async.await.foreach.twitter.doubt.js

import log4js from './common/logger';
var logger = log4js.getLogger('app.js');

function performTask(ms) {
    return new Promise(resolve => setTimeout(resolve, ms));
}

// this wont work as intended

// This code is not working as you spect because the 'forEach' function will never be 
// able to return a promise.

// Due to that, if you run this code (with the logger trace after the addition operation),
// you will obtain that:
// [ INFO] - fn1 0
// [TRACE] - taskSum: 1
// [TRACE] - taskSum: 3
// [TRACE] - taskSum: 6
// [TRACE] - taskSum: 10
// [TRACE] - taskSum: 15
// [ INFO] - fn2 15

// As you can see, the forEach's 'await' is ignored, the final 'fn1' is printed and finally,
// once all forEach's internal async functions are completed, their results are shown.

async function asyncFn1() {
    const someTasks = [1, 2, 3, 4, 5];
    let taskSum = 0;
    await someTasks.forEach(async t => {
        await performTask(t);
        taskSum += t;
        logger.trace(`taskSum: ${taskSum}`);
    });
    logger.info('fn1', taskSum); // fn1 0
}

// This works as intended!

// This code works because the awaiting operation is been applied over an async 
// function which is able to return a promise and not directly on the 'for' sentence.
// Due to that, the 'for' iteration flow is paused every time the 'async' function is run and
// this flow continues after that function's promise is resolved.

// Anyway, in order to work with async/await on iterations, it's recomended directly
// work with plain loops.
// Reference: https://stackoverflow.com/questions/37576685/using-async-await-with-a-foreach-loop/37576787#37576787

async function asyncFn2() {
    const someTasks = [1, 2, 3, 4, 5];
    let taskSum = 0;
    for (let t of someTasks) {
        await performTask(t);
        taskSum += t;
    }
    logger.info('fn2', taskSum); // fn2 15
}

asyncFn1();
asyncFn2();