Best of Modern JavaScript — Async and Promises

Better ways to write async code.

Since 2015, JavaScript has improved immensely.

It’s much more pleasant to use it now than ever.

In this article, we’ll look at JavaScript async programming.

Sync and Async

The synchronous code always runs before the async code.

So if we have:

setTimeout(function() { 
  console.log(2);
}, 0);
console.log(1);

Then we get:

1
2

The console log outside is synchronous, so it runs first.

The callback’s console log runs after the synchronous code, since it’s async.

The event loop may be blocked if we run something that’s synchronous since the whole app is managed by a single process.

Both the user interface and other computations are all in one thread.

So if one thing runs, then whatever comes after it can’t run until that thing is done.

For instance, if we have a synchronous sleep function, then that’ll pause the execution of the whole program.

If we have:

function sleep(ms) {
  const start = Date.now();
  while ((Date.now() - start) < ms);
}
console.log('start');
sleep(2000);
console.log('end');

Then we see 'start' logged first, then wait 2 seconds, then 'end' is logged.

The sleep function uses a while loop, so it’s synchronous.

This means it’ll hold up the whole program from running.

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Async callbacks or promises. Introduction to JavaScript Async Programming