments, obstacles, and decorations, essentially designing and building your game in pieces. More info
See in Glossary; 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:

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

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

To make a call from the global scope of the embedding page, refer to the Code Visibility section.

Calling C functions from Unity scripts

Unity compiles your sources into JavaScript from C/C++ code using Emscripten, so you can also write plug-ins in C/C++ code, and call these functions from C#. Therefore, instead of the .jslib file in the above example, use a C/C++ file in your project, which can be automatically compiled with your scripts, and you can call functions from it.

If you are using C++ (.cpp) to implement the plug-in, then you must declare the functions 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;
}

Note: Unity is using the Emscripten version 2.0.19 toolchain.