How do I return the response from an asynchronous call?

Question

How do I return the response/result from a function foo that makes an asynchronous request?

I am trying to return the value from the callback, as well as assigning the result to a local variable inside the function and returning that one, but none of those ways actually return the response — they all return undefined or whatever the initial value of the variable result is.

Example of an asynchronous function that accepts a callback (using jQuery's ajax function):

function foo() {
    var result;

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

    return result; // It always returns `undefined`
}

Example using Node.js:

function foo() {
    var result;

    fs.readFile("path/to/file", function(err, data) {
        result = data;
        // return data; // <- I tried that one as well
    });

    return result; // It always returns `undefined`
}

Example using the then block of a promise:

function foo() {
    var result;

    fetch(url).then(function(response) {
        result = response;
        // return response; // <- I tried that one as well
    });

    return result; // It always returns `undefined`
}

Answer 1

It has become more simpler:

async function dostuff() {const [err, res] ?= await fetch('https://codingbeautydev.com');}

Answer 2

Async/await provides a cleaner syntax for handling asynchronous code, making it look more synchronous. Here’s how you can implement it:

const fetch = require('node-fetch');

app.get('/data', async function (req, res) {
    try {
        const response = await fetch('https://api.example.com/data');
        const data = await response.json();
        res.send(data);
    } catch (error) {
        res.status(500).send("Error fetching data");
    }
});

Answer 3

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

If you don't use jQuery and want a nice short XMLHttpRequest 2 which works in modern browsers and also in mobile browsers, I suggest using 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 even the XMLHttpRequest variable name. That's wrong.

Check out XMLHttpRequest advanced features.

It supported all *modern browsers. And I can confirm as I have been using this approach since XMLHttpRequest 2 was created. I never had any type of problem in any browsers I used.

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, or 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 and 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 we 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 properly?

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

Error handlers may 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 on the server side if you passed the right post data.

Cross-domain not allowed throws an error automatically.

In the error response, there aren't any error codes.

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

Why add an error handler if you don't have any control over errors? Most of the errors are returned inside this in the callback function displayAjax().

So: There isn't any 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, and 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, or 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.)

Answer 4

In JS, when an asynchronous function is called, it returns a Promise. If you want to return a value from the function, you need to ensure you are properly handling this Promise.

There is an example:

async function foo() {
  var res;
  await asyncRequest().then(function(response) {
    res = response;
  });
  return res;
}

// Call the function and handle it's result:

foo().then(function(res) {
  console.log(res);
});

In the above code, foo is an asynchronous function which makes an asynchronous request. The await keyword is used to pause the execution of the function until the Promise is resolved 🤚. Once the Promise is resolved, the response is assigned to the result variable, that then returned by the function.

When calling the function, since it returns a Promise, you need to use .then() to handle the result after the Promise is resolved.

async function bar() {
  var res = await foo();
  console.log(res);
}

Remember, you can only use await inside an async function(). If you’re not in an async function and want to use await, you will get a syntax error.

Answer 5

1. A first stumbling step

As for many others, my encounter with asynchronous calls was puzzling at first.
I don't remember the details, but I may have tried something like:

let result;

$.ajax({
  url: 'https://jsonplaceholder.typicode.com/todos/1',
  success: function (response) {
    console.log('\nInside $.ajax:');
    console.log(response);
    result = response;
  },
});

console.log('Finally, the result: ' + result);

.as-console-wrapper { max-height: 100% !important; top: 0; }

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.6.0/jquery.min.js"></script>

Whoops!
The output of the line console.log('Finally, the result: ' + result); which I thought would be printed last, is printed before the other output!
– And it doesn't contain the result: it just prints undefined. ¹
How come?

A helpful insight

I distinctly remember my first aha (💡) moment about asynchronous calls. It was :

you actually don't want to get the data out of a callback; you want to get your data-needing action into the callback! ²

This is true in the example above.

2. Plain JavaScript and a callback function

Luckily, it is possible to write code after the asynchronous call that deals with the response once it has completed.

One alternative is the use of a callback function in a continuation-passing style : ³

const url = 'https://jsonplaceholder.typicode.com/todos/2';

function asynchronousFunc(callback) {
  const request = new XMLHttpRequest();
  request.open('GET', url);
  request.send();
  request.onload = function () {
    if (request.readyState === request.DONE) {
      console.log('The request is done. Now calling back.');
      callback(request.responseText);
    }
  };
}

asynchronousFunc(function (result) {
  console.log('This is the start of the callback function. Result:');
  console.log(result);
  console.log('The callback function finishes on this line. THE END!');
});

console.log('LAST in the code, but executed FIRST!');

.as-console-wrapper { max-height: 100% !important; top: 0; }

Note how the function asynchronousFunc is void. It returns nothing.
asynchronousFunc is called with an anonymous callback function,
(asynchronousFunc(function (result) {...});).
This executes the desired actions on the result after the request has completed – when the responseText is available.

Running the above snippet shows how I will probably not want to write any code after the asynchronous call (such as the line LAST in the code, but executed FIRST!).
Why? – Because such code will run before the asynchronous call delivers any response data.
Doing so is bound to cause confusion when comparing the code with the output.

3. Promise with .then()

The .then() construct was introduced in the ECMA-262 6th Edition in June 2015.
The code below is plain JavaScript, replacing the old-school XMLHttpRequest with Fetch. ⁴

fetch('https://api.chucknorris.io/jokes/random')
  .then((response) => response.json())
  .then((responseBody) => {
    console.log('Using .then() :');
    console.log(responseBody.value + '\n');
  });

.as-console-wrapper { max-height: 100% !important; top: 0; }

4. Promise with async/await

The async/await construct was introduced in the ECMA-262 8th Edition in June 2017.

async function awaitAndReceivePromise() {
  const responseBody = (
    await fetch('https://api.quotable.io/quotes/random')
  ).json();
  console.log('Using async/await:');
  const obj = (await responseBody)[0];
  console.log('"' + obj.content + '" – ' + obj.author + '\n');
}

awaitAndReceivePromise();

.as-console-wrapper { max-height: 100% !important; top: 0; }

A word of warning is warranted if you decide to go with the async/await construct. Note in the above snippet how await is needed in two places.
If forgotten in the first place, there will be no output at all. If forgotten in the second place, the only output will be Using async/await: – nothing else gets printed.
Forgetting the async prefix of the function is maybe the worst of all – you'll get a "SyntaxError" – and likely no hint about the missing async keyword.

---

All the above examples succinctly convey how asynchronous calls may be used on toyish APIs. ⁵

References

• Some questions and answers about asynchronous calls
• Using plain JavaScript and a callback function
• Continuation-passing style
• XMLHttpRequest at mdn web docs
• XMLHttpRequest: onload vs. onreadystatechange
• XMLHttpRequest.responseText
• An example demonstrating async/await
• Using the Fetch API
• Using promises
• The XMLHttpRequest Standard
• The Fetch Standard
• The Web Hypertext Application Technology Working Group (WHATWG)
• Links to ECMA specifications
• How can I fetch an array of URLs with Promise.all?

---

¹ Expressed by the asker of the question as they all return undefined.

² Here is more on how asynchronous calls may be confusing at first.

³ Like the X in AJAX, the name XMLHttpRequest is misleading – it can be used to retrieve any type of data, not just XML.
These days, the data format of Web APIs is ubiquitously JSON, not XML.

⁴ Fetch returns a Promise. I was surprised to learn that neither XMLHttpRequest nor Fetch are part of the ECMAScript standard. The reason JavaScript can access them here is that the web browser provides them.
The Fetch Standard and the XMLHttpRequest Standard are both upheld by the Web Hypertext Application Technology Working Group which was formed in June 2004. \

⁵ You might also be interested in How can I fetch an array of URLs with Promise.all?.

Answer 6

I follow these two ways Promises and async/await

Promises:

function makeAsyncCall() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      const response = 'Async response';
      resolve(response);
    }, 2000);
  });
}

makeAsyncCall()
  .then(response => {
    console.log(response); 
  })
  .catch(error => {
    console.error(error); 
  });

And async/await:

async function makeAsyncCall() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      const response = 'Async response';
      resolve(response);
    }, 2000);
  });
}

async function handleAsyncCall() {
  try {
    const response = await makeAsyncCall();
    console.log(response); 
  } catch (error) {
    console.error(error); 
  }
}

handleAsyncCall();

Answer 7

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 JavaScript compilers if it's not defined. Just add this code before your main code and then safely use Promise!

if(typeof Promise === "undefined"){
    function _typeof(obj) { "@babel/helpers - typeof"; return 

    _typeof = "function" == typeof Symbol && "symbol" == typeof Symbol.iterator ? function (obj) { return typeof obj; } : function (obj) { return obj && "function" == typeof Symbol && obj.constructor === Symbol && obj !== Symbol.prototype ? "symbol" : typeof obj; }, _typeof(obj); }
    function _toConsumableArray(arr) { return _arrayWithoutHoles(arr) || _iterableToArray(arr) || _unsupportedIterableToArray(arr) || _nonIterableSpread(); }
    function _nonIterableSpread() { throw new TypeError("Invalid attempt to spread non-iterable instance.\nIn order to be iterable, non-array objects must have a [Symbol.iterator]() method."); }
    function _unsupportedIterableToArray(o, minLen) { if (!o) return; if (typeof o === "string") return _arrayLikeToArray(o, minLen); var n = Object.prototype.toString.call(o).slice(8, -1); if (n === "Object" && o.constructor) n = o.constructor.name; if (n === "Map" || n === "Set") return Array.from(o); if (n === "Arguments" || /^(?:Ui|I)nt(?:8|16|32)(?:Clamped)?Array$/.test(n)) return _arrayLikeToArray(o, minLen); }
    function _iterableToArray(iter) { if (typeof Symbol !== "undefined" && iter[Symbol.iterator] != null || iter["@@iterator"] != null) return Array.from(iter); }
    function _arrayWithoutHoles(arr) { if (Array.isArray(arr)) return _arrayLikeToArray(arr); }
    function _arrayLikeToArray(arr, len) { if (len == null || len > arr.length) len = arr.length; for (var i = 0, arr2 = new Array(len); i < len; i++) arr2[i] = arr[i]; return arr2; }
    function _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError("Cannot call a class as a function"); } }
    function _defineProperties(target, props) { for (var i = 0; i < props.length; i++) { var descriptor = props[i]; descriptor.enumerable = descriptor.enumerable || false; descriptor.configurable = true; if ("value" in descriptor) descriptor.writable = true; Object.defineProperty(target, _toPropertyKey(descriptor.key), descriptor); } }
    function _createClass(Constructor, protoProps, staticProps) { if (protoProps) _defineProperties(Constructor.prototype, protoProps); if (staticProps) _defineProperties(Constructor, staticProps); Object.defineProperty(Constructor, "prototype", { writable: false }); return Constructor; }
    function _toPropertyKey(arg) { var key = _toPrimitive(arg, "string"); return _typeof(key) === "symbol" ? key : String(key); }
    function _toPrimitive(input, hint) { if (_typeof(input) !== "object" || input === null) return input; var prim = input[Symbol.toPrimitive]; if (prim !== undefined) { var res = prim.call(input, hint || "default"); if (_typeof(res) !== "object") return res; throw new TypeError("@@toPrimitive must return a primitive value."); } return (hint === "string" ? String : Number)(input); }
    var Promise = /*#__PURE__*/function () {
  "use strict";

  function Promise(main) {
    _classCallCheck(this, Promise);
    this.main = main;
    this.mainExecuted = false;
    this.resolved = false;
    this.rejected = false;
    this.promiseChain = [];
    this.handleError = function () {};
    this.onResolve = this.onResolve.bind(this);
    this.onReject = this.onReject.bind(this);
  }
  _createClass(Promise, [{
    key: "then",
    value: function then(handleSuccess) {
      if (this.resolved) {
        if (!this.rejected) {
          this.args = handleSuccess(this.args);
        }
      } else {
        this.promiseChain.push(handleSuccess);
        this.main(this.onResolve, this.onReject);
        this.thenExecuted = true;
      }
      return this;
    }
  }, {
    key: "catch",
    value: function _catch(handleError) {
      this.handleError = handleError;
      if (!this.mainExecuted) {
        this.main(this.onResolve, this.onReject);
        this.thenExecuted = true;
      }
      return this;
    }
  }, {
    key: "onResolve",
    value: function onResolve() {
      var _this = this;
      this.resolved = true;
      for (var _len = arguments.length, args = new Array(_len), _key = 0; _key < _len; _key++) {
        args[_key] = arguments[_key];
      }
      this.args = args;
      try {
        this.promiseChain.forEach(function (nextFunction) {
          _this.args = nextFunction.apply(void 0, _toConsumableArray(_this.args));
        });
      } catch (error) {
        this.promiseChain = [];
        this.onReject(error);
      }
    }
  }, {
    key: "onReject",
    value: function onReject(error) {
      this.rejected = true;
      this.handleError(error);
    }
  }]);
  return Promise;
}();
}

Answer 8

→ For a more general explanation of asynchronous behaviour with different examples, 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 the 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 results in a 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 or in a JavaScript module. Top-level await is not supported outside of modules, so you might have to make an async IIFE (Immediately Invoked Function Expression) to start an async context if not using a module.

You can read more about async and await on MDN.

Here is an example that elaborates the delay function findItem() 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 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 when function 1 is passed to function 2. Function 2 can call function 1 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 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 canceled (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 an 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).
  });

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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("https://jsonplaceholder.typicode.com/todos/1")
  .then(function(result) {
    console.log(result); // Code depending on result
  })
  .catch(function() {
    // An error occurred
  });

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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 asynchronous 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, it 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).

Answer 9

Here is an example that works:

const validateName = async userName => {
  const url = "https://jsonplaceholder.typicode.com/todos/1";
  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))

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<script src=
"https://cdnjs.cloudflare.com/ajax/libs/axios/0.21.1/axios.min.js"></script>

Answer 10

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)

})

Stack Snippet that demonstrates the code above.

// The function declaration:
async function foo() {
  var response = await $.ajax({
    url: 'https://jsonplaceholder.typicode.com/todos/1'
  })
  return response;
}

// Execute it like this:
(async function() {
  try {
    var result = await foo()
    console.log(result)
  } catch (e) {}
})()

// Or use Promise syntax:
foo().then(response => {
  console.log(response)
}).catch(error => {
  console.log(error)
})

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<script src=
"https://ajax.googleapis.com/ajax/libs/jquery/3.6.0/jquery.min.js"></script>

Answer 11

There is no way you can directly return the result of an Ajax response from a function. The reason is that an Ajax call ($.get() or $.post()) is asynchronous and calling the function that encapsulates the Ajax call would return even before the response is rendered.

In such scenarios, the only option is to return a promise object, to be resolved when the response arrives.

There are two ways by which the above issue can be resolved. Both make use of a promise.

The code snippets below include a JSON URL. Both work and can be directly copied to JSFiddle and tested.

Option #1 - return the Ajax call directly from the foo method.
In the latest version of jQuery, an Ajax call returns a promise object, which can be resolved using a .then function. In code, the .then function is preceded by the call back function to be resolved, foo() in this case.

   // Declare function foo
   function foo(url)
   {
     return $.get(url);
   }

   // Invoke the foo function, which returns a promise object
   // the 'then' function accepts the call back to the resolve function
   foo('https://jsonplaceholder.typicode.com/todos/1')
     .then(function(response)
     {
       console.log(response);
     })

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<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.6.0/jquery.min.js"></script>

Option #2 - declare a promise object and return it.
Declare a promise object inside the function, encapsulate the Ajax call within that promise function and return the promise object.

   function foo1() {
     var promise = new Promise(function(resolve, reject)
     {
       $.ajax({
       url: 'https://jsonplaceholder.typicode.com/todos/1',
       success: function(response) {
           console.log(response);
           resolve(response);
           // return response; // <- I tried that one as well
         }
       });
     });
     return promise;
   }

   foo1()
   .then(function(response)
   {
     console.log('Promise resolved:');
     console.log(response);
   })

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<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.6.0/jquery.min.js"></script>

Answer 12

Await

A request works in an asynchronous way, so you can't read the data synchronously as in typical code. However, using async/await you can create asynchronous code which looks close/similar to the usual synchronous/sequential style. Code which processes response data needs to be wrapped by an async function (load in the below snippet) and inside it you need to add the await keyword before foo() (which also uses async/await).

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

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

load();

Remember that an async function always (implicitly) wraps its result into a promise (so it returns a promise).

Answer 13

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));
}

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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:", JSON.stringify(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));
}

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(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:", JSON.stringify(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));
}

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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:", JSON.stringify(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));
}

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(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:", JSON.stringify(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));
    });
}

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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:", JSON.stringify(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));
}

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(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:", JSON.stringify(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));
    });
}

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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:", JSON.stringify(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));
    });
}

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...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:", JSON.stringify(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));
    });
}

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Answer 14

JavaScript is single threaded.

The browser can be divided into three parts:

1. Event Loop

2. Web API

3. Event Queue

The event loop runs for forever, i.e., kind of an infinite loop. The event queue is where all your functions are pushed on some event (example: click).

This is one by one carried out of queue and put into the event loop which executes this function and prepares itself for the next one after the first one is executed. This means execution of one function doesn't start until the function before it in the queue is executed in the event loop.

Now let us think we pushed two functions in a queue. One is for getting a data from the server and another utilises that data. We pushed the serverRequest() function in the queue first and then the utiliseData() function. The serverRequest function goes in the 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 the role. It takes this function from the event loop and deals with the server making the event loop free, so that we can execute the next function from the queue.

The next function in the queue is utiliseData() which goes in the loop, but because of no data available, it goes to waste and execution of the next function continues until the end of the queue. (This is called Async calling, i.e., we can do something else until we get data.)

Let us suppose our serverRequest() function had a return statement in code. When we get back data from the server Web API, it will push it in the queue at the end of queue.

As it gets pushed at the end of the queue, we cannot utilise its data as there isn't any function left in our queue to utilise this data. Thus it is not possible to return something from the async call.

Thus the solution to this is callback or promise.

• An image from one of the answers here correctly explains callback use...*

We give our function (function utilising data returned from the server) to a function calling the server.

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

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

In my code it is called as:

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

JavaScript.info callback

Answer 15

async: false

I solved it by setting async to false and restructure my Ajax call:

I set a global function called sendRequest(type, url, data) with three parameters to be called every time everywhere:

function sendRequest(type, url, data) {
    let returnValue = null;
    $.ajax({
        url: url,
        type: type,
        async: false,
        data: data,
        dataType: 'json',
        success: function (resp) {
            returnValue = resp;
        }
    });
    return returnValue;
}

Now call the function:

let password = $("#password").val();
        let email = $("#email").val();
        let data = {
            email: email,
            password: password,
        };
        let  resp =  sendRequest('POST', 'http://localhost/signin')}}", data);
        console.log(resp);

Important Note in code is : async: false

If this solution is not working with you, please note this may not working in some of browsers or jQuery versions.

Answer 16

Originally, callbacks were used for asynchronous operations (e.g., in the XMLHttpRequest API). Now promise-based APIs like the browser's Fetch API have become the default solution and the nicer async/await syntax is supported by all modern browsers and on Node.js (server side).

A common scenario - fetching JSON data from the server - can look like this:

async function fetchResource(url) {
  const res = await fetch(url);
  if (!res.ok) {
    throw new Error(res.statusText);
  }
  return res.json();
}

To use it in another function:

async function doSomething() {
  try {
    const data = await fetchResource("https://example.test/resource/1");
    // ...
  } catch (e) {
    // Handle error
    ...
  }
}

If you design a modern API, it is strongly recommended to prefer promise-based style over callbacks. If you inherited an API that relies on callbacks, it is possible to wrap it as a promise:

function sleep(timeout) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve();
    }, timeout);
  });
}

async function fetchAfterTwoSeconds(url) {
  await sleep(2000);
  return fetchResource(url);
}

In Node.js, which historically relied exclusively on callbacks, that technique is so common that they added a helper function called util.promisify.

Answer 17

After reading all the responses here and with my experiences, I would like to resume the detail of callback, promise and async/await for the asynchronous programming in JavaScript.

1) Callback: The fundamental reason for a callback is to run code in response of an event (see the example below). We use callback in JavaScript every time.

const body = document.getElementsByTagName('body')[0];
function callback() {
  console.log('Hello');
}
body.addEventListener('click', callback);

But if you must use many nested callbacks in the example below, it will be fairy terrible for the code refactoring.

asyncCallOne(function callback1() {
  asyncCallTwo(function callback2() {
    asyncCallThree(function callback3() {
        ...
    })
  })
})

2) Promise: a syntax ES6 - Promise resolves the callback hell issue!

const myFirstPromise = new Promise((resolve, reject) => {
  // We call resolve(...) when what we were doing asynchronously was successful, and reject(...) when it failed.
  // In this example, we use setTimeout(...) to simulate async code.
  // In reality, you will probably be using something like XHR request or an HTML5 API.
  setTimeout(() => {
    resolve("Success!")  // Yay! Everything went well!
  }, 250)
})

myFirstPromise
  .then((res) => {
    return res.json();
  })
  .then((data) => {
    console.log(data);
  })
  .catch((e) => {
    console.log(e);
  });

myFirstPromise is a Promise instance that represents the process of async codes. The resolve function signals that the Promise instance has finished. Afterwards, we can call .then() (a chain of .then as you want) and .catch() on the promise instance:

then — Runs a callback you pass to it when the promise has fulfilled.
catch — Runs a callback you pass to it when something went wrong.

3) Async/Await: a new syntax ES6 - Await is basically syntactic sugar for Promise!

The Async function provides us with a clean and concise syntax that enables us to write less code to accomplish the same outcome we would get with promises. Async/Await looks similar to synchronous code, and synchronous code is much easier to read and write. To catch errors with Async/Await, we can use the block try...catch. In here, you don't need to write a chain of .then() of Promise syntax.

const getExchangeRate = async () => {
  try {
    const res = await fetch('https://getExchangeRateData');
    const data = await res.json();
    console.log(data);
  } catch (err) {
    console.error(err);
  }
}

getExchangeRate();

Conclusion: These are totally the three syntaxes for asynchronous programming in JavaScript that you should well understand. So if possible, I recommend that you should use "promise" or "async/await" for refactoring your asynchronous codes (mostly for XHR requests) !

Answer 18

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)
}

Answer 19

Rather than throwing code at you, there are two concepts that are key to understanding how JavaScript 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 JavaScript 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 JavaScript just waited around for that message synchronously your app would freeze and it will suck. So JavaScript 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. - A function or handler that, when passed into another function, will be executed at a later date. A promise uses callbacks (functions passed to .then() for example) as a way to reason about this asynchronous behaviour in a more linear way. The promise is a way of saying "I promise to return something at some point" and the callback is how we handle that value that is eventually returned. 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.

Answer 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 an 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.)

Let’s 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")

Answer 21

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 a little delay in our function, so that all lines of code are not 'finished' in sequence. Thus, it will emulate the asynchronous behavior of the 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 1 second to return 'wohoo'. But as you can see, the value that's assigned to bar is the immediately returned stuff from foo(), which is nothing, i.e., just undefined.

So, how do we tackle this issue?

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

function foo(){
   return new Promise((resolve, reject) => { // I want foo() to PROMISE me something
    setTimeout ( function(){
      // Promise is RESOLVED, when the 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 => {
          console.error(err);
       })
 }

async/await version:

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

Answer 22

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.

Answer 23

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 rewritten to be a little different:

function foo() {
    var result;

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

    return result;
}

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

Answer 24

2017 answer: you can now do exactly what you want in every current browser and Node.js

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.js 8

Answer 25

Another solution is to execute code via the sequential executor nsynjs.

If the underlying function is promisified

nsynjs will evaluate all promises sequentially, and put the promise result into the 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 the underlying function is not promisified

Step 1. Wrap the function with a callback into the nsynjs-aware wrapper (if it has a 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 the result of some slow function is not ready.

More examples are here.

Answer 26

This is one of the places which two-way 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-way data binding or store concept, this issue is simply fixed for you, so in easy words, 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. The same with callback or the subscribe function on an 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(); // The call happens here
});

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

Answer 27

Use a callback() function inside the foo() success. Try it 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();

Answer 28

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

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

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

Answer 29

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 JavaScript 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. JavaScript, and other declarative languages execute one statement after another without waiting.

The classic JavaScript approach to this problem, taking advantage of the fact that JavaScript 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, "JavaScript 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 JavaScript 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 ECMAScript (ES), but it 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);
}

Answer 30

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

This means AngularJS, jQuery (with deferred), native XHR's replacement (fetch), Ember.js, Backbone.js's save or any Node.js library that returns promises.

Your code should be something along the lines of this:

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

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

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

---

The core issue

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

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

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

Here is a simple analogy for the issue:

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

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

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

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

Quick recap on promises

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

• fulfilled meaning that the computation completed successfully.
• rejected meaning that the computation failed.

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

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

With promises

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

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

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

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

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

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

Applying this

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

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

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

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

ES2015 (ES6)

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

function* foo(){ // Notice the star. This is ES6, so new browsers, Nodes.js, and io.js only
    yield 1;
    yield 2;
    while(true) yield 3;
}

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

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

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

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

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

var main = coroutine(function*(){
   var bar = yield foo(); // Wait our earlier coroutine. It returns a promise
   // The server call is done here, and the code below executes when done
   var baz = yield fetch("/api/users/" + bar.userid); // Depends on foo's result
   console.log(baz); // Runs after both requests are done
});
main();

ES2016 (ES7)

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

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

It still returns a promise just the same :)