By Felix Kling


2013-01-08 17:06:14 8 Comments

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

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`.

30 comments

@Anish K. 2017-10-31 20:12:45

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 is 'finished with its job' before the next one 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 'finished' 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.

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 execution 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.

UPDATE (Promises with async/await)

Apart from using then/catch to work with promises, there exists one more approach. The idea is to recognize an asynchronous function and then wait for the promises to resolve, before moving to the next line of code. It's still just the promises under the hood, but with a different syntactical approach. To make things clearer, you can find a comparison below:

then/catch version:

function saveUsers(){
     getUsers()
      .then(users => {
         saveSomewhere(users);
      })
      .catch(err => {
         throw err;
       })
 }

async/await version:

  async function saveUsers(){
     try{
        let users = await getUsers()
        saveSomewhere(users);
     }
     catch(err){
        throw err;
     }
  }

@edwardsmarkf 2018-09-26 20:58:21

is this still considered the best way to return a value from a promise or async/await?

@Anish K. 2018-10-03 16:12:28

@edwardsmarkf Personally I don't think there's a best way as such. I use promises with then/catch , async/await as well as generators for async portions of my code. It largely depends on the context of usage.

@Alireza 2017-05-24 09:38:10

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

So if you are using Angular, React or any other frameworks which do two ways data binding or store concept 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 be updated and get assigned to the new value which response of your Ajax call...

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.

@steve.sims 2017-07-24 06:14:12

This is fine at global scope, but in some module context you probably want to ensure right context for the callback e.g. $.ajax({url: "api/data", success: fooDone.bind(this)});

@Matthew Brent 2018-05-04 15:57:05

This is actually incorrect as React is one-way data binding

@Alireza 2018-05-14 07:34:28

@MatthewBrent you are not wrong, but not right also, React props are object and if changed, they change throughout the application, but its not a way that React developer recommend to use it...

@nopole 2019-09-10 07:25:53

I think no matter what method or mechanism used, or whatever the framework is (Angular/React) that hides it from you, the following principle holds:

  1. In the flow of the program (think code or even the lowest level: machine code), the data may not arrive back 2 seconds later, 3 seconds later, or may not arrive at all, so there is no usual return to use in order to return the data.

  2. It is the classic "observer pattern". (It can be in the form of a "callback".) It is: "hey, I am interested in knowing a successful arrival of data; would you let me know when it does." So you register an observer to be notified (or a function to be called to notify about the successful arrival of the data.) You also usually register an observer for the failure of arrival of such data.

  3. When it is successful arrival of data, or a failure of the return of such data, the registered observers (or callbacks) are notified together with the data (or called with the data). If the observer is registered in the form of a callback function foo, then foo(data) will be called. If the observer is registered in the form of an object foo, then depending on the interface, it could be that foo.notify(data) is called.

@Kamil Kiełczewski 2019-06-11 07:56:10

Request works in ansynchronous way so you can't read data in synchronous way as typical code. However using async/await you can create async code which looks close/similar to synch code. Code which process request data needs to be wrapped by async function (load in below snippet) and inside it you need to add await keywort before foo() (which also use async/await).

async function foo() {
  var url= 'https://jsonplaceholder.typicode.com/todos/1';
  var result= await (await fetch(url)).text(); // or .json()
  return result;
}

async function load() {
  var result = await foo();
  console.log(result);
}

load();

@Amir Forsati 2018-12-07 14:10:23

Using Promise

The most perfect answer to this question is using Promise.

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

Usage

ajax("GET", "/test", "acrive=1").then(function(result) {
    // Code depending on result
})
.catch(function() {
    // An error occurred
});

But wait...!

There is a problem with using promises!

Why should we use our own custom Promise?

I was using this solution for a while until I figured out there is an error in old browsers:

Uncaught ReferenceError: Promise is not defined

So i decided to implement my own Promise class for ES3 to below js compilers if its not defined. Just add this code before your main code and then safely use Promise!

if(typeof Promise === "undefined"){
    function _classCallCheck(instance, Constructor) {
        if (!(instance instanceof Constructor)) { 
            throw new TypeError("Cannot call a class as a function"); 
        }
    }
    var Promise = function () {
        function Promise(main) {
            var _this = this;
            _classCallCheck(this, Promise);
            this.value = undefined;
            this.callbacks = [];
            var resolve = function resolve(resolveValue) {
                _this.value = resolveValue;
                _this.triggerCallbacks();
            };
            var reject = function reject(rejectValue) {
                _this.value = rejectValue;
                _this.triggerCallbacks();
            };
            main(resolve, reject);
        }
        Promise.prototype.then = function then(cb) {
            var _this2 = this;
            var next = new Promise(function (resolve) {
                _this2.callbacks.push(function (x) {
                    return resolve(cb(x));
                });
            });
            return next;
        };
        Promise.prototype.catch = function catch_(cb) {
            var _this2 = this;
            var next = new Promise(function (reject) {
                _this2.callbacks.push(function (x) {
                    return reject(cb(x));
                });
            });
            return next;
        };
        Promise.prototype.triggerCallbacks = function triggerCallbacks() {
            var _this3 = this;
            this.callbacks.forEach(function (cb) {
                cb(_this3.value);
            });
        };
        return Promise;
    }();
}

@T.J. Crowder 2017-05-03 16:59:06

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.

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).

Parallel

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...)

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).

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:

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.)

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...)

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

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

If you know that doSomethingAsync will ignore a second and third argument, you can just pass it directly to map (map calls its callback with three arguments, but most people only use the first most of the time):

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

Example:

function doSomethingWith(theArray) {
    return Promise.all(theArray.map(doSomethingAsync));
}
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.

Series

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.)

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...)

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:

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;
}

@Sarah 2017-05-28 10:21:35

Could you explain how the if (--expecting === 0) part of the code works please? The callback version of your solution is working great for me, I just don't understand how, with that statement, you are checking the number of responses completed. Appreciate it's just lack of knowledge on my part. Is there an alternative way that check could be written?

@T.J. Crowder 2017-05-28 18:31:11

@Sarah: expecting starts out with the value of array.length, which is how many requests we're going to make. We know the callback won't be called until all of those requests are started. In the callback, if (--expecting === 0) does this: 1. Decrements expecting (we've received a response, so we're expecting one fewer response) and if the value after the decrement is 0 (we're not expecting any more responses), we're done!

@T.J. Crowder 2019-04-26 07:51:43

@PatrickRoberts - Thanks!! Yes, copy-and-paste error, that second argument was completely ignored in that example (which is the only reason it didn't fail, since as you pointed out, results didn't exist). :-) Fixed it.

@Murtaza Hussain 2019-04-16 16:13:16

use of async/await with a transpilers like Babel to get it working in older browsers. You’ll also have to install this Babel preset and polyfill from npm: npm i -D babel-preset-env babel-polyfill.

function getData(ajaxurl) { 
  return $.ajax({
    url: ajaxurl,
    type: 'GET',
  });
};

async test() {
  try {
    const res = await getData('https://api.icndb.com/jokes/random')
    console.log(res)
  } catch(err) {
    console.log(err);
  }
}

test();

or the .then callback is just another way to write the same logic.

getData(ajaxurl).then(function(res) {
    console.log(res)
}

@amaksr 2017-05-27 02:47:04

Another solution is to execute code via sequential executor nsynjs.

If underlying function is promisified

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

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>

If underlying function is not promisified

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

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 nsynjs:

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

@J Morris 2017-06-16 23:55:06

This is interesting. I like how it allows to code async calls the way you'd do it in other languages. But technically it's not real JavaScript?

@Felix Kling 2013-01-08 17:06:14

→ 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.

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

Synchronous

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.

Asynchronous

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:

  • Promises with async/await (ES2017+, available in older browsers if you use a transpiler or regenerator)
  • Callbacks (popular in node)
  • Promises with then() (ES2015+, available in older browsers if you use one of the many promise libraries)

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


ES2017+: Promises with async/await

The 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". The code is still asynchronous, but it's easier to read/understand.

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.

Important: You can only use await inside an async function. Right now, top-level await isn't yet supported, so you might have to make an async IIFE (Immediately Invoked Function Expression) to start an async context.

You can read more about async and await on MDN.

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 3 seconds (just for the sake of this example)
    await delay();
    // 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;
  }
}

// Start an IIFE to use `await` at the top level
(async function(){
  let books = await getAllBooks();
  console.log(books);
})();

Current 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).


Let functions accept callbacks

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

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:

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).

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.


ES2015+: Promises with then()

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

Side note: jQuery's deferred objects

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
});

Side note: Promise gotchas

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.

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
});

Not recommended: Synchronous "Ajax" calls

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:

Without jQuery

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

jQuery

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:

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).

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).

@Felix Kling 2013-01-17 10:47:54

@Pommy: If you want to use jQuery, you have to include it. Please refer to docs.jquery.com/Tutorials:Getting_Started_with_jQuery.

@cssyphus 2013-02-06 21:07:03

In Solution 1, sub jQuery, I could not understand this line: If you use any other jQuery AJAX method, such as $.get, $.getJSON, etc., you have them to $.ajax. (Yes, I realize my nick is a tad ironic in this case)

@Felix Kling 2013-02-06 23:29:52

@gibberish: Mmmh, I don't know how it can be made clearer. Do you see how foo is called and a function is passed to it (foo(function(result) {....});)? result is used inside this function and is the response of the Ajax request. To refer to this function, the first parameter of foo is called callback and assigned to success instead of an anonymous function. So, $.ajax will call callback when the request was successful. I tried to explain it a bit more.

@Chris Moschini 2013-04-16 02:45:22

The Chat for this question is dead so I'm not sure where to propose outlined changes, but I propose: 1) Change the synchronous part to a simple discussion of why it's bad with no code example of how to do it. 2) Remove/merge the callback examples to only show the more flexible Deferred approach, which I think may also be a little easier to follow for those learning Javascript.

@Tracker1 2015-02-19 18:00:31

Just one minor comment on synchronous requests.. if you're implementing something like a SCORM compliant interface, you should cache all values and return immediately for most requests, but you should do flush and exit commands synchronously to purposely block closing the channel. SCORM was a pretty horrible interface for web, and it's about the only case where you should do synchronous calls.

@Felix Kling 2015-09-30 17:06:02

@johnny: Yes. That's why the code that needs to access the response has to go inside the callback, not after the function call (as shown in the very first example of "Let functions accept callbacks").

@Jessi 2015-10-08 17:40:49

I must commend on this most epic of answers. I have only one thing to mention. I feel the suggesting to simply not use $.getJSON seems to side step that matter. What if simple want to test the result of a successful $.getJSON and test for the existence of something inside that object? (My current situation)...stackoverflow.com/questions/33008396/…

@Felix Kling 2015-10-08 17:44:18

@Jessi: I think you misunderstood that part of the answer. You cannot use $.getJSON if you want the Ajax request to be synchronous. However, you should not event want the request to be synchronous, so that doesn't apply. You should be using callbacks or promises to handle the response, as it is explained earlier in the answer.

@Jessi 2015-10-08 17:50:38

I am using the jquery deferred objects of .done and .fail is that ok? I can actually see the Object inside my test via console.dir(result). I'm currently working to figure out how to test for value of something inside the object. Thank you for patience, I am a bit new at this.

@MadeInDreams 2016-01-19 00:10:53

This is well explain but i still cant get mine to work and it is very Basic. The reason i dont get it its because in the foo() example there is nothing pass to the function. In my case I have foo(var1,var2,var3) so how am i suppose to call foo(function(result) { //execution... with var1,var2,var3

@RicardoE 2016-09-08 17:12:52

using callbacks is easier if you want to unit-test your code :)

@Felix Kling 2016-09-08 17:13:51

@RicardoE: what makes you think that? Many testing frameworks support promises.

@Travis Michael Heller 2016-09-09 13:37:14

This was the one of the best explanation of how promises and asynchronous javascript works. Thank you for taking the time to share this.

@RicardoE 2016-09-09 18:08:11

@FelixKling I didn't said it was not supported... I said it was easier :) of course it is supported!

@Patrick Roberts 2016-09-19 11:55:46

Your ajax() Promise implementation is a lot like the fetch API.

@Jo Smo 2016-09-27 11:13:15

Deprecation Notice: The jqXHR.success(), jqXHR.error(), and jqXHR.complete() callbacks are removed as of jQuery 3.0. You can use jqXHR.done(), jqXHR.fail(), and jqXHR.always() instead.

@Manachi 2016-10-05 07:02:17

Just a note of clarification - Dojo implemented "Deferred" before jQuery. So the Deferred concept is not exclusive to, nor created by jQuery

@stom 2016-10-31 17:33:17

You have used callback and myCallback , If i am correct both are same and what you mean is success: myCallback(response) ?

@Felix Kling 2016-10-31 18:33:29

@stom: "both are same" They are referring to the same value in the example. "what you mean is success: myCallback(response)" No. I really do mean success: callback. callback is the name of the parameter of foo, and the value of the parameter is assigned to success. myCallback is the name of the function is passed to foo, but that's irrelevant. Consider this simplified example: var x = 42; function foo(y) { console.log(y); }; foo(x);. y is a parameter of foo, hence console.log(y) is used. We could do console.log(x);, but that defeats the purpose of the function.

@stom 2016-11-01 06:54:24

Thanks for the reply, so that means when response comes from server, success will assign value to foo's callback parameter, which in turn will call foo(myCallback) and pass the value to result parameter of myCallback ?

@Felix Kling 2016-11-01 13:45:50

@stom: foo(myCallback) was already called. That's what initiates the Ajax call. But yes, when the response is received, jQuery will call the function assigned to success and pass the response to it.

@Makyen 2016-12-17 02:38:13

@MadaraUchiha, I strongly disagree with your recent edit (the large TL;DR: Use Promises). This question is a dup target for a large number of general async-issue questions (how to return a variable). Putting that code/info at the start is confusing to the people who we are telling this is a duplicate of their problem. The code you provided only shows how to use the Promise that is auto-returned by jQuery's $.ajax() when you don't provide a callback. You have changed a general purpose answer into one which is now only specific to $.ajax(), or functions which auto-return a Promise.(cont)

@Makyen 2016-12-17 02:38:18

@MadaraUchiha:(cont) This is now seen by newbies as too specific and not applicable to their question (which could be for any async function, not just $.ajax()). $.ajax() is just used as an example of the more general issue. Note that I am only here at this question/answer now because what I describe as the response newbies is what has happened when I suggested this as a dup target on a question. If you were not a moderator, I would just revert the edit while we discuss it, but I don't want to, potentially, get into a rollback contest with a moderator.

@jAndy 2016-12-22 18:10:01

I guess with the presence of BabelJS and therefore ES7 support, this answer deserves an update for being able to actually go back to "synchronous style" using asynchronous functions by applying async / await combos.

@Felix Kling 2016-12-22 18:12:04

@jAndy: async/await is not part of ES7 (ES2016) but it will be part of ES2017. So yes, I agree that talking about it would be valuable. However, one still needs to understand promises. I see if can come up with something in the next few days (and restructure the answer a little to make it not too cluttered).

@DarkMukke 2017-09-07 12:43:55

i think this answer can do with an update since ES8 is out

@Felix Kling 2017-09-07 15:11:09

@DarkMukke: it already mentions async/await. What else do you think is missing?

@DarkMukke 2017-09-07 15:55:04

@FelixKling i'm sorry, you missunderstand, I think the post is perfect, but callback popular in node, afaik everyone is trying to move away from call backs asap in favour of promises. i just feel that statement is outdated, and I figured if 1 bit is, some other bits might too

@Gobliins 2017-11-15 15:46:21

can you also do an empty .then() just to wait until the async function is finished?

@Felix Kling 2017-11-15 16:22:43

@Gobliins: You can add .then() but it doesn't make a difference. The result will be the same as not adding .then().

@Gobliins 2017-11-16 09:06:15

You see, i have mongoose that should connect to a mongodb inside a mocha before block before doing tests. So i have to wait until the connection is established, before the testing can begin.

@loki 2017-12-29 11:25:43

@FelixKling so the response will only be available in the myCallback function and hence the code where I want to do something with result should all be put here right? or I am missing something ? Outside the callback the value of result will be undefined even if the the myCallback returns result ?

@Ben 2018-01-04 12:29:13

Shouldnt it be advertised as "ES1017+: Promises with async/await/then" ? As you name the distincly in the next solution, but still using it there to get the final result ?

@Mâtt Frëëman 2018-03-13 04:59:19

AirAsia the largest LCC carrier in Asia still uses Sync jQuery ajax on their website so definitely in-use outside of SCORM. I got so sick of my browser freezing (due to an equally unimpressive backend) I decided to dive into the code and then found the horror async: false. Incredibily they have won many "travel" awards for their slow, browser crashing, VPN blocking, exception throwing website.

@Estus Flask 2019-02-01 20:20:29

jQuery deferred exposes promise implementation. Which may have its quirks to the point it's unattractive enough to use it instead of ES6 promises. Still, it's thenable, it's possible to do Promise.resolve($.ajax(...).promise()) or await $.ajax(...).promise() instead of dealing with jQuery done and fail.

@cocco 2013-08-19 08:06:04

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:

  1. It's shorter than all other functions Listed.
  2. The callback is set directly (so no extra unnecessary closures).
  3. It uses the new onload (so you don't have to check for readystate && status)
  4. There are some other situations which I don't remember that make the XMLHttpRequest 1 annoying.

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:

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 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.


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]):

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.

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.

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().

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 your 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 web SQL 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.)

@Benjamin Gruenbaum 2013-08-23 05:56:49

While this answer is nice (And we all love XHR2 and posting file data and multipart data is totally awesome) - this shows syntactic sugar for posting XHR with JavaScript - you might want to put this in a blog post (I'd like it) or even in a library (not sure about the name x, ajax or xhr might be nicer :)). I don't see how it addresses returning the response from an AJAX call. (someone could still do var res = x("url") and not understand why it doesn't work ;)). On a side note - it would be cool if you returned c from the method so users can hook on error etc.

@Benjamin Gruenbaum 2013-08-23 17:28:35

2.ajax is meant to be async.. so NO var res=x('url').. That's the entire point of this question and answers :)

@Brian H. 2016-12-21 10:00:52

why is there a 'c' parameter in the functions, if on the first line you're overwriting whatever value it had? am i missing something?

@cocco 2016-12-21 11:05:21

You can use parameters as a placeholder to avoid writing multiple times "var"

@stone 2017-10-08 06:20:55

@cocco So you wrote misleading, unreadable code in a SO answer in order to save a few keystrokes? Please don't do that.

@Sebi 2019-01-10 11:13:33

@cocco I find it much easier to name variables with actual words, instead of using a, b, d, e, c and keeping notes handy explaining the letters with these words. Also, I have found the unused parameter very confusing - at first I was wondering if the result is somehow returned back via the c parameter. I can't see how saving the few keystrokes of const or let boosts the readability. I would think that if you go to such extremes to ' avoid writing multiple times "var" ', you could as well just write it once, and list all variables in the single declaration.

@YakovL 2019-06-15 12:50:33

please update your answer with readable argument/variable names and remove the comments which you use to "fix" the bad naming

@user663031 2016-01-23 03:28:39

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 asynchronicity is waiting for some remote system to respond 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.

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 asynchronous request, which it will then invoke when it has completed 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.

The problem with this callback approach is that it pollutes the normal semantics of a function reporting its result with return; instead, functions must not 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:

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.

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);
}

Enter promises

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.

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() . 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 for 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

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:

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

The async keyword

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() . 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

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);
}

@Sumer 2018-12-02 12:41:39

Simple code example to convert XHR on Node to async-await

var XMLHttpRequest = require("xmlhttprequest").XMLHttpRequest;
var xhttp = new XMLHttpRequest();

function xhrWrapWithPromise() {
  return new Promise((resolve, reject) => {
    xhttp.onreadystatechange = function() {
      if (this.readyState == 4) {
        if (this.status == 200) {
          resolve(this.responseText);
        } else {
          reject(new Error("Couldn't feth data finally"));
        }
      }
    };
    xhttp.open("GET", "https://www.w3schools.com/xml/xmlhttp_info.txt", true);
    xhttp.send();
  });
}

//We need to wrap await in Async function so and anonymous IIFE here
(async _ => {
  try {
    let result = await xhrWrapWithPromise();
    console.log(result);
  } catch (error) {
    console.log(error);
  }
})();

@Felix Kling 2018-12-02 17:01:02

I’d recommend to use fetch instead of wrapping XMLHttpRequest. developer.mozilla.org/en-US/docs/Web/API/…

@loretoparisi 2016-04-12 22:55:39

The following example I have written shows how to

  • Handle asynchronous HTTP calls;
  • Wait for response from each API call;
  • Use Promise pattern;
  • Use Promise.all pattern to join multiple HTTP calls;

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.

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

[
 "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.

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.

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

-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.

@Francisco Carmona 2016-06-02 08:31:13

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).

This is the plnkr:

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

ES6 way (async - await)

(function(){
  async function getJoke(){
    let response = await fetch('http://api.icndb.com/jokes/random');
    let data = await response.json();
    return data.value;
  }

  getJoke().then((joke) => {
    console.log(joke);
  });
})();

@Alex Montoya 2018-07-12 21:48:15

Here is an example that works:

const validateName = async userName => {
  const url = "abc/xyz";
  try {
    const response = await axios.get(url);
    return response.data
  } catch (err) {
    return false;
  }
};

validateName("user")
 .then(data => console.log(data))
 .catch(reason => console.log(reason.message))

@mikemaccana 2017-06-02 09:51:10

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

This is quite simple:

  • Return a Promise
  • Use the 'await', which will tell JavaScript to await the promise to be resolved into a value (like the HTTP response)
  • Add the 'async' keyword to the parent function

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

@Michał Perłakowski 2017-06-08 06:47:10

Unfortunately, this works only with functions that return promises – for example it doesn't work with Node.js API, which uses callbacks. And I wouldn't recommend using it without Babel, because not everyone uses "current browsers".

@mikemaccana 2017-06-09 18:28:16

@MichałPerłakowski node 8 includes nodejs.org/api/util.html#util_util_promisify_original which can be used to make the node.js API return promises. Whether you have the time and money to support non-current browsers obviously depends on your situation.

@Juan Mendes 2018-10-04 14:51:34

IE 11 is still a current browser in 2018, sadly and it doesn't support await/async

@mikemaccana 2018-10-04 14:57:29

IE11 is not a current browser. It was released 5 years ago, has a worldwide market share of 2.5% according to caniuse, and unless someone is doubling your budget to ignore all current tech then it's not worth most people's time.

@James 2018-02-17 15:26:28

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: callback});
    console.log("response is:", response);

    // examples of other things you can do
    yield setTimeout(callback, 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:

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

@James 2018-02-17 17:49:50

I would be interested to know why this is down-voted. It seems a good simple answer that has not been previously given.

@Pieter Jan Bonestroo 2018-01-13 19:13:20

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:

__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")

@Matthew Brent 2018-05-04 15:56:07

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?)

The Event Loop and Concurrency Model

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

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)

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

WHEN is something going to run

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

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()

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.

@Aniket Jha 2018-02-03 06:06:53

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.

CallBack

 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

@Khoa Bui 2017-07-05 20:28:42

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.
}

@Aluan Haddad 2018-03-18 19:48:53

There's nothing inherently asynchronous about callbacks or JavaScript.

@Benjamin Gruenbaum 2013-05-29 23:30:56

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)


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.

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.

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).

Usage would be:

getFive(onComplete);

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

Possible solutions

There are basically two ways how to solve this:

  1. Make the AJAX call synchronous (lets call it SJAX).
  2. Restructure your code to work properly with callbacks.

1. Synchronous AJAX - Don't do it!!

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);
}

2. Restructure code

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.

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.

@Matthew G 2013-08-16 05:54:22

"synchronous requests block the execution of code and can leak memory and events" how can a synchronous request leak memory?

@Benjamin Gruenbaum 2013-08-16 08:28:00

@MatthewG I've added a bounty on it in this question, I'll see what I can fish out. I'm removing the quote from the answer in the mean time.

@Florian Margaine 2013-12-22 21:09:19

Just for the reference, XHR 2 allows us to use the onload handler, which only fires when readyState is 4. Of course, it's not supported in IE8. (iirc, may need confirmation.)

@Robbie Averill 2014-08-07 10:14:22

Your explanation of how to pass an anonymous function as a callback is valid but misleading. The example var bar = foo(); is asking for a variable to be defined, whereas your suggested foo(functim() {}); doesn't define bar

@Fernando Carvajal 2018-01-24 06:18:55

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)

})

@Zum Dummi 2019-02-17 02:02:09

could that second function to reuseable??

@Ken Ingram 2019-06-29 06:47:52

How do you use the results if oncolse,log is called? Doesn't everything just go to the console at that point?

@Mahfuzur Rahman 2017-04-24 08:09:03

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

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();

@Haim Zamir 2017-10-25 03:22:21

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.

  1. Your entry point(s) is executed as the result of an event. For example, a script tag with code is loaded into the browser. (Accordingly, this is why you may need to be concerned with the readiness of the page to run your code if it requires dom elements to be constructed first, etc.)
  2. Your code executes to completion--however many asynchronous calls it makes--without executing any of your callbacks, including XHR requests, set timeouts, dom event handlers, etc. Each of those callbacks waiting to be executed will sit in a queue, waiting their turn to be run after other events that fired have all finished execution.
  3. Each individual callback to an XHR request, set timeout or dom the event once invoked will then run to completion.

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.

@Maleen Abewardana 2014-08-26 08:11:36

Angular1

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

Angular2 and Later

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

@Benjamin Gruenbaum 2014-11-04 02:29:10

This does not explain how promises would solve this issue at all though.

@An Overflowed Stack 2014-11-21 15:35:47

It doesn't work. promiseB will get 'undefined'

@Tracker1 2015-02-19 19:24:49

jQuery and fetch methods both return promises as well. I would suggest revising your answer. Though jQuery's isn't quite the same (then is there, but catch isn't).

@Mohan Dere 2016-08-13 09:36:41

Here are some approaches to work with asynchronous requests:

  1. Browser Promise object
  2. Q - A promise library for JavaScript
  3. A+ Promises.js
  4. jQuery deferred
  5. XMLHttpRequest API
  6. Using callback concept - As implementation in first answer

Example: jQuery deferred implementation to work with multiple requests

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();

@Hemant Bavle 2014-02-18 18:58:37

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

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);    
}); 

@Hemant Bavle 2014-03-28 18:12:35

I don't know who voted it negative. But this is a work around which has worked in fact i used this approach to create a whole application. The jquery.ajax don't return data so its better to use the above approach. If it's wrong then please explain and suggest better way to do it.

@Benjamin Gruenbaum 2014-04-10 09:18:07

Sorry, I forgot to leave a comment (I usually do!). I downvoted it. Downvotes don't indicate factual correctness or lack of, they indicate usefulness in the context or lack of. I don't find your answer useful given Felix's which already explains this only in much more detail. On a side note, why would you stringify the response if it's JSON?

@Hemant Bavle 2014-04-10 10:27:10

ok.. @Benjamin i used stringify, to convert a JSON Object to string. And thanks for clarifying your point. Will keep in mind to post more elaborate answers.

@pesho hristov 2016-02-19 16:02:57

And what if you want to return the "responseObj" outside of "successCallback" ... :) ... how will you do it ... ? ... cause a simple return will return it to the "success" callback of the ajax ... and not outside of "successCallback" ...

@jsbisht 2015-09-02 12:54:37

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.

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.

@Felix Kling 2015-09-02 13:18:11

There is nothing special about using an object here. It would work as well if you assigned he response directly to result. It works because you are reading the variable after the async function is complete.

@Vinoth Rajendran 2016-05-26 13:26:40

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)
});

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