building_promise.all.js

function Promise_all(promises) {
	return new Promise((resolve, reject) => {
		let results = [];
		let pending = promises.length;
		for (let i = 0; i < promises.length; i++) {
			promises[i]
				.then((result) => {
					results[i] = result;
					pending--;
					if (pending == 0) resolve(results);
				})
				.catch(reject);
		}
		if (promises.length == 0) resolve(results);
	});
}

// Test code.
Promise_all([]).then((array) => {
	console.log('This should be []:', array);
});
function soon(val) {
	return new Promise((resolve) => {
		setTimeout(() => resolve(val), Math.random() * 500);
	});
}
Promise_all([soon(1), soon(2), soon(3)]).then((array) => {
	console.log('This should be [1, 2, 3]:', array);
});
Promise_all([soon(1), Promise.reject('X'), soon(3)])
	.then((array) => {
		console.log('We should not get here');
	})
	.catch((error) => {
		if (error != 'X') {
			console.log('Unexpected failure:', error);
		}
	});