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Promises

A promise in JavaScript is an object representing the eventual completion or failure of an asynchronous operation, with three states: pending, fulfilled, and rejected. Promises simplify handling asynchronous tasks by using resolve and reject functions to manage outcomes, and can be processed using 'then' for success and 'catch' for errors. The 'async' keyword indicates that a function returns a promise, while 'await' pauses execution until the promise settles.

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33 views9 pages

Promises

A promise in JavaScript is an object representing the eventual completion or failure of an asynchronous operation, with three states: pending, fulfilled, and rejected. Promises simplify handling asynchronous tasks by using resolve and reject functions to manage outcomes, and can be processed using 'then' for success and 'catch' for errors. The 'async' keyword indicates that a function returns a promise, while 'await' pauses execution until the promise settles.

Uploaded by

Sai Bindu
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Promises

Key points to remember


What is a promise ?

“Imagine you are a kid. Your dad promises


you that he will buy you a new toy next
week” that is a promise.
A promise has 3 states

1. Pending: You don’t know if you will get the toy

2. Fulfilled: Dad is happy and he will get you a toy

3. Rejected: Your dad is not happy, he withholds the toy


What is a promise in javascript ?

A promise is an object that may produce a single value some time in the future: either a
resolved value, or a reason that it’s not resolved.

Here also a Promise can have one of three states


● fulfilled
● rejected
● pending
Syntax of promise and stages of promise
Creation of Promises

Promises are generally used for easier handling of


asynchronous operations.

If the asynchronous operations are successful then the


expected result is returned by calling the resolve function
by the creator of the promise.

Similarly if there was some unexpected error the reasons


is passed on by calling the reject function.

Note- every promise has a PromiseState and


PromiseResult.

When we create a promise it initializes in its pending


state
Working with promises
We need to find what is the result of the promised once the executor runs.
In order to find, we have then and catch.

● 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.

● 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. output
Promise using async and await

The word “async” before a function means one simple thing: a


function always returns a promise. Other values are wrapped in
a resolved promise automatically.

The keyword await makes JavaScript wait until that promise


settles and returns its result.

Note- The function execution “pauses” at the line (*) and resumes when the promise settles, with result becoming its result. So the code
above shows “done!” in one second.

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