bridge.js

// Initially, the __go_wasm__ object will be an empty object. 
const g = global || window || self;
if (!g.__go_wasm__) {
    g.__go_wasm__ = {};
}

/**
 * The maximum amount of time that we would expect Wasm to take to initialize.
 * If it doesn't initialize after this time, we send a warning to console.
 * Most likely something has gone wrong if it takes more than 3 seconds to initialize.
 */
const maxTime = 3 * 1000;

/**
 * bridge is an easier way to refer to the Go WASM object.
 */
const bridge = g.__go_wasm__;

/**
 * Wrapper is used by Go to run all Go functions in JS.
 * 
 * @param {Function} goFunc a function that is expected to return an object of the following specification:
 * {
 *  result:  undefined | any         // undefined when error is returned, or function returns undefined.
 *  error:       Error | undefined   // undefined when no error is present.
 * }
 * 
 * @returns {Function} returns a function that take arguments which are used to call the Go function.
 */
function wrapper(goFunc) {
    return (...args) => {
        const result = goFunc.apply(undefined, args);
        if (result.error instanceof Error) {
            throw result.error;
        }
        return result.result;
    }
}

/**
 * Sleep is used when awaiting for Go Wasm to initialize.
 * It uses the lowest possible sane delay time (via requestAnimationFrame).
 * However, if the window is not focused, requestAnimationFrame never returns.
 * A timeout will ensure to be called after 50 ms, regardless of whether or not the tab is in focus.
 * 
 * @returns {Promise} an always-resolving promise when a tick has been completed.
 */
function sleep() {
    return new Promise((res) => {
        requestAnimationFrame(() => res());
        setTimeout(() => {
            res();
        }, 50);
    });
}

/**
 * @param {ArrayBuffer} getBytes a promise that is bytes of the Go Wasm object.
 * 
 * @returns {Proxy} an object that can be used to call WASM's objects and properly parse their results.
 * 
 * All values that want to be retrieved from the proxy, regardless of if they are a function or not, must be retrieved
 * as if they are from a function call. 
 * 
 * If a non-function value is returned however arguments are provided, a warning will be printed.
 */
export default function (getBytes) {
    let proxy;
    let go;

    async function init() {
        bridge.__wrapper__ = wrapper;

        go = new g.Go();
        let bytes = await getBytes;
        let result = await WebAssembly.instantiate(bytes, go.importObject);
        go.run(result.instance);
    }

    init();
    setTimeout(() => {
        if (bridge.__ready__ !== true) {
            console.warn("Golang WASM Bridge (__go_wasm__.__ready__) still not true after max time");
        }
    }, maxTime);


    proxy = new Proxy(
        {},
        {
            get: (_, key) => {
                return (...args) => {
                    return new Promise(async (res, rej) => {
                        if (!go || go.exited) {
                            return rej(new Error("The Go instance is not active."));
                        }
                        while (bridge.__ready__ !== true) {
                            await sleep();
                        }

                        if (typeof bridge[key] !== 'function') {
                            res(bridge[key]);

                            if (args.length !== 0) {
                                console.warn("Retrieved value from WASM returned function type, however called with arguments.")
                            }
                            return;
                        }

                        try {
                            res(bridge[key].apply(undefined, args));
                        } catch (e) {
                            rej(e);
                        }
                    })
                };
            }
        }
    );

    bridge.__proxy__ = proxy;
    return proxy;
}