worker.js

// The purpose of this file is two-fold:
// First: Expose a function to start a web worker. This function must
// not be inlined into the Rust lib, as otherwise bundlers could not
// bundle it -- huh.
export function startWorker(module, memory, state, opts, helper) {
    const worker = new Worker(new URL('./worker.js',
        import.meta.url), opts);
    worker.postMessage([module, memory, state, helper.mainJS()]);

    return new Promise((res, rej) => {
      worker.onmessage = ev => {
        if (ev.data === 'started') res();
      };
      worker.onerror = rej;
    });
}

// Second: Entry script for the actual web worker.
if ('WorkerGlobalScope' in self &&
    self instanceof WorkerGlobalScope) {

    // Initialize wasm module, and memory. `state` is the shared state,
    // to be used with `worker_entry_point`.
    self.onmessage = async event => {
        let [module, memory, state, mainJS] = event.data;
        // This crate only works with bundling via webpack or not
        // using a bundler at all:
        // When bundling with webpack, this file is relative to the wasm
        // module file (or package.json) located in `../..` generated by
        // wasm-pack.
        // When using it without any bundlers, the module that
        // provided the `helper` object below is loaded; in other words
        // the main wasm module.
        const importFrom = (typeof __webpack_require__ === 'function') ? import('../..') : import(mainJS);
        try {
            const {
                default: init,
                worker_entry_point
            } = await importFrom;
            await init(module, memory);

            worker_entry_point(state);
            postMessage('started');
            // There shouldn't be any additional messages after the first.
            self.onmessage = event => {
                console.error("Unexpected message", event);
            }
        } catch (err) {
            // Propagate to main `onerror`:
            setTimeout(() => {
                throw err;
                //Terminate the worker
                close();
            });
            throw err;
        }
    }
}