I am experiencing inconsistent behavior in Google Chrome 60.0.3112.78 (Official Build) (64-bit) when using modern ES6+ async/await, depending on whether I use brackets in the arrow function that returns a Promise. The same happens in Node.js. I am having trouble understanding why.
I understand that this is not how to implement a sleep() function but it is the easiest way to demonstrate. Consider the following example code snippet.
function sleep(ms = 0) {
return new Promise(resolve => setTimeout(resolve, ms));
}
(async () => {
console.log('a');
await sleep(5000);
console.log('b');
})()
As expected, this will write a to the console, wait 5 seconds and then write b to the console.
Shorter notation using an arrow function to return the Promise.
const sleep = ms => { return new Promise(resolve => setTimeout(resolve, ms)) }
(async () => {
console.log('a');
await sleep(5000);
console.log('b');
})()
As expected this code behaves the same. a and b are written to the console with a 5000 millisecond interval in between.
The following code does not work. The only difference is that I am not wrapping the return of the Promise in brackets on the first line.
const sleep = ms => new Promise(resolve => setTimeout(resolve, ms))
(async () => {
console.log('a');
await sleep(5000);
console.log('b');
})()
In this case, await sleep does not work. In fact, this code does absolutely nothing at all. It does not log anything to the console, not a and not b.
I consider myself fairly experienced but I don't currently understand this. Why do the brackets matter in this particular case? The return value is identical, right? And how come not even the character a is console logged?
Somebody please explain to me exactly and specifically why this is the way it is. Is this a bug or do I just need sleep myself?
Thank you very much.
