JavaScript Promise Reference

JavaScript Promise is used to handle asynchronous operations JavaScript. They are easy to manage when dealing with multiple asynchronous operations where callbacks can create callback hell leading to unmanageable code. Promises are the ideal choice for handling asynchronous operations in the simplest manner. They can handle multiple asynchronous operations easily and provide better error handling than callbacks and events.

Syntax

Promise.function();

Example:

Javascript

// Illustration of Promise.allSettled()
// Method in Javascript with Example

const p1 = Promise.resolve(50);
const p2 = new Promise((resolve, reject) =>
setTimeout(reject, 100, 'geek'));
const prm = [p1, p2];

Promise.allSettled(prm)
.then((results) => results.forEach((result) =>
console.log(result.status,result.value)));

Output:

"fulfilled"
50
"rejected" 
undefined

The complete list of JavaScript Promise is listed below:

JavaScript Promise Constructor: In JavaScript, a constructor gets called when an object is created using the new keyword.

Constructor	Description	Example
Promise	Handle asynchronous operations in JavaScript.	Try

JavaScript Promise Properties: A JavaScript property is a member of an object that associates a key with a value.

• Instance Property: An instance property is a property that has a new copy for every new instance of the class.

Instance Properties	Description	Example
constructor	Return the promise constructor function for the object.	Try

JavaScript Promise Methods: JavaScript methods are actions that can be performed on objects.

• Static Method: If the method is called using the array class itself then it is called a static method.

Static Methods	Description	Example
all()	Handle all the asynchronous operations), that take an array of promises(an iterable) as input.	Try
allSettled()	Get a promise when all inputs are settled that is either fulfilled or rejected.	Try
any()	Return a single promise that fulfills with the value from that promise.	Try
race()	Returns a single Promise after receiving an iterable of promises as input.	Try
reject()	Returns a Promise object that is rejected with a given reason.	Try
resolve()	Returns a Promise object that is resolved with a given value.	Try

• Instance Method: If the method is called on an instance of a promise then it is called an instance method.

Instance Methods	Description	Example
catch()	The return value of the callback if it is called.	Try
then()	Deal with asynchronous tasks such as API calls.	Try
finally()	Return a Promise when a Promise is settled, that is, it is either fulfilled or rejected.	Try