I was testing the Promise object and wrote some code that simulates a long running task that is synchronous. I was comparing Promise and setTimeout - see fiddle:
<!DOCTYPE html>
<html>
<head>
<link rel="stylesheet" href="style.css">
</head>
<body>
<h2>Promise vs setTimeout</h2>
<div><button id="settimeout-test">setTimeout with slow running function</button></div>
<div><button id="promise-test">Promise and slow running function</button></div>
<div><button id="clear">Clear Results</button></div>
<h5>Results</h5>
<div id="result"></div>
<script>
const slow = function() {
let nu = Date.now();
while (Date.now() - nu < 1000) {}
}
const getSlowPromise = () => new Promise(resolve => {
slow();
resolve();
});
const resultsElement = document.getElementById('result')
const log = (message) => {
resultsElement.innerText += message;
}
const settimeoutButton = document.getElementById('settimeout-test');
settimeoutButton.addEventListener('click', () => {
const now = Date.now();
log(`
setTimeout test starts after ${Date.now() - now} ms`);
setTimeout(() => {
slow();
log(`
Slow function completes after ${Date.now() - now} ms`);
}, 0);
log(`
Event listener completes after ${Date.now() - now} ms`);
});
const promiseButton = document.getElementById('promise-test');
promiseButton.addEventListener('click', () => {
const now = Date.now();
log(`
setTimeout test starts after ${Date.now() - now} ms`);
getSlowPromise().then(res => log(`
Promise completes after ${Date.now() - now} ms`));
log(`
event listener completes after ${Date.now() - now} ms`);
})
const clear = () => resultsElement.innerText = '';
const clearButton = document.getElementById('clear');
clearButton.addEventListener('click', () => clear());
</script>
</body>
</html>
I thought Promise and setTimeout would behave in a similar way, add the code to the task queue and then continue the execution. The order of the results are the same but the promise with a long running task seems to block the rendering until the long running task completes. Can someone explain this?
The example runs best in Chrome.
Update:
I'm not trying to make the long running task to run in parallel, I just want to understand why Promise and setTimeout behave differently in my example. But if you do want to run the task in parallel then Web Workers / Worker threads is the way to go as Quentin suggests.
But the answer to my question seems to be that Promise constructors are synchronous as Bergi writes in a comment.
Here is a longer explanation
See Question&Answers more detail:
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