Making a for loop wait WITHOUT a library in JS

Question

How can one create a for loop that waits for an asynchronous call to complete prior to starting a new iteration of a loop WITHOUT a library (like jQuery)?

Example:

var items = [1,2,3,4,5];

for (var i = 0; i < items.length; i++) {

    var promise = new Promise(function(resolve, reject){
        asyncAPIcall({
            body : item[i]
        }, function(error, response){
            if(error) {
                reject();
            } else {
                resolve();
            }
        });

    promise.then(function() {
            //continue loop
        }, function() {
            //break loop
        });

}

Thanks

UPDATE (4/29)

I thought of this solution, where I create a function that calls itself:

var items = [1,2,3,4,5];

var counter = items.length - 1; //minus one since array is zero based.

function myLoop(){

    asyncAPIcall({
        body : item[counter]
    }, function(error, response){
        if(error) {
            // Error message.
        } else {
            counter = counter - 1;
            if(counter == -1){
                //Done
            }
            else {
                myLoop();
            }
        }
    });

}

A `for` loop is synchronous. It can never wait for an asynchronous operation. You can have a queue though or something similar. — Felix Kling, Apr 27 '16 at 21:48
@Amit I need to fire of a long series of API calls and if one fails, stop the chain of an additional calls. I am interacting with an API that only accepts one object at a time, so I have to hit it like 20 times to send all my objects. — Ben Rondeau, Apr 27 '16 at 21:51
You could have used `Array#reduce()` to mimic sequential operation. — dvlsg, Apr 27 '16 at 21:52

dvlsg · Answer 1 · 2016-04-27T22:05:49.563

You can use reduce to make them process sequentially (or set up the promise chain using a regular for loop -- I prefer reduce, myself).

let promise = items.reduce((carry, current) => {
    return carry.then(arr => {
        return asyncAPIcall({ body: current }).then(result => arr.concat([ result ]));
    });
}, Promise.resolve([]));

promise.then(finalResult => {
    console.log('final result:', finalResult);
});

This may be more than you need, if you don't actually need to capture the results of those promise resolutions, though. Also note that you'll still have a promise at the end of it, which will contain an array of the results of each promise, corresponding to their original array position.

Also, here is a mocked version of the asyncAPIcall, which should assist with showing the order of operations here, if you want to trace through how / where the methods are called.

function asyncAPIcall(obj) {
    console.log('asyncAPIcall for:', obj);
    return new Promise((resolve) => {
        setTimeout(() => {
            let resolution = obj.body + 5; // change the value in some way, just to show that input !== output
            console.log('resolving with:', resolution);
            return resolve(resolution);
        }, 100);
    });
}

Good luck! If you don't need the results at the end, a lot of the array shenanigans can go away entirely. If you need, I can add a simpler version that doesn't care about the results. — dvlsg, Apr 27 '16 at 22:06

Making a for loop wait WITHOUT a library in JS

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