JavaScript Wait until all async calls finish

Question

I need some help with handling async calls in JavaScript. I have a for loop, each loop calls an async HttpRequest, and adds its response to an array. I want the program to wait

Answer 1

With promises, you should not use a callback parameter. Call the resolve/reject functions from the promise instead.

Instead of passing a callback to the call, chain the things you want to do with the result of the promise in a .then handler.

function httpGet(theUrl) {
    return new Promise(function(resolve, reject) {
        var xmlRequest = new XMLHttpRequest();
        xmlRequest.onreadystatechange = function() {
            if (xmlRequest.readyState == 4) {
                if (xmlRequest.status == 200) 
                    resolve(xmlRequest.responseText);
    //              ^^^^^^^
                else
                    reject(new Error(xmlRequest.statusText)); // or something
            }
        }
        xmlRequest.open("GET", theUrl, true);
        xmlRequest.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
        xmlRequest.setRequestHeader("Accept", "application/json");
        xmlRequest.send(null);
    });
}
$(document).ready(function() {    
    var channels = ["freecodecamp", "storbeck", "terakilobyte", "habathcx", "RobotCaleb", "thomasballinger", "noobs2ninjas", "beohoff"];
    var urls = channels.map((x) => "https://api.twitch.tv/kraken/channels/" + x);
    var promises = urls.map(function(url) {
//                      ^^^ simpler than forEach+push
        var promise = httpGet(url); // <-- no callback
        return promise.then(JSON.parse);
    });

    Promise.all(promises).then(function(data) {
//                                      ^^^^
        console.log(data);
    });
})

Answer 2

Since you are using jQuery you can use the Deferred Object to chain promises.

Collect all the promises and use $.when with spread operator to wait for all promises to resolve. You can use then to run a function after all ajax requests are resolved.

ES5 Example

$(document).ready(function () {

    var channels = ["freecodecamp", "storbeck", "terakilobyte", "habathcx", "RobotCaleb", "thomasballinger", "noobs2ninjas", "beohoff"];
    var urls = channels.map(function (x) {
        return "https://api.twitch.tv/kraken/channels/" + x;
    });
    var data = [];
    var promises = urls.map(function (url) {
        return $.get(url).then(function (response) {
            data.push(response);
        });
    });

    $.when.apply($, promises).then(function () {
        console.log('done', data);
    });
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

ES6 Example

$(document).ready(function() {    
    var channels = ["freecodecamp", "storbeck", "terakilobyte", "habathcx","RobotCaleb","thomasballinger","noobs2ninjas","beohoff"];
    var urls = channels.map((x) => "https://api.twitch.tv/kraken/channels/" + x);
    var data = [];
    var promises = urls.map((url) => $.get(url).then((response) => {
        data.push(response);
    }));

    $.when(...promises).then(function() {
        console.log('done', data);
    });
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

Answer 3

Can't it be done by just keeping the count of ajax requests as a variable:

var urls_count, data_count = 0;
function httpGet(theUrl, callback, onComplete) {
    var xmlRequest = new XMLHttpRequest();
    xmlRequest.onreadystatechange = function() {
        if (xmlRequest.readyState == 4 && xmlRequest.status == 200) {
            callback(xmlRequest.responseText);
        }
        if(xmlRequest.readyState == 4){
            data_count += 1
            if(urls_count == data_count){
                //this is called when all ajax calls complete
                onComplete();
            }
        }
    }
    xmlRequest.open("GET", theUrl, true);
    xmlRequest.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
    xmlRequest.setRequestHeader("Accept", "application/json");
    xmlRequest.send(null);
}
$(document).ready(function() {    
    var channels = ["freecodecamp", "storbeck", "terakilobyte", "habathcx","RobotCaleb","thomasballinger","noobs2ninjas","beohoff"];
    var urls = channels.map((x) => "https://api.twitch.tv/kraken/channels/" + x);
    var data = [];
    urls_count = urls.length;
    var onComplete = function(){
        //your code after all ajax completes.
    }
    (function(urls, data) {
        urls.forEach(function(url) {  
            function(resolve, reject) {
                httpGet(url, function(response) {
                    data.push(JSON.parse(response));
                }, onComplete)
            };
        })
    })(urls, data);
})