Implement your own Promises in JavaScript
Custom implementation of then, catch and finally methods
Promises are one of the most fundamental concepts in JavaScript that all of us have used many times in our applications but can we implement our own Promise API?
Don’t worry it is not as complicated as it looks.
In this post, we will implement a basic Promise API ourselves.
What is a Promise?
The Promise object represents the eventual completion (or failure) of an asynchronous operation and its resulting value.
It can be in one of the three states:
PENDING, initial state when an operation is in progress
FULFILLED, define that the operation was successful
REJECTED, denotes a failure in an operation
Note: A promise is said to be settled when it is either fulfilled or rejected. (we are going to use this term a lot in this article)
How do you use a Promise?
Let’s first look at its skeleton for implementing promises, essentially the input it takes, and the methods it exposes.
It has a constructor function that takes a callback, and methods like then, catch, and finally.
const promise = new Promise((resolve, reject) => {
/*
Your code logic goes here and you call resolve(value)
or reject(error) to resolve or reject the promise
*/
})
promise.then((value) => {
// Code logic on success of an operation
}).catch(error => {
// Code logic on failure of an operation
}).finally(() => {
// Code logic to be executed after completion of operation
})
1. Defining the skeleton
We start by defining our Promise class MyPromise.
Following properties are defined in the constructor:
state
: can be either bePENDING
,FULFILLED
orREJECTED
handlers
: stores callbacks of then, catch, finally methods. (Handlers will only be executed when a promise is settled.)value
: resolve or rejected value.
Note: A promise is executed as soon as it is created, which means our promise callback function will be called inside the constructor with reject
and resolve
methods passed as parameters to it.
const STATE = {
PENDING: 'PENDING',
FULFILLED: 'FULFILLED',
REJECTED: 'REJECTED',
}
class MyPromise {
constructor(callback) {
// Initial state of Promise is empty
this.state = STATE.PENDING;
this.value = undefined;
this.handlers = [];
// Invoke callback by passing the _resolve and the _reject function of our class
try {
callback(this._resolve, this._reject);
} catch (err) {
this._reject(err)
}
}
_resolve = (value) => {}
_reject = (error) => {}
then(onSuccess, onFail) {
}
catch(onFail) {
}
finally(callback) {
}
}
2. _resolve() and _reject() method implementation
_resolve()
or _reject()
set the state
of promise to FULFILLED
or REJECTED
respectively, updates the value
property and executes the attached handlers.
Note: Nothing happens if we try to call
_resolve()
or_reject()
on an already settled Promise.
_resolve = (value) => {
this.updateResult(value, STATE.FULFILLED);
}
_reject = (error) => {
this.updateResult(error, STATE.REJECTED);
}
updateResult(value, state) {
// This is to make the processing async
setTimeout(() => {
/*
Process the promise if it is still in a pending state.
An already rejected or resolved promise is not processed
*/
if (this.state !== STATE.PENDING) {
return;
}
// check is value is also a promise
if (isThenable(value)) {
return value.then(this._resolve, this._reject);
}
this.value = value;
this.state = state;
// execute handlers if already attached
this.executeHandlers();
}, 0);
}
Wondering what is isThenable(value)
in the above code?
Well for a case where a Promise is resolved/rejected with another promise, we have to wait for it to complete and then process our current Promise.
isThenable() function implementation
An isThenable
function checks if value is an instance of MyPromise
or it is an object containing a then
function.
function isThenable(val) {
return val instanceof MyPromise;
}
// or
function isThenable(value) {
if (typeof value === "object" && value !== null && value.then && typeof value.then === "function") {
return true;
}
return false;
}
3. then() method implementation
then()
method takes two arguments as callbacks onSuccess
and onFail
. onSuccess
is called if Promise was fulfilled and onFail
is called if Promise was rejected.
“Remember that Promises can be chained”. The essence of Promise chaining is that the
then()
method returns a new Promise object. That is how promises can be chained. This is especially useful in scenarios where we need to execute two or more asynchronous operations back to back, where each subsequent operation starts when the previous operation succeeds, with the result from the previous step.
Callbacks passed to then()
are stored in handlers
array using addHandlers
function. A handler is an object {onSuccess, onFail}
which will be executed when a promise is settled.
Our implementation of then()
looks like this:
then(onSuccess, onFail) {
return new MyPromise((res, rej) => {
this.addHandlers({
onSuccess: function(value) {
// if no onSuccess provided, resolve the value for the next promise chain
if (!onSuccess) {
return res(value);
}
try {
return res(onSuccess(value))
} catch(err) {
return rej(err);
}
},
onFail: function(value) {
// if no onFail provided, reject the value for the next promise chain
if (!onFail) {
return rej(value);
}
try {
return res(onFail(value))
} catch(err) {
return rej(err);
}
}
});
});
}
addHandlers(handlers) {
this.handlers.push(handlers);
this.executeHandlers();
}
executeHandlers() {
// Don't execute handlers if promise is not yet fulfilled or rejected
if (this.state === STATE.PENDING) {
return null;
}
// We have multiple handlers because add them for .finally block too
this.handlers.forEach((handler) => {
if (this.state === STATE.FULFILLED) {
return handler.onSuccess(this.value);
}
return handler.onFail(this.value);
});
// After processing all handlers, we reset it to empty.
this.handlers = [];
}
4. catch() method implementation
catch()
is implemented using then()
. We call then()
method with the onSuccess
callback as null
and pass onFail
callback as second argument.
/*
Since then method take the second function as onFail,
we can leverage it while implementing catch
*/
catch(onFail) {
return this.then(null, onFail);
}
5. finally() method implementation
Before we start implementing the finally()
method, let us understand its behaviour first (Took me sometime to understand it myself).
From MDN docs:
The
finally()
method returns aPromise
. When the promise is settled, i.e either fulfilled or rejected, the specified callback function is executed. This provides a way for code to be run whether the promise was fulfilled successfully or rejected once thePromise
has been dealt with.The
finally()
method is very similar to calling.then(onFinally, onFinally)
however there are a couple of differences:When creating a function inline, you can pass it once, instead of being forced to either declare it twice, or create a variable for it
Unlike
Promise.resolve(2).then(() => {}, () => {})
(which will be resolved withundefined
),Promise.resolve(2).finally(() => {})
will be resolved with2
.Similarly, unlike
Promise.reject(3).then(() => {}, () => {})
(which will be fulfilled withundefined
),Promise.reject(3).finally(() => {})
will be rejected with3
.
finally()
method returns a Promise which will be settled with previous fulfilled
or rejected
value.
// Finally block returns a promise which fails or succeedes with the previous promise resove value
finally(callback) {
return new MyPromise((res, rej) => {
let val;
let wasRejected;
this.then((value) => {
wasRejected = false;
val = value;
return callback();
}, (err) => {
wasRejected = true;
val = err;
return callback();
}).then(() => {
// If the callback didn't have any error we resolve/reject the promise based on promise state
if(!wasRejected) {
return res(val);
}
return rej(val);
})
})
}
Check out the full code implementation in the codepen below:
Summary
We emulated the basic implementation of Promises. There is a lot more to it than then()
, catch()
, finally()
methods which are the instance methods. There are static methods as well which I will try to cover in my future posts.
I hope you enjoyed the article.
Thank you for reading...
If you have any suggestions or questions, please feel free to comment or DM me on Twitter