Run Web Worker Async

/**
 * Runs a function in a separate thread by using a Web Worker, allowing long
 * running functions to not block the UI.
 *
 * @param {Function} fn
 * @return {Promise}
 */
const runAsync = fn => {
    const worker = new Worker(
        URL.createObjectURL(new Blob([`postMessage((${fn})());`]), {
            type: "application/javascript; charset=utf-8"
        })
    );
    return new Promise((res, rej) => {
        worker.onmessage = ({ data }) => {
            res(data), worker.terminate();
        };
        worker.onerror = err => {
            rej(err), worker.terminate();
        };
    });
};

//
// Example Usage:

const longRunningFunction = () => {
    let result = 0;
    for (let i = 0; i < 1000; i++) {
        for (let j = 0; j < 700; j++) {
            for (let k = 0; k < 300; k++) {
                result = result + i + j + k;
            }
        }
    }
    return result;
};

/*
  NOTE: Since the function is running in a different context, closures are not supported.
  The function supplied to `runAsync` gets stringified, so everything becomes literal.
  All variables and functions must be defined inside.
*/
runAsync(longRunningFunction).then(console.log); // 209685000000
runAsync(() => 10 ** 3).then(console.log); // 1000

let outsideVariable = 50;
runAsync(() => typeof outsideVariable).then(console.log); // 'undefined'