What does the function then() mean in JavaScript?

What does the function then() mean in JavaScript?

I’ve been seeing code that looks like:
myObj.doSome(“task”).then(function(env) {
// logic

Where does then() come from?


Solution 1:

The traditional way to deal with asynchronous calls in JavaScript has been with callbacks.
Say we had to make three calls to the server, one after the other, to set up our
application. With callbacks, the code might look something like the following (assuming
a xhrGET function to make the server call):

// Fetch some server configuration
    xhrGET('/api/server-config', function(config) {
        // Fetch the user information, if he's logged in
        xhrGET('/api/' + config.USER_END_POINT, function(user) {
            // Fetch the items for the user
            xhrGET('/api/' + user.id + '/items', function(items) {
                // Actually display the items here

In this example, we first fetch the server configuration. Then based on that, we fetch
information about the current user, and then finally get the list of items for the current
user. Each xhrGET call takes a callback function that is executed when the server

Now of course the more levels of nesting we have, the harder the code is to read, debug,
maintain, upgrade, and basically work with. This is generally known as callback hell.
Also, if we needed to handle errors, we need to possibly pass in another function to each
xhrGET call to tell it what it needs to do in case of an error. If we wanted to have just one
common error handler, that is not possible.

The Promise API was designed to solve this nesting problem and the
problem of error handling.

The Promise API proposes the following:

  1. Each asynchronous task will return a promise object.
  2. Each promise object will have a then function that can take two arguments, a success
    handler and an error handler.
  3. The success or the error handler in the then function will be called only once, after
    the asynchronous task finishes.
  4. The then function will also return a promise, to allow chaining multiple calls.
  5. Each handler (success or error) can return a value, which will be passed to the next
    function as an argument, in the chain of promises.
  6. If a handler returns a promise (makes another asynchronous request), then the next
    handler (success or error) will be called only after that request is finished.
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So the previous example code might translate to something like the following, using
promises and the $http service(in AngularJs):

    function(configResponse) {
        return $http.get('/api/' + configResponse.data.USER_END_POINT);
    function(userResponse) {
        return $http.get('/api/' + userResponse.data.id + '/items');
    function(itemResponse) {
        // Display items here
    function(error) {
        // Common error handling

Propagating Success and Error

Chaining promises is a very powerful technique that allows us to accomplish a lot of
functionality, like having a service make a server call, do some postprocessing of the
data, and then return the processed data to the controller. But when we work with
promise chains, there are a few things we need to keep in mind.

Consider the following hypothetical promise chain with three promises, P1, P2, and P3.
Each promise has a success handler and an error handler, so S1 and E1 for P1, S2 and
E2 for P2, and S3 and E3 for P3:

  .then(S1, E1) //P1
  .then(S2, E2) //P2
  .then(S3, E3) //P3

In the normal flow of things, where there are no errors, the application would flow
through S1, S2, and finally, S3. But in real life, things are never that smooth. P1 might
encounter an error, or P2 might encounter an error, triggering E1 or E2.

Consider the following cases:

• We receive a successful response from the server in P1, but the data returned is not
correct, or there is no data available on the server (think empty array). In such a
case, for the next promise P2, it should trigger the error handler E2.

• We receive an error for promise P2, triggering E2. But inside the handler, we have
data from the cache, ensuring that the application can load as normal. In that case,
we might want to ensure that after E2, S3 is called.

So each time we write a success or an error handler, we need to make a call—given our
current function, is this promise a success or a failure for the next handler in the promise

If we want to trigger the success handler for the next promise in the chain, we can just
return a value from the success or the error handler

If, on the other hand, we want to trigger the error handler for the next promise in the
chain, we can do that using a deferred object and calling its reject() method

Now What is deferred object?

Deferred objects in jQuery represents a unit of work that will be
completed later, typically asynchronously. Once the unit of work
completes, the deferred object can be set to resolved or failed.

A deferred object contains a promise object. Via the promise object
you can specify what is to happen when the unit of work completes. You
do so by setting callback functions on the promise object.

Deferred objects in Jquery : https://api.jquery.com/jquery.deferred/

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Deferred objects in AngularJs : https://docs.angularjs.org/api/ng/service/$q

Solution 2:

then() function is related to “Javascript promises” that are used in some libraries or frameworks like jQuery or AngularJS.

A promise is a pattern for handling asynchronous operations. The promise allows you to call a method called “then” that lets you specify the function(s) to use as the callbacks.

For more information see: http://wildermuth.com/2013/8/3/JavaScript_Promises

And for Angular promises: http://liamkaufman.com/blog/2013/09/09/using-angularjs-promises/

Solution 3:

To my knowledge, there isn’t a built-in then() method in javascript (at the time of this writing).

It appears that whatever it is that doSome("task") is returning has a method called then.

If you log the return result of doSome() to the console, you should be able to see the properties of what was returned.

console.log( myObj.doSome("task") ); // Expand the returned object in the
                                     //   console to see its properties.

UPDATE (As of ECMAScript6) :-

The .then() function has been included to pure javascript.

From the Mozilla documentation here,

The then() method returns a Promise. It takes two arguments: callback
functions for the success and failure cases of the Promise.

The Promise object, in turn, is defined as

The Promise object is used for deferred and asynchronous
computations. A Promise represents an operation that hasn’t completed
yet, but is expected in the future.

That is, the Promise acts as a placeholder for a value that is not yet computed, but shall be resolved in the future. And the .then() function is used to associate the functions to be invoked on the Promise when it is resolved – either as a success or a failure.

Solution 4:

Here is a thing I made for myself to clear out how things work. I guess others too can find this concrete example useful:

doit().then(function() { log('Now finally done!') });
log('---- But notice where this ends up!');

// For pedagogical reasons I originally wrote the following doit()-function so that 
// it was clear that it is a promise. That way wasn't really a normal way to do 
// it though, and therefore Slikts edited my answer. I therefore now want to remind 
// you here that the return value of the following function is a promise, because 
// it is an async function (every async function returns a promise). 
async function doit() {
  log('Calling someTimeConsumingThing');
  await someTimeConsumingThing();
  log('Ready with someTimeConsumingThing');

function someTimeConsumingThing() {
  return new Promise(function(resolve,reject) {
    setTimeout(resolve, 2000);

function log(txt) {
  document.getElementById('msg').innerHTML += txt + '<br>'
<div id='msg'></div>

Solution 5:

I suspect doSome returns this, which is myObj, which also has a then method. Standard method chaining…

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if doSome is not returning this, being the object on which doSome was executed, rest assured it is returning some object with a then method…

as @patrick points out, there is no then() for standard js

Solution 6:

Here is a small JS_Fiddle.

then is a method callback stack which is available after a promise is resolved it is part of library like jQuery but now it is available in native JavaScript and below is the detail explanation how it works

You can do a Promise in native JavaScript : just like there are promises in jQuery, Every promise can be stacked and then can be called with Resolve and Reject callbacks, This is how you can chain asynchronous calls.

I forked and Edited from MSDN Docs on Battery charging status..

What this does is try to find out if user laptop or device is charging battery. then is called and you can do your work post success.

    .then(function(battery) {
       var charging = battery.charging;
    .then(function(){alert("YeoMan : SINGH is King !!");});

Another es6 Example

function fetchAsync (url, timeout, onData, onError) {
let fetchPromised = (url, timeout) => {
    return new Promise((resolve, reject) => {
        fetchAsync(url, timeout, resolve, reject)
    fetchPromised("http://backend/foo.txt", 500),
    fetchPromised("http://backend/bar.txt", 500),
    fetchPromised("http://backend/baz.txt", 500)
]).then((data) => {
    let [ foo, bar, baz ] = data
    console.log(`success: foo=${foo} bar=${bar} baz=${baz}`)
}, (err) => {
    console.log(`error: ${err}`)

Definition :: then is a method used to solve Asynchronous callbacks

this is introduced in ES6

Please find the proper documentation here Es6 Promises