How to call async/await JavaScript function in Unity/C# WebGL platform?

Question

I followed this doc to call JavaScript function from my C# script in Unity to make a WebGL game.

But there is a problem if the js code contains async/await, for example:

C# script:

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

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

    void Start(){
      Foo();
      Boo();
    }

JavaScript mylib.jslib

mergeInto(LibraryManager.library, {
  // works well
  Foo: function () {
    window.alert("Hello, world!");
  },

  // error: can't compile
  Boo: async function (){
    var s = function (ms) {
        return new Promise(resolve => setTimeout(resolve, ms));
    };
    await s(2000);
    window.alert("Boo!");
  }
});

When I tried to build these code, it showed error as below:

Failed process stderr log:
error: failure to execute js library "D:\CloudLinProject\Unity\My project\Assets\Plugins\Javascripts\PhantomAPI.jslib": SyntaxError: Unexpected token function,,SyntaxError: Unexpected token function
    at Object.load (eval at globalEval (D:\Program Files\Unity\2020.3.22f1\Editor\Data\PlaybackEngines\WebGLSupport\BuildTools\Emscripten\src\compiler.js:105:8), <anonymous>:179:14)
    at JSify (eval at globalEval (D:\Program Files\Unity\2020.3.22f1\Editor\Data\PlaybackEngines\WebGLSupport\BuildTools\Emscripten\src\compiler.js:105:8), <anonymous>:87:20)
    at D:\CloudLinProject\Unity\My project\Assets\Plugins\Javascripts\PhantomAPI.jslib (D:\Program Files\Unity\2020.3.22f1\Editor\Data\PlaybackEngines\WebGLSupport\BuildTools\Emscripten\src\compiler.js:221:3)
    at Module._compile (module.js:570:32)
    at Object.Module._extensions..js (module.js:579:10)
    at Module.load (module.js:487:32)
    at tryModuleLoad (module.js:446:12)
    at Function.Module._load (module.js:438:3)
    at Module.runMain (module.js:604:10)
    at run (bootstrap_node.js:394:7)
preprocessed source (you can run a js engine on this to get a clearer error message sometimes):


Internal compiler error in src/compiler.js! Please raise a bug report at https://github.com/kripken/emscripten/issues/ with a log of the build and the input files used to run. Exception message: "SyntaxError: Unexpected token function" | SyntaxError: Unexpected token function
    at Object.load (eval at globalEval (D:\Program Files\Unity\2020.3.22f1\Editor\Data\PlaybackEngines\WebGLSupport\BuildTools\Emscripten\src\compiler.js:105:8), <anonymous>:179:14)
    at JSify (eval at globalEval (D:\Program Files\Unity\2020.3.22f1\Editor\Data\PlaybackEngines\WebGLSupport\BuildTools\Emscripten\src\compiler.js:105:8), <anonymous>:87:20)
    at Object.<anonymous> (D:\Program Files\Unity\2020.3.22f1\Editor\Data\PlaybackEngines\WebGLSupport\BuildTools\Emscripten\src\compiler.js:221:3)
    at Module._compile (module.js:570:32)
    at Object.Module._extensions..js (module.js:579:10)
    at Module.load (module.js:487:32)
    at tryModuleLoad (module.js:446:12)
    at Function.Module._load (module.js:438:3)
    at Module.runMain (module.js:604:10)
    at run (bootstrap_node.js:394:7)
Traceback (most recent call last):
  File "D:\Program Files\Unity\2020.3.22f1\Editor\Data\PlaybackEngines\WebGLSupport\BuildTools\Emscripten\emcc.py", line 3063, in <module>
    sys.exit(run())
  File "D:\Program Files\Unity\2020.3.22f1\Editor\Data\PlaybackEngines\WebGLSupport\BuildTools\Emscripten\emcc.py", line 1780, in run
    final = shared.Building.emscripten(final, append_ext=False, extra_args=extra_args)
  File "D:\Program Files\Unity\2020.3.22f1\Editor\Data\PlaybackEngines\WebGLSupport\BuildTools\Emscripten\tools\shared.py", line 2274, in emscripten
    emscripten._main(cmdline)
  File "D:\Program Files\Unity\2020.3.22f1\Editor\Data\PlaybackEngines\WebGLSupport\BuildTools\Emscripten\emscripten.py", line 2233, in _main
    return temp_files.run_and_clean(lambda: main(
  File "D:\Program Files\Unity\2020.3.22f1\Editor\Data\PlaybackEngines\WebGLSupport\BuildTools\Emscripten\tools\tempfiles.py", line 93, in run_and_clean
    return func()
  File "D:\Program Files\Unity\2020.3.22f1\Editor\Data\PlaybackEngines\WebGLSupport\BuildTools\Emscripten\emscripten.py", line 2238, in <lambda>
    DEBUG=DEBUG,
  File "D:\Program Files\Unity\2020.3.22f1\Editor\Data\PlaybackEngines\WebGLSupport\BuildTools\Emscripten\emscripten.py", line 2164, in main
    temp_files=temp_files, DEBUG=DEBUG)
  File "D:\Program Files\Unity\2020.3.22f1\Editor\Data\PlaybackEngines\WebGLSupport\BuildTools\Emscripten\emscripten.py", line 86, in emscript
    glue, forwarded_data = compiler_glue(metadata, libraries, compiler_engine, temp_files, DEBUG)
  File "D:\Program Files\Unity\2020.3.22f1\Editor\Data\PlaybackEngines\WebGLSupport\BuildTools\Emscripten\emscripten.py", line 218, in compiler_glue
    glue, forwarded_data = compile_settings(compiler_engine, libraries, temp_files)
  File "D:\Program Files\Unity\2020.3.22f1\Editor\Data\PlaybackEngines\WebGLSupport\BuildTools\Emscripten\emscripten.py", line 541, in compile_settings
    cwd=path_from_root('src'), error_limit=300)
  File "D:\Program Files\Unity\2020.3.22f1\Editor\Data\PlaybackEngines\WebGLSupport\BuildTools\Emscripten\tools\jsrun.py", line 132, in run_js
    raise Exception('Expected the command ' + str(command) + ' to finish with return code ' + str(assert_returncode) + ', but it returned with code ' + str(proc.returncode) + ' instead! Output: ' + str(ret)[:error_limit])
Exception: Expected the command ['D:/Program Files/Unity/2020.3.22f1/Editor/Data\\Tools\\nodejs\\node.exe', '--stack_size=8192', '--max-old-space-size=4096', 'D:\\Program Files\\Unity\\2020.3.22f1\\Editor\\Data\\PlaybackEngines\\WebGLSupport\\BuildTools\\Emscripten\\src\\compiler.js', 'C:\\Users\\eucyl\\AppData\\Local\\Temp\\tmpoqhbko.txt', 'D:\\Program Files\\Unity\\2020.3.22f1\\Editor\\Data\\PlaybackEngines\\WebGLSupport\\BuildTools\\lib\\Audio.js', 'D:\\Program Files\\Unity\\2020.3.22f1\\Editor\\Data\\PlaybackEngines\\WebGLSupport\\BuildTools\\lib\\case_1174367_workaround.js', 'D:\\Program Files\\Unity\\2020.3.22f1\\Editor\\Data\\PlaybackEngines\\WebGLSupport\\BuildTools\\lib\\case_1179945_workaround.js', 'D:\\Program Files\\Unity\\2020.3.22f1\\Editor\\Data\\PlaybackEngines\\WebGLSupport\\BuildTools\\lib\\case_1187965_workaround.js', 'D:\\Program Files\\Unity\\2020.3.22f1\\Editor\\Data\\PlaybackEngines\\WebGLSupport\\BuildTools\\lib\\case_1208971_workaround.js', 'D:\\Program Files\\Unity\\2020.3.22f1\\Editor\\Data\\PlaybackEngines\\WebGLSupport\\BuildTools\\lib\\Cursor.js', 'D:\\Program Files\\Unity\\2020.3.22f1\\Editor\\Data\\PlaybackEngines\\WebGLSupport\\BuildTools\\lib\\Eval.js', 'D:\\Program Files\\Unity\\2020.3.22f1\\Editor\\Data\\PlaybackEngines\\WebGLSupport\\BuildTools\\lib\\FileSystem.js', 'D:\\Program Files\\Unity\\2020.3.22f1\\Editor\\Data\\PlaybackEngines\\WebGLSupport\\BuildTools\\lib\\Logging.js', 'D:\\Program Files\\Unity\\2020.3.22f1\\Editor\\Data\\PlaybackEngines\\WebGLSupport\\BuildTools\\lib\\Profiler.js', 'D:\\Program Files\\Unity\\2020.3.22f1\\Editor\\Data\\PlaybackEngines\\WebGLSupport\\BuildTools\\lib\\SystemInfo.js', 'D:\\Program Files\\Unity\\2020.3.22f1\\Editor\\Data\\PlaybackEngines\\WebGLSupport\\BuildTools\\lib\\UnetWebSocket.js', 'D:\\Program Files\\Unity\\2020.3.22f1\\Editor\\Data\\PlaybackEngines\\WebGLSupport\\BuildTools\\lib\\Video.js', 'D:\\Program Files\\Unity\\2020.3.22f1\\Editor\\Data\\PlaybackEngines\\WebGLSupport\\BuildTools\\lib\\WebCam.js', 'D:\\Program Files\\Unity\\2020.3.22f1\\Editor\\Data\\PlaybackEngines\\WebGLSupport\\BuildTools\\lib\\WebRequest.js', 'D:\\CloudLinProject\\Unity\\My project\\Assets\\Plugins\\Javascripts\\PhantomAPI.jslib', 'D:\\Program Files\\Unity\\2020.3.22f1\\Editor\\Data\\PlaybackEngines\\WebGLSupport\\BuildTools\\Emscripten\\src\\library_pthread_stub.js'] to finish with return code 0, but it returned with code 1 instead! Output: // The Module object: Our interface to the outside world. We import
// and export values on it. There are various ways Module can be used:
// 1. Not defined. We create it here
// 2. A function parameter, function(Module) { ..generated code.. }
// 3. pre-run appended it, var Module = {}; ..generated
UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&)

So how should I handle js async/await functions in C#?

Answer 1

tl;dr: This is how. c# doesn't need to be aware of the async and it should work.

---

I just made a little test using

Assets/Plugins/mylib.jslib

mergeInto(LibraryManager.library, {

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

    Boo: async function () {
        var s = function (ms) {
            return new Promise(resolve => setTimeout(resolve, ms));
        };
        await s(2000);
        window.alert("Boo!");
    }
});

and

Assets/Exmaple.cs

public class Example : MonoBehaviour
{
    [DllImport("__Internal")]
    private static extern void Foo();

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

    void Awake()
    {
        Foo();
        Boo();
    }
}

And this is how it looks like without any issue

So without further information I would claim: You issue is not the async but something else.