PromiseWorker.js

/**
 * Promise wrapper for web workers.
 *
 * @author Lara Sophie Schütt (@literallylara)
 * @license MIT
 */

export default class PromiseWorker
{
    constructor(fn, ...args)
    {
        return new Promise((resolve, reject) =>
        {
            if (typeof fn != "function")
            {
                reject("Expected function at first argument")
                return
            }

            const script = `onmessage = e => postMessage((${fn})(...e.data))`
            const url = "data:application/javascript;base64," + btoa(script)
            const worker = new Worker(url)

            worker.onmessage = e =>
            {
                resolve(e.data)
                worker.terminate()
            }

            worker.onmessageerror = worker.onerror = e =>
            {
                reject(e)
                worker.terminate()
            }

            worker.postMessage(args)
        })
    }
}

Example usage:

import PromiseWorker from "./promise-worker.js"

// Calculate all primes up to a million
new PromiseWorker(limit =>
{
    const primes = []
    
    function isPrime(n)
    {
        let d = Math.floor(Math.sqrt(n)) + 1

        while (d > 1) if (n%(d--) == 0) return false

        return true
    }
    
    for (let i = 1; i < limit; i++)
    {
        if (isPrime(i)) primes.push(i)
    }
    
    return primes

}, 1e6)
.then(primes =>
{
    // [1, 3, 5, 7, ..., 999983]
    console.log(primes)
})
.catch(err =>
{
    console.log(err)
})