An example of how differently `Promise.all(arr.map(iteratee))` behaves when iteratee returns a synchronous result (value) versus an asynchronous result (promise).

promise-all-map_sync-vs-async.js

const chalk = require('chalk');

/**
 * sleep given number of milliseconds
 *
 * @param {Integer} ms
 * @return {Void}
 */
function sleep(ms) {
	const start = new Date().getTime();
	for (let i = 0; i < 1e7; i++) {
		if ((new Date().getTime() - start) > ms) {
			break;
		}
	}
}

/**
 * generate a random number up to given max
 *
 * @param {Integer} max
 * @return {Integer}
 */
function random(max) {
	return Math.floor(Math.random() * Math.floor(max));
}

/**
 * perform a randomly timed synchronous task (up to 2000ms)
 *
 * @param {Integer} index
 * @return {Integer} index
 */
function iterateSync(index) {
	console.log(chalk.yellow(`sync started, index ${index}`));
	const delay = random(2000);
	sleep(delay);
	console.log(chalk.yellow(`sync complete, index ${index} took ${delay}ms`));
	return index;
}

/**
 * perform a randomly timed asynchronous task (up to 2000ms)
 *
 * @param {Integer} index
 * @return {Promise} `.then(index)`
 */
async function iterateAsync(index) {
	console.log(chalk.green(`async started, index ${index}`));
	return new Promise((r) => {
		const delay = Math.floor(Math.random() * Math.floor(2000));
		setTimeout(() => {
			console.log(chalk.green(`async complete, index ${index} took ${delay}ms`));
			r(index);
		}, delay);
	});
}

const indexes = [1, 2, 3, 4, 5];

syncItems = Promise.all(indexes.map(iterateSync));
asyncItems = Promise.all(indexes.map(iterateAsync));