worker.js

// The worker has its own scope and no direct access to functions/objects of the
// global scope. We import the generated JS file to make `wasm_bindgen`
// available which we need to initialize our Wasm code.
importScripts('./pkg/wasm_in_web_worker.js');

console.log('Initializing worker')

// In the worker, we have a different struct that we want to use as in
// `index.js`.
const {NumberEval} = wasm_bindgen;

async function init_wasm_in_worker() {
    // Load the Wasm file by awaiting the Promise returned by `wasm_bindgen`.
    await wasm_bindgen('./pkg/wasm_in_web_worker_bg.wasm');

    // Create a new object of the `NumberEval` struct.
    var num_eval = NumberEval.new();

    // Set callback to handle messages passed to the worker.
    self.onmessage = async event => {
        // By using methods of a struct as reaction to messages passed to the
        // worker, we can preserve our state between messages.
        var worker_result = num_eval.is_even(event.data);

        // Send response back to be handled by callback in main thread.
        self.postMessage(worker_result);
    };
};

init_wasm_in_worker();