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. |
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. |
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. | |
| allSettled() | Get a promise when all inputs are settled that is either fulfilled or rejected. | |
| any() | Return a single promise that fulfills with the value from that promise. | |
| race() | Returns a single Promise after receiving an iterable of promises as input. | |
| reject() | Returns a Promise object that is rejected with a given reason. | |
| resolve() | Returns a Promise object that is resolved with a given value. |
- 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. | |
| then() | Deal with asynchronous tasks such as API calls. | |
| finally() | Return a Promise when a Promise is settled, that is, it is either fulfilled or rejected. |
