Async / Await - Return values


When reading async / await code you come across different styles of returning a value in an async function.

Some just return the plain value:

async function test1() {
  return 5

Others return a promise:

async function test2() {
  return Promise.resolve(5);

While the third group don’t just use and return a Promise, they await it:

async function test3() {
  return await Promise.resolve(5);

So, you might wonder what the correct way to do this is, or if there even is a difference. But there is no right way to do it, in fact the behavior is exactly the same in all three cases. The important thing is that async functions always return a Promise`.

the return value of an async function is implicitly wrapped in Promise.resolve. - MDN

So if you chain a then on the Promise of any of these functions (or await these functions if you want to use async/await), it will only be called once the (implicitly wrapped) Promise is resolved.

const delayedLog = message =>
  new Promise(resolve =>
    setTimeout(() => {
    }, 1000)

async function test1() {
  await delayedLog(1);
  return 1;

async function test2() {
  return delayedLog(2);

async function test3() {
  return await delayedLog(3);

test1().then(val => console.log("finished ", val));
test2().then(val => console.log("finished ", val));
test3().then(val => console.log("finished ", val));

// Output after 1 second:
// 1
// finished 1 
// 2
// finished 2 
// 3
// finished 3 

Without needing to understand how exactly async / await is transpiled to ES5, intuitively, the second example test2, returning a Promise instead of just the value, looks more like this after conversion:

function test2() {
  return Promise.resolve(Promise.resolve(5));

However the inner Promise is automatically unwrapped, so in the end it again returns just a single Promise resolving to 5. The way to use any of these functions is the same.

Hi, I'm Christoph Michel πŸ‘‹

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