Difference between Javascript async functions and Web workers?

In contrast to WebWorkers, async functions are never guaranteed to be executed on a separate thread.

They just don't block the whole thread until their response arrives. You can think of them as being registered as waiting for a result, let other code execute and when their response comes through they get executed; hence the name asynchronous programming.

This is achieved through a message queue, which is a list of messages to be processed. Each message has an associated function which gets called in order to handle the message.

Doing this:

setTimeout(() => {
  console.log('foo')
}, 1000)

will simply add the callback function (that logs to the console) to the message queue. When it's 1000ms timer elapses, the message is popped from the message queue and executed.

While the timer is ticking, other code is free to execute. This is what gives the illusion of multithreading.

The setTimeout example above uses callbacks. Promises and async work the same way at a lower level — they piggyback on that message-queue concept, but are just syntactically different.

async functions are just syntactic sugar around Promises and they are wrappers for callbacks.

// v await is just syntactic sugar
//                 v Promises are just wrappers
//                                         v functions taking callbacks are actually the source for the asynchronous behavior
   await new Promise(resolve => setTimeout(resolve));
 

Now a callback could be called back immediately by the code, e.g. if you .filter an array, or the engine could store the callback internally somewhere. Then, when a specific event occurs, it executes the callback. One could say that these are asynchronous callbacks, and those are usually the ones we wrap into Promises and await them.

To make sure that two callbacks do not run at the same time (which would make concurrent modifications possible, which causes a lot of trouble) whenever an event occurs the event does not get processed immediately, instead a Job (callback with arguments) gets placed into a Job Queue. Whenever the JavaScript Agent (= thread²) finishes execution of the current job, it looks into that queue for the next job to process¹.

Therefore one could say that an async function is just a way to express a continuous series of jobs.

 async function getPage() {
   // the first job starts fetching the webpage
   const response = await fetch("https://stackoverflow.com"); // callback gets registered under the hood somewhere, somewhen an event gets triggered
   // the second job starts parsing the content
   const result = await response.json(); // again, callback and event under the hood
   // the third job logs the result
   console.log(result);
}

// the same series of jobs can also be found here:
fetch("https://stackoverflow.com") // first job
   .then(response => response.json()) // second job / callback
   .then(result => console.log(result)); // third job / callback

Although two jobs cannot run in parallel on one agent (= thread), the job of one async function might run between the jobs of another. Therefore, two async functions can run concurrently.

Now who does produce these asynchronous events? That depends on what you are awaiting in the async function (or rather: what callback you registered). If it is a timer (setTimeout), an internal timer is set and the JS-thread continues with other jobs until the timer is done and then it executes the callback passed. Some of them, especially in the Node.js environment (fetch, fs.readFile) will start another thread internally. You only hand over some arguments and receive the results when the thread is done (through an event).

To get real parallelism, that is running two jobs at the same time, multiple agents are needed. WebWorkers are exactly that - agents. The code in the WebWorker therefore runs independently (has it's own job queues and executor).

Agents can communicate with each other via events, and you can react to those events with callbacks. For sure you can await actions from another agent too, if you wrap the callbacks into Promises:

const workerDone = new Promise(res => window.onmessage = res);

(async function(){
    const result = await workerDone;
        //...
})();

TL;DR:

JS  <---> callbacks / promises <--> internal Thread / Webworker

---

¹ There are other terms coined for this behavior, such as event loop / queue and others. The term Job is specified by ECMA262.

² How the engine implements agents is up to the engine, though as one agent may only execute one Job at a time, it very much makes sense to have one thread per agent.

Async functions have nothing to do with web workers or node child processes - unlike those, they are not a solution for parallel processing on multiple threads.

An async function is just¹ syntactic sugar for a function returning a promise then() chain.

async function example() {
    await delay(1000);
    console.log("waited.");
}

is just the same as

function example() {
    return Promise.resolve(delay(1000)).then(() => {
        console.log("waited.");
    });
}

These two are virtually indistinguishable in their behaviour. The semantics of await or a specified in terms of promises, and every async function does return a promise for its result.

_{1: The syntactic sugar gets a bit more elaborate in the presence of control structures such as if/else or loops which are much harder to express as a linear promise chain, but it's still conceptually the same.}

Are such functions threaded in the same way as a function executed through setInterval is?

Yes, the asynchronous parts of async functions run as (promise) callbacks on the standard event loop. The delay in the example above would implemented with the normal setTimeout - wrapped in a promise for easy consumption:

function delay(t) {
    return new Promise(resolve => {
        setTimeout(resolve, t);
    });
}

Workers are also accessed by asynchronous code (i.e. Promises) however Workers are a solution to the CPU intensive tasks which would block the thread that the JS code is being run on; even if this CPU intensive function is invoked asynchronously.

So if you have a CPU intensive function like renderThread(duration) and if you do like

new Promise((v,x) => setTimeout(_ => (renderThread(500), v(1)),0)
    .then(v => console.log(v);
new Promise((v,x) => setTimeout(_ => (renderThread(100), v(2)),0)
    .then(v => console.log(v);

Even if second one takes less time to complete it will only be invoked after the first one releases the CPU thread. So we will get first 1 and then 2 on console.

However had these two function been run on separate Workers, then the outcome we expect would be 2 and 1 as then they could run concurrently and the second one finishes and returns a message earlier.

So for basic IO operations standard single threaded asynchronous code is very efficient and the need for Workers arises from need of using tasks which are CPU intensive and can be segmented (assigned to multiple Workers at once) such as FFT and whatnot.