JavaScript: async/await with forEach()

async-foreach.js

const waitFor = (ms) => new Promise(r => setTimeout(r, ms))
const asyncForEach = async (array, callback) => {
  for (let index = 0; index < array.length; index++) {
    await callback(array[index], index, array)
  }
}

const start = async () => {
  await asyncForEach([1, 2, 3], async (num) => {
    await waitFor(50)
    console.log(num)
  })
  console.log('Done')
}

start()

Object.defineProperty(Object.prototype, 'eachObjectKey', {writable: true, value:
	async function (p, f, next) {
		var promises = [];
		for (var key in this) {
			if (p=='await') {
				//(in order)
				promises.push(await new Promise( (resolve, reject) => f(key, this[key], resolve)));
			} else {
				//(in parallel)
				promises.push(new Promise( (resolve, reject) => f(key, this[key], resolve)));
			}
		}
		await Promise.all(promises).then(next);
	}
});

var obj = [1, 2, 3, 4];
var f = function(key, value, next) {
	setTimeout(function() {
		console.log(key, value);
		if (value==2) return next('error');
		next('OK');
	}, value*100);
}

console.time('app');
console.log('\nstart in order');
obj.eachObjectKey('await', f, function(results) {
	console.log('Done!');
	console.log(results);
	console.timeEnd('app');

	console.time('app');
	console.log('\nstart in parallel');
	obj.eachObjectKey('not await', f, function(results) {
		console.log('Done!');
		console.log(results);
		console.timeEnd('app')
	});
});

I don´t get why callback reicive 3 arguments, in the start function the call of asyncForEach method the second parameter is callback and is just sending one argument ... can some explain step by step?

Here's my typescript version (Basically @wodCZ's codeblock) with the ability to return data which is readable in the form of an array

async function asyncForEach<T = any>(array: T[], callback: (value: T, index?: number, array?: T[]) => any): Promise<any[]> {
  const data: any[] = [];
  for (let index = 0; index < array.length; index++) {
    await (async () => {
      const item = await callback(array[index], index, array);
      if (item !== undefined) data.push(item);
      return;
    })();
  }
  return Promise.resolve(data);
}

I feel like this is just a glorified promise.all at this point.

Why not use for await()?

const waitFor = (ms) => new Promise(r => setTimeout(r, ms))
const start = async () => {
  for await (num of [1, 2, 3]) {
    await waitFor(50)
    console.log(num)
  };
  console.log('Done')
}
start()

More info here