Understand JavaScript promises actually work from the inside out

Understand JavaScript promises actually work from the inside out

Understand JavaScript promises actually work from the inside out

  • by admin
  • Web Development

One of the most important questions I faced in interviews was how promises are implemented. Since async/await is becoming more popular, you need to understand promises.

What is a Promise?

A promise is an object which represents the result of an asynchronous operation which is either resolved or rejected (with a reason).

There are 3 states

  • Fulfilled: onFulfilled() will be called (e.g., resolve() was called)
  • Rejected: onRejected() will be called (e.g., reject() was called)
  • Pending: not yet fulfilled or rejected

So let’s see how’s it is implemented:


According to the definition at Mozilla: It takes an executor function as an argument.

function noop() {} 
function Promise(executor) {
  if (typeof this !== 'object') {
    throw new TypeError('Promises must be constructed via new');
 if (typeof executor !== 'function') {
   throw new TypeError('Promise constructor\'s argument is not a function');
  this._deferredState = 0;
  this._state = 0;
  this._value = null;
  this._deferreds = null;
  if (executor === noop) return;
  doResolve(executor, this);

Looks like a simple function with some properties initialized to 0 or null. Here are a few things to notice:

this._state property can have three possible values as described above:

0 - pending
1 - fulfilled with _value
2 - rejected with _value
3 - adopted the state of another promise, _value

Its value is0 (pending) when you create a new promise.

Later doResolve(executor, this) is invoked with executor and promiseobject.

Let’s move on to the definition of doResolve and see how it’s implemented.

* Take a potentially misbehaving resolver function and make sure
* onFulfilled and onRejected are only called once.
* Makes no guarantees about asynchrony.
function doResolve(fn, promise) {
  var done = false;
  var resolveCallback = function(value) {
      if (done) return;
      done = true;
      resolve(promise, value);
 var rejectCallback = function(reason) {
   if (done) return;
   done = true;
   reject(promise, reason);
var res = tryCallTwo(fn, resolveCallback, rejectCallback);
  if (!done && res === IS_ERROR) {
    done = true;
    reject(promise, LAST_ERROR);

Here it is again calling tryCallTwo function with executor and 2 callbacks. The callbacks are again calling resolve and reject

The done variable is used here to make sure the promise is resolved or rejected only once, so if you try to reject or resolve a promise more than once then it will return because done = true.

function tryCallTwo(fn, a, b) {
   try {
    fn(a, b);
   } catch (ex) {
     LAST_ERROR = ex;
     return IS_ERROR;

This function indirectly calls the main executor callback with 2 arguments. These arguments contain logic on how resolve or reject should be called. You can check resolveCallback and rejectCallback in doResolve function above.

If there is an error during execution it will store the error in LAST_ERROR and return the error.

Before we jump to the resolve function definition, let’s check out the .thenfunction first:

Promise.prototype.then = function(onFulfilled, onRejected) {
   if (this.constructor !== Promise) {
     return safeThen(this, onFulfilled, onRejected);
   var res = new Promise(noop);
   handle(this, new Handler(onFulfilled, onRejected, res));
   return res;
function Handler(onFulfilled, onRejected, promise) {
   this.onFulfilled = typeof onFulfilled === "function" ? onFulfilled  : null;
   this.onRejected = typeof onRejected === "function" ? onRejected :  null;
   this.promise = promise;

So in the above function, then is creating new promise and assigning it as a property to a new function called Handler. The Handler function has arguments onFulfilled and onRejected. Later it will use this promise to resolve or reject with value/reason.

As you can see, the .then function is calling again another function:

handle(this, new Handler(onFulfilled, onRejected, res));


function handle(self, deferred) {
  while (self._state === 3) {
    self = self._value;
  if (Promise._onHandle) {
  if (self._state === 0) {
     if (self._deferredState === 0) {
         self._deferredState = 1;
         self._deferreds = deferred;
    if (self._deferredState === 1) {
       self._deferredState = 2;
       self._deferreds = [self._deferreds, deferred];
   handleResolved(self, deferred);
  • There is a while loop which will keep assigning the resolved promise object to the current promise which is also a promise for _state === 3
  • If _state = 0(pending) and promise state has been deferred until another nested promise is resolved, its callback is stored in self._deferreds
function handleResolved(self, deferred) {
   asap(function() { // asap is external lib used to execute cb immediately
   var cb = self._state === 1 ? deferred.onFulfilled :     deferred.onRejected;
   if (cb === null) {
       if (self._state === 1) {
           resolve(deferred.promise, self._value);
       } else {
         reject(deferred.promise, self._value);
  var ret = tryCallOne(cb, self._value);
    if (ret === IS_ERROR) {
       reject(deferred.promise, LAST_ERROR);
    } else {
      resolve(deferred.promise, ret);

What's happening:

  • If the state is 1(fulfilled) then call the resolve else reject
  • If onFulfilled or onRejected is null or if we used an empty .then()resolved or reject will be called respectively
  • If cb is not empty then it is calling another function tryCallOne(cb, self._value)
function tryCallOne(fn, a) {
   try {
     return fn(a);
   } catch (ex) {
      LAST_ERROR = ex;
     return IS_ERROR;

tryCallOne : This function only calls the callback that is passed into the argument self._value. If there is no error it will resolve the promise, otherwise it will reject it.

Every promise must supply a .then() method with the following signature:

  onFulfilled?: Function,
  onRejected?: Function
) => Promise
  • Both onFulfilled() and onRejected() are optional.
  • If the arguments supplied are not functions, they must be ignored.
  • onFulfilled() will be called after the promise is fulfilled, with the promise’s value as the first argument.
  • onRejected() will be called after the promise is rejected, with the reason for rejection as the first argument.
  • Neither onFulfilled() nor onRejected() may be called more than once.
  • .then() may be called many times on the same promise. In other words, a promise can be used to aggregate callbacks.
  • .then() must return a new promise.

Promise Chaining

.then should return a promise. That's why we can create a chain of promises like this:

.then(() => 
  Promise.then(() => 
   Promise.then(result => result) 

Resolving a promise

Let’s see the resolve function definition that we left earlier before moving on to .then():

function resolve(self, newValue) {
   if (newValue === self) {
      return reject(
        new TypeError("A promise cannot be resolved with itself.")
   if (
      newValue &&
     (typeof newValue === "object" || typeof newValue === "function")
   ) {
    var then = getThen(newValue);
    if (then === IS_ERROR) {
      return reject(self, LAST_ERROR);
   if (then === self.then && newValue instanceof Promise) {
      self._state = 3;
     self._value = newValue;
   } else if (typeof then === "function") {
      doResolve(then.bind(newValue), self);
   self._state = 1;
   self._value = newValue;
  • We check if the result is a promise or not. If it’s a function, then call that function with value using doResolve().
  • If the result is a promise then it will be pushed to the deferreds array. You can find this logic in the finale function.

Rejecting a promise:

Promise.prototype['catch'] = function (onRejected) {
   return this.then(null, onRejected);

The above function can be found in ./es6-extensions.js.

Whenever we reject a promise, the .catch callback is called which is a sugar coat for then(null, onRejected).

Here is the basic rough diagram that I have created which is a birds-eye view of what's happening inside:

Let’s see once again how everything is working:

For example, we have this promise:

new Promise((resolve, reject) => {
   setTimeout(() => {
    resolve("Time is out");
  }, 3000)
.then(console.log.bind(null, 'Promise is fulfilled'))
.catch(console.error.bind(null, 'Something bad happened: '))
  1. Promise constructor is called and an instance is created with new Promise
  2. executor function is passed to doResolve(executor, this) and callback where we have defined setTimeout will be called by tryCallTwo(executor, resolveCallback, rejectCallback)so it will take 3 seconds to finish
  3. We are calling .then() over the promise instance so before our timeout is completed or any async api returns, Promise.prototype.then will be called as .then(cb, null)
  4. .then creates a new promise and passes it as an argument to new Handler(onFulfilled, onRejected, promise)
  5. handle function is called with the original promise instance and the handler instance we created in point 4.
  6. Inside the handle function, current self._state = 0 and self._deferredState = 0 so self_deferredState will become 1 and handler instance will be assigned to self.deferreds after that control will return from there
  7. After .then() we are calling .catch() which will internally call .then(null, errorCallback) — again the same steps are repeated from point 4 to point 6 and skip point 7 since we called .catch once
  8. Current promise state is pending and it will wait until it is resolved or rejected. So in this example, after 3 seconds, setTimeout callback is called and we are resolving this explicitly which will call resolve(value).
  9. resolveCallback will be called with value Time is out :) and it will call the main resolve function which will check if value !== null && value == 'object' && value === 'function'
  10. It will fail in our case since we passed string and self._state will become 1 with self._value = 'Time is out' and later finale(self) is called.
  11. finale will call handle(self, self.deferreds) once because self._deferredState = 1, and for the chain of promises, it will call handle() for each deferred function.
  12. In the handle function, since promise is resolved already, it will call handleResolved(self, deferred)
  13. handleResolved function will check if _state === 1 and assign cb = deferred.onFulfilled which is our then callback. Later tryCallOne(cb, self._value) will call that callback and we get the final result. While doing this if any error occurred then promise will be rejected.

When a promise is rejected

In this case, all the steps will remain the same — but in point 8 we call reject(reason). This will indirectly call rejectCallback defined in doResolve() and self._state will become 2. In the finale function cbwill be equal to deferred.onRejected which will be called later by tryCallOne. That’s how the .catch callback will be called.

That's all for now! I hope you enjoyed the article and it helps in your next JavaScript interview.

If you encounter any problem checkout JavaScript TutorialsI would be happy to help 🙂

Don’t hesitate to clap if you considered this a worthwhile read!

Tags: JavaScript