How do I return the response from an asynchronous call?

I have a function foo which makes an Ajax request. How can I return the response from foo?

foo

foo

I tried returning the value from the success callback as well as assigning the response to a local variable inside the function and returning that one, but none of those ways actually return the response.

success

function foo() {
var result;

$.ajax({
url: '...',
success: function(response) {
result = response;
// return response; // <- I tried that one as well
}
});

return result;
}

var result = foo(); // It always ends up being `undefined`.


$http

$.ajax

async: false

31 Answers
31

-> For a more general explanation of async behaviour with different examples, please see Why is my variable unaltered after I modify it inside of a function? - Asynchronous code reference

->

-> If you already understand the problem, skip to the possible solutions below.

->

The problem

The A in Ajax stands for asynchronous . That means sending the request (or rather receiving the response) is taken out of the normal execution flow. In your example, $.ajax returns immediately and the next statement, return result;, is executed before the function you passed as success callback was even called.

$.ajax

return result;

success

Here is an analogy which hopefully makes the difference between synchronous and asynchronous flow clearer:

Imagine you make a phone call to a friend and ask him to look something up for you. Although it might take a while, you wait on the phone and stare into space, until your friend gives you the answer that you needed.

The same is happening when you make a function call containing "normal" code:

function findItem() {
var item;
while(item_not_found) {
// search
}
return item;
}

var item = findItem();

// Do something with item
doSomethingElse();


Even though findItem might take a long time to execute, any code coming after var item = findItem(); has to wait until the function returns the result.

findItem

var item = findItem();

You call your friend again for the same reason. But this time you tell him that you are in a hurry and he should call you back on your mobile phone. You hang up, leave the house and do whatever you planned to do. Once your friend calls you back, you are dealing with the information he gave to you.

That's exactly what's happening when you do an Ajax request.

findItem(function(item) {
// Do something with item
});
doSomethingElse();


Instead of waiting for the response, the execution continues immediately and the statement after the Ajax call is executed. To get the response eventually, you provide a function to be called once the response was received, a callback (notice something? call back ?). Any statement coming after that call is executed before the callback is called.

Solution(s)

Embrace the asynchronous nature of JavaScript! While certain asynchronous operations provide synchronous counterparts (so does "Ajax"), it's generally discouraged to use them, especially in a browser context.

Why is it bad do you ask?

JavaScript runs in the UI thread of the browser and any long running process will lock the UI, making it unresponsive. Additionally, there is an upper limit on the execution time for JavaScript and the browser will ask the user whether to continue the execution or not.

All of this is really bad user experience. The user won't be able to tell whether everything is working fine or not. Furthermore, the effect will be worse for users with a slow connection.

In the following we will look at three different solutions that are all building on top of each other:

async/await

then()

All three are available in current browsers, and node 7+.

async/await

The new ECMAScript version released in 2017 introduced syntax-level support for asynchronous functions. With the help of async and await, you can write asynchronous in a "synchronous style". Make no mistake though: The code is still asynchronous, but it's easier to read/understand.

async

await

async/await builds on top of promises: an async function always returns a promise. await "unwraps" a promise and either result in the value the promise was resolved with or throws an error if the promise was rejected.

async/await

async

await

Important: You can only use await inside an async function. That means that at the very top level, you still have to work directly with the promise.

await

async

You can read more about async and await on MDN.

async

await

Here is an example that builds on top of delay above:

// Using 'superagent' which will return a promise.
var superagent = require('superagent')

// This is isn't declared as `async` because it already returns a promise
function delay() {
// `delay` returns a promise
return new Promise(function(resolve, reject) {
// Only `delay` is able to resolve or reject the promise
setTimeout(function() {
resolve(42); // After 3 seconds, resolve the promise with value 42
}, 3000);
});
}


async function getAllBooks() {
try {
// GET a list of book IDs of the current user
var bookIDs = await superagent.get('/user/books');
// wait for a second (just for the sake of this example)
await delay(1000);
// GET information about each book
return await superagent.get('/books/ids='+JSON.stringify(bookIDs));
} catch(error) {
// If any of the awaited promises was rejected, this catch block
// would catch the rejection reason
return null;
}
}

// Async functions always return a promise
getAllBooks()
.then(function(books) {
console.log(books);
});


Newer browser and node versions support async/await. You can also support older environments by transforming your code to ES5 with the help of regenerator (or tools that use regenerator, such as Babel).

async/await

A callback is simply a function passed to another function. That other function can call the function passed whenever it is ready. In the context of an asynchronous process, the callback will be called whenever the asynchronous process is done. Usually, the result is passed to the callback.

In the example of the question, you can make foo accept a callback and use it as success callback. So this

foo

success

var result = foo();
// Code that depends on 'result'


becomes

foo(function(result) {
// Code that depends on 'result'
});


Here we defined the function "inline" but you can pass any function reference:

function myCallback(result) {
// Code that depends on 'result'
}

foo(myCallback);


foo itself is defined as follows:

foo

function foo(callback) {
$.ajax({
// ...
success: callback
});
}


callback will refer to the function we pass to foo when we call it and we simply pass it on to success. I.e. once the Ajax request is successful, $.ajax will call callback and pass the response to the callback (which can be referred to with result, since this is how we defined the callback).

callback

foo

success

$.ajax

callback

result

You can also process the response before passing it to the callback:

function foo(callback) {
$.ajax({
// ...
success: function(response) {
// For example, filter the response
callback(filtered_response);
}
});
}


It's easier to write code using callbacks than it may seem. After all, JavaScript in the browser is heavily event-driven (DOM events). Receiving the Ajax response is nothing else but an event.
Difficulties could arise when you have to work with third-party code, but most problems can be solved by just thinking through the application flow.

The Promise API is a new feature of ECMAScript 6 (ES2015), but it has good browser support already. There are also many libraries which implement the standard Promises API and provide additional methods to ease the use and composition of asynchronous functions (e.g. bluebird).

Promises are containers for future values. When the promise receives the value (it is resolved) or when it is cancelled (rejected), it notifies all of its "listeners" who want to access this value.

The advantage over plain callbacks is that they allow you to decouple your code and they are easier to compose.

Here is a simple example of using a promise:

function delay() {
// `delay` returns a promise
return new Promise(function(resolve, reject) {
// Only `delay` is able to resolve or reject the promise
setTimeout(function() {
resolve(42); // After 3 seconds, resolve the promise with value 42
}, 3000);
});
}

delay()
.then(function(v) { // `delay` returns a promise
console.log(v); // Log the value once it is resolved
})
.catch(function(v) {
// Or do something else if it is rejected
// (it would not happen in this example, since `reject` is not called).
});


Applied to our Ajax call we could use promises like this:

function ajax(url) {
return new Promise(function(resolve, reject) {
var xhr = new XMLHttpRequest();
xhr.onload = function() {
resolve(this.responseText);
};
xhr.onerror = reject;
xhr.open('GET', url);
xhr.send();
});
}

ajax("/echo/json")
.then(function(result) {
// Code depending on result
})
.catch(function() {
// An error occurred
});


Describing all the advantages that promise offer is beyond the scope of this answer, but if you write new code, you should seriously consider them. They provide a great abstraction and separation of your code.

More information about promises: HTML5 rocks - JavaScript Promises

Deferred objects are jQuery's custom implementation of promises (before the Promise API was standardized). They behave almost like promises but expose a slightly different API.

Every Ajax method of jQuery already returns a "deferred object" (actually a promise of a deferred object) which you can just return from your function:

function ajax() {
return $.ajax(...);
}

ajax().done(function(result) {
// Code depending on result
}).fail(function() {
// An error occurred
});


Keep in mind that promises and deferred objects are just containers for a future value, they are not the value itself. For example, suppose you had the following:

function checkPassword() {
return $.ajax({
url: '/password',
data: {
username: $('#username').val(),
password: $('#password').val()
},
type: 'POST',
dataType: 'json'
});
}

if (checkPassword()) {
// Tell the user they're logged in
}


This code misunderstands the above asynchrony issues. Specifically, $.ajax() doesn't freeze the code while it checks the '/password' page on your server - it sends a request to the server and while it waits, immediately returns a jQuery Ajax Deferred object, not the response from the server. That means the if statement is going to always get this Deferred object, treat it as true, and proceed as though the user is logged in. Not good.

$.ajax()

if

true

But the fix is easy:

checkPassword()
.done(function(r) {
if (r) {
// Tell the user they're logged in
} else {
// Tell the user their password was bad
}
})
.fail(function(x) {
// Tell the user something bad happened
});


As I mentioned, some(!) asynchronous operations have synchronous counterparts. I don't advocate their use, but for completeness' sake, here is how you would perform a synchronous call:

If you directly use a XMLHTTPRequest object, pass false as third argument to .open.

XMLHTTPRequest

false

.open

If you use jQuery, you can set the async option to false. Note that this option is deprecated since jQuery 1.8.
You can then either still use a success callback or access the responseText property of the jqXHR object:

async

false

success

responseText

function foo() {
var jqXHR = $.ajax({
//...
async: false
});
return jqXHR.responseText;
}


If you use any other jQuery Ajax method, such as $.get, $.getJSON, etc., you have to change it to $.ajax (since you can only pass configuration parameters to $.ajax).

$.get

$.getJSON

$.ajax

$.ajax

Heads up! It is not possible to make a synchronous JSONP request. JSONP by its very nature is always asynchronous (one more reason to not even consider this option).

If you use any other jQuery AJAX method, such as $.get, $.getJSON, etc., you have them to $.ajax.

foo

foo(function(result) {....});

result

callback

success

$.ajax

callback

$.getJSON

If you're not using jQuery in your code, this answer is for you

Your code should be something along the lines of this:

function foo() {
var httpRequest = new XMLHttpRequest();
httpRequest.open('GET', "/echo/json");
httpRequest.send();
return httpRequest.responseText;
}

var result = foo(); // always ends up being 'undefined'


Felix Kling did a fine job writing an answer for people using jQuery for AJAX, I've decided to provide an alternative for people who aren't.

(Note, for those using the new fetch API, Angular or promises I've added another answer below)

fetch

What you're facing

This is a short summary of "Explanation of the problem" from the other answer, if you're not sure after reading this, read that.

The A in AJAX stands for asynchronous. That means sending the request (or rather receiving the response) is taken out of the normal execution flow. In your example, .send returns immediately and the next statement, return result;, is executed before the function you passed as success callback was even called.

.send

return result;

success

This means when you're returning, the listener you've defined did not execute yet, which means the value you're returning has not been defined.

Here is a simple analogy

function getFive(){
var a;
setTimeout(function(){
a=5;
},10);
return a;
}


(Fiddle)

The value of a returned is undefined since the a=5 part has not executed yet. AJAX acts like this, you're returning the value before the server got the chance to tell your browser what that value is.

a

undefined

a=5

One possible solution to this problem is to code re-actively , telling your program what to do when the calculation completed.

function onComplete(a){ // When the code completes, do this
alert(a);
}

function getFive(whenDone){
var a;
setTimeout(function(){
a=5;
whenDone(a);
},10);
}


This is called CPS. Basically, we're passing getFive an action to perform when it completes, we're telling our code how to react when an event completes (like our AJAX call, or in this case the timeout).

getFive

Usage would be:

getFive(onComplete);


Which should alert "5" to the screen. (Fiddle).

Possible solutions

There are basically two ways how to solve this:

As for synchronous AJAX, don't do it! Felix's answer raises some compelling arguments about why it's a bad idea. To sum it up, it'll freeze the user's browser until the server returns the response and create a very bad user experience. Here is another short summary taken from MDN on why:

XMLHttpRequest supports both synchronous and asynchronous communications. In general, however, asynchronous requests should be preferred to synchronous requests for performance reasons.

In short, synchronous requests block the execution of code... ...this can cause serious issues...

If you have to do it, you can pass a flag: Here is how:

var request = new XMLHttpRequest();
request.open('GET', 'yourURL', false); // `false` makes the request synchronous
request.send(null);

if (request.status === 200) {// That's HTTP for 'ok'
console.log(request.responseText);
}


Let your function accept a callback. In the example code foo can be made to accept a callback. We'll be telling our code how to react when foo completes.

foo

foo

So:

var result = foo();
// code that depends on `result` goes here


Becomes:

foo(function(result) {
// code that depends on `result`
});


Here we passed an anonymous function, but we could just as easily pass a reference to an existing function, making it look like:

function myHandler(result) {
// code that depends on `result`
}
foo(myHandler);


For more details on how this sort of callback design is done, check Felix's answer.

Now, let's define foo itself to act accordingly

function foo(callback) {
var httpRequest = new XMLHttpRequest();
httpRequest.onload = function(){ // when the request is loaded
callback(httpRequest.responseText);// we're calling our method
};
httpRequest.open('GET', "/echo/json");
httpRequest.send();
}


(fiddle)

We have now made our foo function accept an action to run when the AJAX completes successfully, we can extend this further by checking if the response status is not 200 and acting accordingly (create a fail handler and such). Effectively solving our issue.

If you're still having a hard time understanding this read the AJAX getting started guide at MDN.

onload

readyState

4

result

XMLHttpRequest 2 (first of all read the answers from Benjamin Gruenbaum & Felix Kling)

If you don't use jQuery and want a nice short XMLHttpRequest 2 which works on the modern browsers and also on the mobile browsers I suggest to use it this way:

function ajax(a, b, c){ // URL, callback, just a placeholder
c = new XMLHttpRequest;
c.open('GET', a);
c.onload = b;
c.send()
}


As you can see:

There are two ways to get the response of this Ajax call (three using the XMLHttpRequest var name):

The simplest:

this.response


Or if for some reason you bind() the callback to a class:

bind()

e.target.response


Example:

function callback(e){
console.log(this.response);
}
ajax('URL', callback);


Or (the above one is better anonymous functions are always a problem):

ajax('URL', function(e){console.log(this.response)});


Nothing easier.

Now some people will probably say that it's better to use onreadystatechange or the even the XMLHttpRequest variable name. That's wrong.

Check out XMLHttpRequest advanced features

It supported on all *modern browsers. And I can confirm as I'm using this approach since XMLHttpRequest 2 exists. I never had any type of problem on all browsers I use.

onreadystatechange is only useful if you want to get the headers on state 2.

Using the XMLHttpRequest variable name is another big error as you need to execute the callback inside the onload/oreadystatechange closures else you lost it.

XMLHttpRequest

Now if you want something more complex using post and FormData you can easily extend this function:

function x(a, b, e, d, c){ // URL, callback, method, formdata or {key:val},placeholder
c = new XMLHttpRequest;
c.open(e||'get', a);
c.onload = b;
c.send(d||null)
}


Again ... it's a very short function, but it does get & post.

Examples of usage:

x(url, callback); // By default it's get so no need to set
x(url, callback, 'post', {'key': 'val'}); // No need to set post data


Or pass a full form element (document.getElementsByTagName('form')[0]):

document.getElementsByTagName('form')[0]

var fd = new FormData(form);
x(url, callback, 'post', fd);


Or set some custom values:

var fd = new FormData();
fd.append('key', 'val')
x(url, callback, 'post', fd);


As you can see I didn't implement sync... it's a bad thing.

Having said that ... why don't do it the easy way?

As mentioned in the comment the use of error && synchronous does completely break the point of the answer. Which is a nice short way to use Ajax in the proper way?

Error handler

function x(a, b, e, d, c){ // URL, callback, method, formdata or {key:val}, placeholder
c = new XMLHttpRequest;
c.open(e||'get', a);
c.onload = b;
c.onerror = error;
c.send(d||null)
}

function error(e){
console.log('--Error--', this.type);
console.log('this: ', this);
console.log('Event: ', e)
}
function displayAjax(e){
console.log(e, this);
}
x('WRONGURL', displayAjax);


In the above script, you have an error handler which is statically defined so it does not compromise the function. The error handler can be used for other functions too.

But to really get out an error the only way is to write a wrong URL in which case every browser throws an error.

Error handlers are maybe useful if you set custom headers, set the responseType to blob array buffer or whatever....

Even if you pass 'POSTAPAPAP' as the method it won't throw an error.

Even if you pass 'fdggdgilfdghfldj' as formdata it won't throw an error.

In the first case the error is inside the displayAjax() under this.statusText as Method not Allowed.

displayAjax()

this.statusText

Method not Allowed

In the second case, it simply works. You have to check at the server side if you passed the right post data.

cross-domain not allowed throws error automatically.

In the error response, there are no error codes.

There is only the this.type which is set to error.

this.type

Why add an error handler if you totally have no control over errors?
Most of the errors are returned inside this in the callback function displayAjax().

displayAjax()

So: No need for error checks if you're able to copy and paste the URL properly. ;)

PS: As the first test I wrote x('x', displayAjax)..., and it totally got a response...??? So I checked the folder where the HTML is located, and there was a file called 'x.xml'. So even if you forget the extension of your file XMLHttpRequest 2 WILL FIND IT. I LOL'd

Read a file synchronous

Don't do that.

If you want to block the browser for a while load a nice big txt file synchronous.

function omg(a, c){ // URL
c = new XMLHttpRequest;
c.open('GET', a, true);
c.send();
return c; // Or c.response
}


Now you can do

var res = omg('thisIsGonnaBlockThePage.txt');


There is no other way to do this in a non-asynchronous way. (Yeah, with setTimeout loop... but seriously?)

Another point is... if you work with APIs or just you own list's files or whatever you always use different functions for each request...

Only if you have a page where you load always the same XML/JSON or whatever you need only one function. In that case, modify a little the Ajax function and replace b with your special function.

The functions above are for basic use.

If you want to EXTEND the function...

Yes, you can.

I'm using a lot of APIs and one of the first functions I integrate into every HTML page is the first Ajax function in this answer, with GET only...

But you can do a lot of stuff with XMLHttpRequest 2:

I made a download manager (using ranges on both sides with resume, filereader, filesystem), various image resizers converters using canvas, populate websql databases with base64images and much more... But in these cases you should create a function only for that purpose... sometimes you need a blob, array buffers, you can set headers, override mimetype and there is a lot more...

But the question here is how to return an Ajax response... (I added an easy way.)

x

ajax

xhr

var res = x("url")

c

error

2.ajax is meant to be async.. so NO var res=x('url')..

If you're using promises, this answer is for you.

This means AngularJS, jQuery (with deferred), native XHR's replacement (fetch), EmberJS, BackboneJS's save or any node library that returns promises.

Your code should be something along the lines of this:

function foo() {
var data;
// or $.get(...).then, or request(...).then, or query(...).then
fetch("/echo/json").then(function(response){
data = response.json();
});
return data;
}

var result = foo(); // result is always undefined no matter what.


Felix Kling did a fine job writing an answer for people using jQuery with callbacks for AJAX. I have an answer for native XHR. This answer is for generic usage of promises either on the frontend or backend.

The core issue

The JavaScript concurrency model in the browser and on the server with NodeJS/io.js is asynchronous and reactive.

Whenever you call a method that returns a promise, the then handlers are always executed asynchronously - that is, after the code below them that is not in a .then handler.

then

.then

This means when you're returning data the then handler you've defined did not execute yet. This in turn means that the value you're returning has not been set to the correct value in time.

data

then

Here is a simple analogy for the issue:

function getFive(){
var data;
setTimeout(function(){ // set a timer for one second in the future
data = 5; // after a second, do this
}, 1000);
return data;
}
document.body.innerHTML = getFive(); // `undefined` here and not 5

The value of data is undefined since the data = 5 part has not executed yet. It will likely execute in a second but by that time it is irrelevant to the returned value.

data

undefined

data = 5

Since the operation did not happen yet (AJAX, server call, IO, timer) you're returning the value before the request got the chance to tell your code what that value is.

One possible solution to this problem is to code re-actively , telling your program what to do when the calculation completed. Promises actively enable this by being temporal (time-sensitive) in nature.

Quick recap on promises

A Promise is a value over time. Promises have state, they start as pending with no value and can settle to:

A promise can only change states once after which it will always stay at the same state forever. You can attach then handlers to promises to extract their value and handle errors. then handlers allow chaining of calls. Promises are created by using APIs that return them. For example, the more modern AJAX replacement fetch or jQuery's $.get return promises.

then

then

fetch

$.get

When we call .then on a promise and return something from it - we get a promise for the processed value. If we return another promise we'll get amazing things, but let's hold our horses.

.then

With promises

Let's see how we can solve the above issue with promises. First, let's demonstrate our understanding of promise states from above by using the Promise constructor for creating a delay function:

function delay(ms){ // takes amount of milliseconds
// returns a new promise
return new Promise(function(resolve, reject){
setTimeout(function(){ // when the time is up
resolve(); // change the promise to the fulfilled state
}, ms);
});
}


Now, after we converted setTimeout to use promises, we can use then to make it count:

then

function delay(ms){ // takes amount of milliseconds
// returns a new promise
return new Promise(function(resolve, reject){
setTimeout(function(){ // when the time is up
resolve(); // change the promise to the fulfilled state
}, ms);
});
}

function getFive(){
// we're RETURNING the promise, remember, a promise is a wrapper over our value
return delay(100).then(function(){ // when the promise is ready
return 5; // return the value 5, promises are all about return values
})
}
// we _have_ to wrap it like this in the call site, we can't access the plain value
getFive().then(function(five){
document.body.innerHTML = five;
});

Basically, instead of returning a value which we can't do because of the concurrency model - we're returning a wrapper for a value that we can unwrap with then. It's like a box you can open with then.

then

then

This stands the same for your original API call, you can:

function foo() {
// RETURN the promise
return fetch("/echo/json").then(function(response){
return response.json(); // process it inside the `then`
});
}

foo().then(function(response){
// access the value inside the `then`
})


So this works just as well. We've learned we can't return values from already asynchronous calls but we can use promises and chain them to perform processing. We now know how to return the response from an asynchronous call.

ES2015 (ES6)

ES6 introduces generators which are functions that can return in the middle and then resume the point they were at. This is typically useful for sequences, for example:

function* foo(){ // notice the star, this is ES6 so new browsers/node/io only
yield 1;
yield 2;
while(true) yield 3;
}


Is a function that returns an iterator over the sequence 1,2,3,3,3,3,.... which can be iterated. While this is interesting on its own and opens room for a lot of possibility there is one particular interesting case.

1,2,3,3,3,3,....

If the sequence we're producing is a sequence of actions rather than numbers - we can pause the function whenever an action is yielded and wait for it before we resume the function. So instead of a sequence of numbers, we need a sequence of future values - that is: promises.

This somewhat tricky but very powerful trick lets us write asynchronous code in a synchronous manner. There are several "runners" that do this for you, writing one is a short few lines of code but is beyond the scope of this answer. I'll be using Bluebird's Promise.coroutine here, but there are other wrappers like co or Q.async.

Promise.coroutine

co

Q.async

var foo = coroutine(function*(){
var data = yield fetch("/echo/json"); // notice the yield
// code here only executes _after_ the request is done
return data.json(); // data is defined
});


This method returns a promise itself, which we can consume from other coroutines. For example:

var main = coroutine(function*(){
var bar = yield foo(); // wait our earlier coroutine, it returns a promise
// server call done here, code below executes when done
var baz = yield fetch("/api/users/"+bar.userid); // depends on foo's result
console.log(baz); // runs after both requests done
});
main();


ES2016 (ES7)

In ES7, this is further standardized, there are several proposals right now but in all of them you can await promise. This is just "sugar" (nicer syntax) for the ES6 proposal above by adding the async and await keywords. Making the above example:

await

async

await

async function foo(){
var data = await fetch("/echo/json"); // notice the await
// code here only executes _after_ the request is done
return data.json(); // data is defined
}


It still returns a promise just the same :)

You are using Ajax incorrectly. The idea is not to have it return anything, but instead hand off the data to something called a callback function, which handles the data.

That is:

function handleData( responseData ) {

// Do what you want with the data
console.log(responseData);
}

$.ajax({
url: "hi.php",
...
success: function ( data, status, XHR ) {
handleData(data);
}
});


Returning anything in the submit handler will not do anything. You must instead either hand off the data, or do what you want with it directly inside the success function.

success: handleData

success

$.ajax

The simplest solution is create a JavaScript function and call it for the Ajax success callback.

success

function callServerAsync(){
$.ajax({
url: '...',
success: function(response) {

successCallback(response);
}
});
}

function successCallback(responseObj){
// Do something like read the response and show data
alert(JSON.stringify(responseObj)); // Only applicable to JSON response
}

function foo(callback) {

$.ajax({
url: '...',
success: function(response) {
return callback(null, response);
}
});
}

var result = foo(function(err, result){
if (!err)
console.log(result);
});


I will answer with a horrible-looking, hand-drawn comic. The second image is the reason why result is undefined in your code example.

result

undefined

For people who are using AngularJS, can handle this situation using Promises.

Promises

Here it says,

Promises can be used to unnest asynchronous functions and allows one to chain multiple functions together.

You can find a nice explanation here also.

Example found in docs mentioned below.

promiseB = promiseA.then(
function onSuccess(result) {
return result + 1;
}
,function onError(err) {
//Handle error
}
);

// promiseB will be resolved immediately after promiseA is resolved
// and its value will be the result of promiseA incremented by 1.


In Angular2 with look at the following example, but its recommended to use Observables with Angular2.

Angular2

Observables

Angular2

search(term: string) {
return this.http
.get(`https://api.spotify.com/v1/search?q=${term}&type=artist`)
.map((response) => response.json())
.toPromise();


}

You can consume that in this way,

search() {
this.searchService.search(this.searchField.value)
.then((result) => {
this.result = result.artists.items;
})
.catch((error) => console.error(error));
}


See the original post here. But Typescript does not support native es6 Promises, if you want to use it, you might need plugin for that.

Additionally here is the promises spec define here.

Most of the answers here give useful suggestions for when you have a single async operation, but sometimes, this comes up when you need to do an asynchronous operation for each entry in an array or other list-like structure. The temptation is to do this:

// WRONG
var results = ;
theArray.forEach(function(entry) {
doSomethingAsync(entry, function(result) {
results.push(result);
});
});
console.log(results); // E.g., using them, returning them, etc.


Example:

// WRONG
var theArray = [1, 2, 3];
var results = ;
theArray.forEach(function(entry) {
doSomethingAsync(entry, function(result) {
results.push(result);
});
});
console.log("Results:", results); // E.g., using them, returning them, etc.

function doSomethingAsync(value, callback) {
console.log("Starting async operation for " + value);
setTimeout(function() {
console.log("Completing async operation for " + value);
callback(value * 2);
}, Math.floor(Math.random() * 200));
}

.as-console-wrapper {
max-height: 100% !important;
}

The reason that doesn't work is that the callbacks from doSomethingAsync haven't run yet by the time you're trying to use the results.

doSomethingAsync

So, if you have an array (or list of some kind) and want to do async operations for each entry, you have two options: Do the operations in parallel (overlapping), or in series (one after another in sequence).

You can start all of them and keep track of how many callbacks you're expecting, and then use the results when you've gotten that many callbacks:

var results = ;
var expecting = theArray.length;
theArray.forEach(function(entry, index) {
doSomethingAsync(entry, function(result) {
results[index] = result;
if (--expecting === 0) {
// Done!
console.log("Results:", results); // E.g., using the results
}
});
});


Example:

var theArray = [1, 2, 3];
var results = ;
var expecting = theArray.length;
theArray.forEach(function(entry, index) {
doSomethingAsync(entry, function(result) {
results[index] = result;
if (--expecting === 0) {
// Done!
console.log("Results:", results); // E.g., using the results
}
});
});

function doSomethingAsync(value, callback) {
console.log("Starting async operation for " + value);
setTimeout(function() {
console.log("Completing async operation for " + value);
callback(value * 2);
}, Math.floor(Math.random() * 200));
}

.as-console-wrapper {
max-height: 100% !important;
}

(We could do away with expecting and just use results.length === theArray.length, but that leaves us open to the possibility that theArray is changed while the calls are outstanding...)

expecting

results.length === theArray.length

theArray

Notice how we use the index from forEach to save the result in results in the same position as the entry it relates to, even if the results arrive out of order (since async calls don't necessarily complete in the order in which they were started).

index

forEach

results

But what if you need to return those results from a function? As the other answers have pointed out, you can't; you have to have your function accept and call a callback (or return a Promise). Here's a callback version:

function doSomethingWith(theArray, callback) {
var results = ;
var expecting = theArray.length;
theArray.forEach(function(entry, index) {
doSomethingAsync(entry, function(result) {
results[index] = result;
if (--expecting === 0) {
// Done!
callback(results);
}
});
});
}
doSomethingWith(theArray, function(results) {
console.log("Results:", results);
});


Example:

function doSomethingWith(theArray, callback) {
var results = ;
var expecting = theArray.length;
theArray.forEach(function(entry, index) {
doSomethingAsync(entry, function(result) {
results[index] = result;
if (--expecting === 0) {
// Done!
callback(results);
}
});
});
}
doSomethingWith([1, 2, 3], function(results) {
console.log("Results:", results);
});

function doSomethingAsync(value, callback) {
console.log("Starting async operation for " + value);
setTimeout(function() {
console.log("Completing async operation for " + value);
callback(value * 2);
}, Math.floor(Math.random() * 200));
}

.as-console-wrapper {
max-height: 100% !important;
}

Or here's a version returning a Promise instead:

Promise

function doSomethingWith(theArray) {
return new Promise(function(resolve) {
var results = ;
var expecting = theArray.length;
theArray.forEach(function(entry, index) {
doSomethingAsync(entry, function(result) {
results[index] = result;
if (--expecting === 0) {
// Done!
resolve(results);
}
});
});
});
}
doSomethingWith(theArray).then(function(results) {
console.log("Results:", results);
});


Of course, if doSomethingAsync passed us errors, we'd use reject to reject the promise when we got an error.)

doSomethingAsync

reject

Example:

function doSomethingWith(theArray) {
return new Promise(function(resolve) {
var results = ;
var expecting = theArray.length;
theArray.forEach(function(entry, index) {
doSomethingAsync(entry, function(result) {
results[index] = result;
if (--expecting === 0) {
// Done!
resolve(results);
}
});
});
});
}
doSomethingWith([1, 2, 3]).then(function(results) {
console.log("Results:", results);
});

function doSomethingAsync(value, callback) {
console.log("Starting async operation for " + value);
setTimeout(function() {
console.log("Completing async operation for " + value);
callback(value * 2);
}, Math.floor(Math.random() * 200));
}

.as-console-wrapper {
max-height: 100% !important;
}

(Or alternately, you could make a wrapper for doSomethingAsync that returns a promise, and then do the below...)

doSomethingAsync

If doSomethingAsync gives you a Promise, you can use Promise.all:

doSomethingAsync

Promise.all

function doSomethingWith(theArray) {
return Promise.all(theArray.map(function(entry) {
return doSomethingAsync(entry, function(result) {
results.push(result);
});
}));
}
doSomethingWith(theArray).then(function(results) {
console.log("Results:", results);
});


Example:

function doSomethingWith(theArray) {
return Promise.all(theArray.map(function(entry) {
return doSomethingAsync(entry, function(result) {
results.push(result);
});
}));
}
doSomethingWith([1, 2, 3]).then(function(results) {
console.log("Results:", results);
});

function doSomethingAsync(value) {
console.log("Starting async operation for " + value);
return new Promise(function(resolve) {
setTimeout(function() {
console.log("Completing async operation for " + value);
resolve(value * 2);
}, Math.floor(Math.random() * 200));
});
}

.as-console-wrapper {
max-height: 100% !important;
}

Note that Promise.all resolves its promise with an array of the results of all of the promises you give it when they are all resolved, or rejects its promise when the first of the promises you give it rejects.

Promise.all

Suppose you don't want the operations to be in parallel? If you want to run them one after another, you need to wait for each operation to complete before you start the next. Here's an example of a function that does that and calls a callback with the result:

function doSomethingWith(theArray, callback) {
var results = ;
doOne(0);
function doOne(index) {
if (index < theArray.length) {
doSomethingAsync(theArray[index], function(result) {
results.push(result);
doOne(index + 1);
});
} else {
// Done!
callback(results);
}
}
}
doSomethingWith(theArray, function(results) {
console.log("Results:", results);
});


(Since we're doing the work in series, we can just use results.push(result) since we know we won't get results out of order. In the above we could have used results[index] = result;, but in some of the following examples we don't have an index to use.)

results.push(result)

results[index] = result;

Example:

function doSomethingWith(theArray, callback) {
var results = ;
doOne(0);
function doOne(index) {
if (index < theArray.length) {
doSomethingAsync(theArray[index], function(result) {
results.push(result);
doOne(index + 1);
});
} else {
// Done!
callback(results);
}
}
}
doSomethingWith([1, 2, 3], function(results) {
console.log("Results:", results);
});

function doSomethingAsync(value, callback) {
console.log("Starting async operation for " + value);
setTimeout(function() {
console.log("Completing async operation for " + value);
callback(value * 2);
}, Math.floor(Math.random() * 200));
}

.as-console-wrapper {
max-height: 100% !important;
}

(Or, again, build a wrapper for doSomethingAsync that gives you a promise and do the below...)

doSomethingAsync

If doSomethingAsync gives you a Promise, if you can use ES2017+ syntax (perhaps with a transpiler like Babel), you can use an async function with for-of and await:

doSomethingAsync

async

for-of

await

async function doSomethingWith(theArray) {
const results = ;
for (const entry of theArray) {
results.push(await doSomethingAsync(entry));
}
return results;
}
doSomethingWith(theArray).then(results => {
console.log("Results:", results);
});


Example:

async function doSomethingWith(theArray) {
const results = ;
for (const entry of theArray) {
results.push(await doSomethingAsync(entry));
}
return results;
}
doSomethingWith([1, 2, 3]).then(function(results) {
console.log("Results:", results);
});

function doSomethingAsync(value) {
console.log("Starting async operation for " + value);
return new Promise(function(resolve) {
setTimeout(function() {
console.log("Completing async operation for " + value);
resolve(value * 2);
}, Math.floor(Math.random() * 200));
});
}

.as-console-wrapper {
max-height: 100% !important;
}

If you can't use ES2017+ syntax (yet), you can use a variation on the "Promise reduce" pattern (this is more complex than the usual Promise reduce because we're not passing the result from one into the next, but instead gathering up their results in an array):

function doSomethingWith(theArray) {
return theArray.reduce(function(p, entry) {
return p.then(function(results) {
return doSomethingAsync(entry).then(function(result) {
results.push(result);
return results;
});
});
}, Promise.resolve());
}
doSomethingWith(theArray).then(function(results) {
console.log("Results:", results);
});


Example:

function doSomethingWith(theArray) {
return theArray.reduce(function(p, entry) {
return p.then(function(results) {
return doSomethingAsync(entry).then(function(result) {
results.push(result);
return results;
});
});
}, Promise.resolve());
}
doSomethingWith([1, 2, 3]).then(function(results) {
console.log("Results:", results);
});

function doSomethingAsync(value) {
console.log("Starting async operation for " + value);
return new Promise(function(resolve) {
setTimeout(function() {
console.log("Completing async operation for " + value);
resolve(value * 2);
}, Math.floor(Math.random() * 200));
});
}

.as-console-wrapper {
max-height: 100% !important;
}

...which is less cumbersome with ES2015+ arrow functions:

function doSomethingWith(theArray) {
return theArray.reduce((p, entry) => p.then(results => doSomethingAsync(entry).then(result => {
results.push(result);
return results;
})), Promise.resolve());
}
doSomethingWith(theArray).then(results => {
console.log("Results:", results);
});


Example:

function doSomethingWith(theArray) {
return theArray.reduce((p, entry) => p.then(results => doSomethingAsync(entry).then(result => {
results.push(result);
return results;
})), Promise.resolve());
}
doSomethingWith([1, 2, 3]).then(function(results) {
console.log("Results:", results);
});

function doSomethingAsync(value) {
console.log("Starting async operation for " + value);
return new Promise(function(resolve) {
setTimeout(function() {
console.log("Completing async operation for " + value);
resolve(value * 2);
}, Math.floor(Math.random() * 200));
});
}

.as-console-wrapper {
max-height: 100% !important;
}

if (--expecting === 0)

expecting

array.length

if (--expecting === 0)

expecting

Have a look at this example:

var app = angular.module('plunker', );

app.controller('MainCtrl', function($scope,$http) {

var getJoke = function(){
return $http.get('http://api.icndb.com/jokes/random').then(function(res){
return res.data.value;
});
}

getJoke().then(function(res) {
console.log(res.joke);
});
});


As you can see getJoke is returning a resolved promise (it is resolved when returning res.data.value). So you wait until the $http.get request is completed and then console.log(res.joke) is executed (as a normal asynchronous flow).

getJoke

res.data.value

This is the plnkr:

http://embed.plnkr.co/XlNR7HpCaIhJxskMJfSg/

Another approach to return a value from an asynchronous function, is to pass in an object that will store the result from the asynchronous function.

Here is an example of the same:

var async = require("async");

// This wires up result back to the caller
var result = {};
var asyncTasks = ;
asyncTasks.push(function(_callback){
// some asynchronous operation
$.ajax({
url: '...',
success: function(response) {
result.response = response;
_callback();
}
});
});

async.parallel(asyncTasks, function(){
// result is available after performing asynchronous operation
console.log(result)
console.log('Done');
});


I am using the result object to store the value during the asynchronous operation. This allows the result be available even after the asynchronous job.

result

I use this approach a lot. I would be interested to know how well this approach works where wiring the result back through consecutive modules is involved.

result

This is one of the places which two ways data binding that's used in many new JavaScript frameworks will work greatly for you...

So if you are using Angular, React or any other frameworks which do two ways data binding, this issue is simply fixed for you, so in easy word, your result is undefined at the first stage, so you have got result = undefined before you receive the data, then as soon as you get the result, it will updated and get assigned to the new value which is respond of your Ajax call...

undefined

result = undefined

But how you can do it in pure javascript or jQuery for example as you asked in this question?

You can use a callback, promise and recently observable to handle it for you, for example in promises we have some function like success() or then() which will be executed when your data is ready for you, same with callback or subscribe function on observable.

For example in your case which you are using jQuery, you can do something like this:

$(document).ready(function(){
function foo() {
$.ajax({url: "api/data", success: function(data){
fooDone(data); //after we have data, we pass it to fooDone
}});
};

function fooDone(data) {
console.log(data); //fooDone has the data and console.log it
};

foo(); //call happens here
});


For more information study about promises and observables which are newer ways to do this async stuffs.

$.ajax({url: "api/data", success: fooDone.bind(this)});

While promises and callbacks work fine in many situations, it is a pain in the rear to express something like:

if (!name) {
name = async1();
}
async2(name);


You'd end up going through async1; check if name is undefined or not and call the callback accordingly.

async1

name

async1(name, callback) {
if (name)
callback(name)
else {
doSomething(callback)
}
}

async1(name, async2)


While it is okay in small examples it gets annoying when you have a lot of similar cases and error handling involved.

Fibers helps in solving the issue.

Fibers

var Fiber = require('fibers')

function async1(container) {
var current = Fiber.current
var result
doSomething(function(name) {
result = name
fiber.run()
})
Fiber.yield()
return result
}

Fiber(function() {
var name
if (!name) {
name = async1()
}
async2(name)
// Make any number of async calls from here
}


You can checkout the project here.

async-await

Short answer is, you have to implement a callback like this:

function callback(response) {
// Here you can do what ever you want with the response object.
console.log(response);
}

$.ajax({
url: "...",
success: callback
});


The following example I have written shows how to

This working example is self-contained. It will define a simple request object that uses the window XMLHttpRequest object to make calls. It will define a simple function to wait for a bunch of promises to be completed.

XMLHttpRequest

Context. The example is querying the Spotify Web API endpoint in order to search for playlist objects for a given set of query strings:

playlist

[
"search?type=playlist&q=%22doom%20metal%22",
"search?type=playlist&q=Adele"
]


For each item, a new Promise will fire a block - ExecutionBlock, parse the result, schedule a new set of promises based on the result array, that is a list of Spotify user objects and execute the new HTTP call within the ExecutionProfileBlock asynchronously.

ExecutionBlock

user

ExecutionProfileBlock

You can then see a nested Promise structure, that lets you spawn multiple and completely asynchronous nested HTTP calls, and join the results from each subset of calls through Promise.all.

Promise.all

NOTE
Recent Spotify search APIs will require an access token to be specified in the request headers:

search

-H "Authorization: Bearer {your access token}"


So, you to run the following example you need to put your access token in the request headers:

var spotifyAccessToken = "YourSpotifyAccessToken";
var console = {
log: function(s) {
document.getElementById("console").innerHTML += s + "<br/>"
}
}

// Simple XMLHttpRequest
// based on https://davidwalsh.name/xmlhttprequest
SimpleRequest = {
call: function(what, response) {
var request;
if (window.XMLHttpRequest) { // Mozilla, Safari, ...
request = new XMLHttpRequest();
} else if (window.ActiveXObject) { // Internet Explorer
try {
request = new ActiveXObject('Msxml2.XMLHTTP');
}
catch (e) {
try {
request = new ActiveXObject('Microsoft.XMLHTTP');
} catch (e) {}
}
}

// State changes
request.onreadystatechange = function() {
if (request.readyState === 4) { // Done
if (request.status === 200) { // Complete
response(request.responseText)
}
else
response();
}
}
request.open('GET', what, true);
request.setRequestHeader("Authorization", "Bearer " + spotifyAccessToken);
request.send(null);
}
}

//PromiseAll
var promiseAll = function(items, block, done, fail) {
var self = this;
var promises = ,
index = 0;
items.forEach(function(item) {
promises.push(function(item, i) {
return new Promise(function(resolve, reject) {
if (block) {
block.apply(this, [item, index, resolve, reject]);
}
});
}(item, ++index))
});
Promise.all(promises).then(function AcceptHandler(results) {
if (done) done(results);
}, function ErrorHandler(error) {
if (fail) fail(error);
});
}; //promiseAll

// LP: deferred execution block
var ExecutionBlock = function(item, index, resolve, reject) {
var url = "https://api.spotify.com/v1/"
url += item;
console.log( url )
SimpleRequest.call(url, function(result) {
if (result) {

var profileUrls = JSON.parse(result).playlists.items.map(function(item, index) {
return item.owner.href;
})
resolve(profileUrls);
}
else {
reject(new Error("call error"));
}
})
}

arr = [
"search?type=playlist&q=%22doom%20metal%22",
"search?type=playlist&q=Adele"
]

promiseAll(arr, function(item, index, resolve, reject) {
console.log("Making request [" + index + "]")
ExecutionBlock(item, index, resolve, reject);
}, function(results) { // Aggregated results

console.log("All profiles received " + results.length);
//console.log(JSON.stringify(results[0], null, 2));

///// promiseall again

var ExecutionProfileBlock = function(item, index, resolve, reject) {
SimpleRequest.call(item, function(result) {
if (result) {
var obj = JSON.parse(result);
resolve({
name: obj.display_name,
followers: obj.followers.total,
url: obj.href
});
} //result
})
} //ExecutionProfileBlock

promiseAll(results[0], function(item, index, resolve, reject) {
//console.log("Making request [" + index + "] " + item)
ExecutionProfileBlock(item, index, resolve, reject);
}, function(results) { // aggregated results
console.log("All response received " + results.length);
console.log(JSON.stringify(results, null, 2));
}

, function(error) { // Error
console.log(error);
})

/////

},
function(error) { // Error
console.log(error);
});

<div id="console" />

I have extensively discussed this solution here.

You can use this custom library (written using Promise) to make a remote call.

function $http(apiConfig) {
return new Promise(function (resolve, reject) {
var client = new XMLHttpRequest();
client.open(apiConfig.method, apiConfig.url);
client.send();
client.onload = function () {
if (this.status >= 200 && this.status < 300) {
// Performs the function "resolve" when this.status is equal to 2xx.
// Your logic here.
resolve(this.response);
}
else {
// Performs the function "reject" when this.status is different than 2xx.
reject(this.statusText);
}
};
client.onerror = function () {
reject(this.statusText);
};
});
}


Simple usage example:

$http({
method: 'get',
url: 'google.com'
}).then(function(response) {
console.log(response);
}, function(error) {
console.log(error)
});


Js is a single threaded.

Browser can be divided into three parts:

1)Event Loop

2)Web API

3)Event Queue

Event Loop runs for forever i.e kind of infinite loop.Event Queue is where all your function are pushed on some event(example:click) this is one by one carried out of queue and put into Event loop which execute this function and prepares it self for next one after first one is executed.This means Execution of one function doesn't starts till the function before it in queue is executed in event loop.

Now let us think we pushed two functions in a queue one is for getting a data from server and another utilises that data.We pushed the serverRequest() function in queue first then utiliseData() function. serverRequest function goes in event loop and makes a call to server as we never know how much time it will take to get data from server
so this process is expected to take time and so we busy our event loop thus hanging our page, that's where Web API come into role it take this function from event loop and deals with server making event loop free so that we can execute next function from queue.The next function in queue is utiliseData() which goes in loop but because of no data available it goes waste and execution of next function continues till end of the queue.(This is called Async calling i.e we can do something else till we get data)

Let suppose our serverRequest() function had a return statement in a code, when we get back data from server Web API will push it in queue at the end of queue.
As it get pushed at end in queue we cannot utilise its data as there is no function left in our queue to utilise this data.Thus it is not possible to return something from Async Call.

Thus Solution to this is callback or promise.

A Image from one of the answers here, Correctly explains callback use...
We give our function(function utilising data returned from server) to function calling server.

function doAjax(callbackFunc, method, url) {
var xmlHttpReq = new XMLHttpRequest();
xmlHttpReq.open(method, url);
xmlHttpReq.onreadystatechange = function() {

if (xmlHttpReq.readyState == 4 && xmlHttpReq.status == 200) {
callbackFunc(xmlHttpReq.responseText);
}


}
xmlHttpReq.send(null);

}


In my Code it is called as

function loadMyJson(categoryValue){
if(categoryValue==="veg")
doAjax(print,"GET","http://localhost:3004/vegetables");
else if(categoryValue==="fruits")
doAjax(print,"GET","http://localhost:3004/fruits");
else
console.log("Data not found");
}


Read here for new methods in ECMA(2016/17) for making async call(@Felix Kling Answer on Top)
https://stackoverflow.com/a/14220323/7579856

Another solution is to execute code via sequential executor nsynjs.

nsynjs will evaluate all promises sequentially, and put promise result into data property:

data

function synchronousCode() {

var getURL = function(url) {
return window.fetch(url).data.text().data;
};

var url = 'https://ajax.googleapis.com/ajax/libs/jquery/2.0.0/jquery.min.js';
console.log('received bytes:',getURL(url).length);

};

nsynjs.run(synchronousCode,{},function(){
console.log('synchronousCode done');
});

<script src="https://rawgit.com/amaksr/nsynjs/master/nsynjs.js"></script>

Step 1. Wrap function with callback into nsynjs-aware wrapper (if it has promisified version, you can skip this test):

var ajaxGet = function (ctx,url) {
var res = {};
var ex;
$.ajax(url)
.done(function (data) {
res.data = data;
})
.fail(function(e) {
ex = e;
})
.always(function() {
ctx.resume(ex);
});
return res;
};
ajaxGet.nsynjsHasCallback = true;


Step 2. Put synchronous logic into function:

function process() {
console.log('got data:', ajaxGet(nsynjsCtx, "data/file1.json").data);
}


Step 3. Run function in synchronous manner via nnsynjs:

nsynjs.run(process,this,function () {
console.log("synchronous function finished");
});


Nsynjs will evaluate all operators and expressions step-by-step, pausing execution in case if result of some slow function is not ready.

More examples here: https://github.com/amaksr/nsynjs/tree/master/examples

2017 answer: you can now do exactly what you want in every current browser and node

This is quite simple:

Here's a working version of your code:

(async function(){

var response = await superagent.get('...')
console.log(response)

})()


await is supported in all current browsers and node 8

var App = App || {};

App = {
getDataFromServer: function(){

var self = this,
deferred = $.Deferred(),
requests = ;

requests.push($.getJSON('request/ajax/url/1'));
requests.push($.getJSON('request/ajax/url/2'));

$.when.apply(jQuery, requests).done(function(xhrResponse) {
return deferred.resolve(xhrResponse.result);
});
return deferred;
},

init: function(){

this.getDataFromServer().done(_.bind(function(resp1, resp2) {

// Do the operations which you wanted to do when you
// get a response from Ajax, for example, log response.
}, this));
}
};
App.init();

Use a callback() function inside the foo() success.
Try in this way. It is simple and easy to understand.  

callback()

foo()

var lat = "";
var lon = "";
function callback(data) {
lat = data.lat;
lon = data.lon;
}
function getLoc() {
var url = "http://ip-api.com/json"
$.getJSON(url, function(data) {
callback(data);
});
}

getLoc();


Short answer: Your foo() method returns immediately, while the $ajax() call executes asynchronously after the function returns. The problem is then how or where to store the results retrieved by the async call once it returns.

foo()

$ajax()

Several solutions have been given in this thread. Perhaps the easiest way is to pass an object to the foo() method, and to store the results in a member of that object after the async call completes.

foo()

function foo(result) {
$.ajax({
url: '...',
success: function(response) {
result.response = response; // Store the async result
}
});
}

var result = { response: null }; // Object to hold the async result
foo(result); // Returns before the async completes


Note that the call to foo() will still return nothing useful. However, the result of the async call will now be stored in result.response.

foo()

result.response

It's a very common issue we face while struggling with the 'mysteries' of JavaScript. Let me try demystifying this mystery today.

Let's start with a simple JavaScript function:

function foo(){
// do something
return 'wohoo';
}

let bar = foo(); // bar is 'wohoo' here


That's a simple synchronous function call (where each line of code executed one after other in sequence), and the result is same as expected.

Now let's add a bit of twist, by introducing little delay in our function, so that all lines of code are not executed in sequence. Thus, it will emulate the asynchronous behavior of function :

function foo(){
setTimeout( ()=>{
return 'wohoo';
}, 1000 )
}

let bar = foo() // bar is undefined here


So there you go, that delay just broke the functionality we expected ! But what exactly happened ? Well, it's actually pretty logical if you look at the code. the function foo(), upon execution, returns nothing (thus returned value is undefined), but it does start a timer, which executes a function after 1s to return 'wohoo'. But as you can see, the value that's assigned to bar is the immediately returned stuff from foo(), not anything else that comes later.

foo()

undefined

So, how do we tackle this issue?

Let's ask our function for a PROMISE.
Promise is really about what it means : it means that the function guarantees you to provide with any output it gets in future. so let's see it in action for our little problem above :

function foo(){
return new Promise( (resolve, reject) => { // I want foo() to PROMISE me something
setTimeout ( function(){
// promise is RESOLVED , when exececution reaches this line of code
resolve('wohoo')// After 1 second, RESOLVE the promise with value 'wohoo'
}, 1000 )
})
}

let bar ;
foo().then( res => {
bar = res;
console.log(bar) // will print 'wohoo'
});


Thus, the summary is - to tackle the asynchronous functions like ajax based calls etc., you can use a promise to resolve the value (which you intend to return). Thus, in short you resolve value instead of returning, in asynchronous functions.

resolve

ECMAScript 6 has 'generators' which allow you to easily program in an asynchronous style.

function* myGenerator() {
const callback = yield;
let [response] = yield $.ajax("https://stackoverflow.com", {complete: asyncCallback});
console.log("response is:", response);

// examples of other things you can do
yield setTimeout(asyncCallback, 1000);
console.log("it delayed for 1000ms");
while (response.statusText === "error") {
[response] = yield* anotherGenerator();
}
}


To run the above code you do this:

const gen = myGenerator(); // Create generator
gen.next(); // Start it
gen.next((...args) => gen.next([...args])); // Set its callback function


If you need to target browsers that don't support ES6 you can run the code through Babel or closure-compiler to generate ECMAScript 5.

The callback ...args are wrapped in an array and destructured when you read them so that the pattern can cope with callbacks that have multiple arguments. For example with node fs:

...args

const [err, data] = yield fs.readFile(filePath, "utf-8", callback);


Of course there are many approaches like synchronous request, promise, but from my experience I think you should use the callback approach. It's natural to asynchronous behavior of Javascript.
So, your code snippet can be rewrite a little different:

function foo() {
var result;

$.ajax({
url: '...',
success: function(response) {
myCallback(response);
}
});

return result;
}

function myCallback(response) {
// Does something.
}


The question was:

How do I return the response from an asynchronous call?

which CAN be interpreted as:

How to make asynchronous code look synchronous?

The solution will be to avoid callbacks, and use a combination of Promises and async/await.

I would like to give an example for a Ajax request.

(Although it can be written in Javascript, I prefer to write it in Python, and compile it to Javascript using Transcrypt. It will be clear enough.)

Lets first enable JQuery usage, to have $ available as S:

$

S

__pragma__ ('alias', 'S', '$')


Define a function which returns a Promise, in this case an Ajax call:

def read(url: str):
deferred = S.Deferred()
S.ajax({'type': "POST", 'url': url, 'data': { },
'success': lambda d: deferred.resolve(d),
'error': lambda e: deferred.reject(e)
})
return deferred.promise()


Use the asynchronous code as if it were synchronous:

async def readALot():
try:
result1 = await read("url_1")
result2 = await read("url_2")
except Exception:
console.warn("Reading a lot failed")


We find ourselves in a universe which appears to progress along a dimension we call "time". We don't really understand what time is, but we have developed abstractions and vocabulary that let us reason and talk about it: "past", "present", "future", "before", "after".

The computer systems we build--more and more--have time as an important dimension. Certain things are set up to happen in the future. Then other things need to happen after those first things eventually occur. This is the basic notion called "asynchronicity". In our increasingly networked world, the most common case of asynchonicity is waiting for some remote system to response to some request.

Consider an example. You call the milkman and order some milk. When it comes, you want to put it in your coffee. You can't put the milk in your coffee right now, because it is not here yet. You have to wait for it to come before putting it in your coffee. In other words, the following won't work:

var milk = order_milk();
put_in_coffee(milk);


Because JS has no way to know that it needs to wait for order_milk to finish before it executes put_in_coffee. In other words, it does not know that order_milk is asynchronous--is something that is not going to result in milk until some future time. JS, and other declarative languages, execute one statement after another without waiting.

order_milk

put_in_coffee

order_milk

The classic JS approach to this problem, taking advantage of the fact that JS supports functions as first-class objects which can be passed around, is to pass a function as a parameter to the asynchonous request, which it will then invoke when it has complete its task sometime in the future. That is the "callback" approach. It looks like this:

order_milk(put_in_coffee);


order_milk kicks off, orders the milk, then, when and only when it arrives, it invokes put_in_coffee.

order_milk

put_in_coffee

The problem with this callback approach is that it pollutes the normal semantics of a function reporting its result with return; instead, functions must nost reports their results by calling a callback given as a parameter. Also, this approach can rapidly become unwieldy when dealing with longer sequences of events. For example, let's say that I want to wait for the milk to be put in the coffee, and then and only then perform a third step, namely drinking the coffee. I end up needing to write something like this:

return

order_milk(function(milk) { put_in_coffee(milk, drink_coffee); }


where I am passing to put_in_coffee both the milk to put in it, and also the action (drink_coffee) to execute once the milk has been put in. Such code becomes hard to write, and read, and debug.

put_in_coffee

drink_coffee

In this case, we could rewrite the code in the question as:

var answer;
$.ajax('/foo.json') . done(function(response) {
callback(response.data);
});

function callback(data) {
console.log(data);
}


This was the motivation for the notion of a "promise", which is a particular type of value which represents a future or asynchronous outcome of some sort. It can represent something that already happened, or that is going to happen in the future, or might never happen at all. Promises have a single method, named then, to which you pass an action to be executed when the outcome the promise represents has been realized.

then

In the case of our milk and coffee, we design order_milk to return a promise for the milk arriving, then specify put_in_coffee as a then action, as follows:

order_milk

put_in_coffee

then

order_milk() . then(put_in_coffee)


One advantage of this is that we can string these together to create sequences of future occurrences ("chaining"):

order_milk() . then(put_in_coffee) . then(drink_coffee)


Let's apply promises to your particular problem. We will wrap our request logic inside a function, which returns a promise:

function get_data() {
return $.ajax('/foo.json');
}


Actually, all we've done is added a return to the call to $.ajax. This works because jQuery's $.ajax already returns a kind of promise-like thing. (In practice, without getting into details, we would prefer to wrap this call so as return a real promise, or use some alternative to $.ajax that does so.) Now, if we want to load the file and wait for it to finish and then do something, we can simply say

return

$.ajax

$.ajax

$.ajax

get_data() . then(do_something)


for instance,

get_data() .
then(function(data) { console.log(data); });


When using promises, we end up passing lots of functions into then, so it's often helpful to use the more compact ES6-style arrow functions:

then

get_data() .
then(data => console.log(data));


async

But there's still something vaguely dissatisfying about having to write code one way if synchronous and a quite different way if asynchronous. For synchronous, we write

a();
b();


but if a is asynchronous, with promises we have to write

a

a() . then(b);


Above, we said "JS has no way to know that it needs to wait for the first call to finish before it executes the second". Wouldn't it be nice if there was some way to tell JS that? It turns out that there is--the await keyword, used inside a special type of function called an "async" function. This feature is part of the upcoming version of ES, but is already available in transpilers such as Babel given the right presets. This allows us to simply write

await

async function morning_routine() {
var milk = await order_milk();
var coffee = await put_in_coffee(milk);
await drink(coffee);
}


In your case, you would be able to write something like

async function foo() {
data = await get_data();
console.log(data);
}


Using ES2017 you should have this as the function declaration

async function foo() {
var response = await $.ajax({url: '...'})
return response;
}


And executing it like this.

(async function() {
try {
var result = await foo()
console.log(result)
} catch (e) {}
})()


Or the Promise syntax

foo().then(response => {
console.log(response)

}).catch(error => {
console.log(error)

})


Rather than throwing code at you, there are 2 concepts that are key to understanding how JS handles callbacks and asynchronicity. (is that even a word?)

There are three things you need to be aware of; The queue; the event loop and the stack

In broad, simplistic terms, the event loop is like the project manager, it is constantly listening for any functions that want to run and communicates between the queue and the stack.

while (queue.waitForMessage()) {
queue.processNextMessage();
}


Once it receives a message to run something it adds it to the queue. The queue is the list of things that are waiting to execute (like your AJAX request). imagine it like this:

1. call foo.com/api/bar using foobarFunc
2. Go perform an infinite loop
... and so on


When one of these messages is going to execute it pops the message from the queue and creates a stack, the stack is everything JS needs to execute to perform the instruction in the message. So in our example it's being told to call foobarFunc

foobarFunc

function foobarFunc (var) {
console.log(anotherFunction(var));
}


So anything that foobarFunc needs to execute (in our case anotherFunction) will get pushed onto the stack. executed, and then forgotten about - the event loop will then move onto the next thing in the queue (or listen for messages)

anotherFunction

The key thing here is the order of execution. That is

When you make a call using AJAX to an external party or run any asynchronous code (a setTimeout for example), Javascript is dependant upon a response before it can proceed.

The big question is when will it get the response? The answer is we don't know - so the event loop is waiting for that message to say "hey run me". If JS just waited around for that message synchronously your app would freeze and it will suck. So JS carries on executing the next item in the queue whilst waiting for the message to get added back to the queue.

That's why with asynchronous functionality we use things called callbacks. It's kinda like a promise quite literally. As in I promise to return something at some point jQuery uses specific callbacks called deffered.done deffered.fail and deffered.always (amongst others). You can see them all here

deffered.done

deffered.fail

deffered.always

So what you need to do is pass a function that is promised to execute at some point with data that is passed to it.

Because a callback is not executed immediately but at a later time it's important to pass the reference to the function not it executed. so

function foo(bla) {
console.log(bla)
}


so most of the time (but not always) you'll pass foo not foo()

foo

foo()

Hopefully that will make some sense. When you encounter things like this that seem confusing - i highly recommend reading the documentation fully to at least get an understanding of it. It will make you a much better developer.

Let's see the forest first before looking at the trees.

There are many informative answers with great details here, I won't repeat any of them. The key to programming in JavaScript is having first the correct mental model of overall execution.

The good news is that if you understand this point well, you will never have to worry about race conditions. You should first and foremost thing of how you want to organize your code as essentially the response to different discrete events, and how you want to thread them together into a logical sequence. You can use promises or higher level new async/await as tools to that end, or you can roll your own.

But you shouldn't use any tactical tools to solve a problem until you are comfortable with the actual problem domain. Draw a map of these dependencies to know what needs to run when. Attempting an ad-hoc approach to all these callbacks is just not going to serve you well.

Thank you for your interest in this question.
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