JavaScript Promises for Beginners

#javascript #beginners #webdev #tutorial

To get the most out of this article, please make sure that you know basics about Synchronous and Asynchronous Programming and, if possible, about javascript callbacks.
However, I will try to keep most of the stuff away from these topics so that you can understand atleast 85% of it.

Understanding Promises

Suppose a friend comes to you for financial help and you promise him to give some cash after taking it out from an ATM.
--You go to the ATM, swipe your card, and find out that your account has been frozen due to some reasons and you can't take out your money.
-- Then, you go back to your friend and tell him that you can't give him money because your account is frozen. Hence, breaking your promise.

//You have made promise to your friend and you leave for bank
if (everythingGoesWell) {
return theMoney //Promise is fulfilled
}
else {
return "Account_Frozen" //Promise failed
}

Promises are used in JavaScript to handle async operations. They are special objects that link the actual output (in above example, it's required money) and the reason due to which we may not get the output (Account Frozen).

States of a Promise

A JavaScript promise is either settled or pending. We can use the Promise constructor to create a promise.

var thePromise = new Promise(executor())

The executor function is callback of the promise constructor. It takes two callbacks: resolve and reject, as arguments.

The resolve callback is used when the promise is actually fulfilled. It takes the output value as its argument.
The reject callback is used when the promise couldn't be fulfilled. It takes the reason as its argument.

When we create a promise it initializes in its pending state. Once the executor runs, the state changes to settled which can be either resolved or rejected.

Implementation: Code Snippet

Let's create a promise for our problem here:

var thePromise = new Promise(function(resolve,reject){
// Try to take out money from atm
withdraw(function(error,money) {
// If withdrawal failed, tell why
if (error) reject (error)
// else return the money
else resolve (money)
})
})

Promises can be bit of hectic stuff if you don't have experience with callbacks and callback hells. But once you grasp this concept, there is no going back !

The `then` and `catch` clauses

The catch function is attached to promise which executes when the promise is rejected. It takes the error sent through reject method as an argument.
The then function is attached to promise which executes when the promise is resolved. It sends the value sent through resolve method as an argument.
There is a finally clause too. It is executed no matter the promise resolves or rejects. It takes no arguments.

var thePromise = new Promise(function(resolve,reject){
// Try to take out money from atm
withdraw(function(error,money) {
// If withdrawal failed, tell why
if (error) reject (error)
// else return the money
else resolve (money)
})
})

thePromise.then(function(money) {
// This function is executed when the above promise resolves and returns an //amount of money
})

thePromise.catch(function(error) {
// This function is executed when the above promise rejects due to an error
})

thePromise.finally(function() {
// This function is executed after the promise is resolved or rejected.
})