Simplest JS promise tutorial using await / async

Rabu, 24 April 2024, 4:30 pm

I often forgot how to use output of a function that uses Promise. Which callback to use? .then() / .catch() / .finally() / .all() ?

So here’s the easiest way to use it & remember.

	async function get_name(value) {
	return new Promise((resolve, reject) => { // must return Promise instance from async function
	setTimeout(() => { // emulate asynchronous, long running task
	if (value) {
	resolve(value); // success
	} else {
	reject(false); // fail
	}
	}, 3000);
	});
	}

	// await can only be used inside an async function
	async function main() {
	try {
	var name = await get_name();
	console.log('name=', name);
	} catch (e) { // to get rejected output, need to try-catch
	console.log('e=', e);
	}
	try {
	var name2 = await get_name('john');
	console.log('name2=', name2);
	} catch (e2) {
	console.log('e2=', e2);
	}
	}

	main();
	// output after 3 secs:
	// e=false
	// output after 6 secs:
	// name2=john