Implement Promise.all by hand.

Promise.all.js

function promiseAll(promises) {
	return new Promise((resolve, reject) => {
		let	results = [],
			position = 0,
			fulfilled = 0,
			resolved = false
		for(let promise of promises) {
			let promisePosition = position
			if(promise instanceof Promise) {
				promise.then(value => {
					results[promisePosition] = value
					fulfilled++
					if(!resolved && fulfilled == promises.length) {
						resolved = true
						resolve(results)
					}
				}, reason => reject(reason))
			} else {
				results[promisePosition] = promise
				fulfilled++
			}
			position++
		}

		/* 1. `promises` is empty, or
		 * 2. `promises` contains no Promises.
		 **/
		if(position == 0 || fulfilled == position) {
			resolve(results)
		}
	})
}