Why you should return in callback?

test_promises.js

/**
 * Note that you don't have a "return_err" in your console.
 * But you have "noreturn_err".
 *
 * Even if you resolve/reject your promise, it's still a function that'll
 * keep its execution until it is returned.
 */

const log = tag => (...args) => console.log(tag, ...args);

const p = (tag, err) => new Promise((resolve, reject) => {
	if (err) reject(err); // no return, it'll keep processing even if it's rejected/resolved.

	console.log(tag, 'Shouldn\'t display if error');

	resolve(tag + ' resolved');
});

p('noreturn_noerr')
	.then(log('noerr_then'))
	.catch(log('noerr_catch'));

p('noreturn_err', new Error('error: no_return'))
	.then(log('err_then'))
	.catch(log('err_catch'));

const pR = (tag, err) => new Promise((resolve, reject) => {
	if (err) return reject(err); // Promise is rejected and execution stops

	console.log(tag, 'Shouldn\'t display if error');

	return resolve(tag + ' resolved');
});

pR('return_noerr')
	.then(log('noerr_then'))
	.catch(log('noerr_catch'));

pR('return_err', new Error('error: return'))
	.then(log('err_then'))
	.catch(log('err_catch'));