CODEWITHSUNDEEP
javascript
Simran Kaur
Simran Kaur

Reputation: 1101

difference between resolve and onFulfilled

What is the difference between

resolve and onFulfilled in javascript promises ?

Similarly, what is the difference between

reject and onRejected ? In simple words, I would just ask how does onsuccess callback of Promise.then(onsuccess, onreject) differs from Promise.resolve ()?

I am reading Javascript with Promises by Daniel Parker. The book has mentioned both of them but I have not realized the difference between two yet.

While describing then in promises:

promise.then

promise.then([onFulfilled], [onRejected]) returns promise

The promise.then() method accepts an onFulfilled callback and an onRejected callback. People generally register onRejected callbacks using promise.catch() instead of passing a second argument to then . The function then returns a promise that is resolved by the return value of the onFulfilled or onRejected callback. Any error thrown inside the callback rejects the new promise with that error.

Also,

Promise.resolve

Promise.resolve([value|promise]) returns promise The Promise.resolve() function is a convenience function for creating a promise that is already resolved with a given value. If you pass a promise as the argument to Promise.resolve(), the new promise is bound to the promise you provided and it will be fulfilled or rejected accordingly.

Code:

function settled(promises) {
 var alwaysFulfilled = promises.map(function (p) {
 return p.then(
 function onFulfilled(value) {
 return { state: 'fulfilled', value: value };
 },
 function onRejected(reason) {

 return { state: 'rejected', reason: reason };
 }
 );
 });
 return Promise.all(alwaysFulfilled);
}
};

Upvotes: 5

Views: 6018

Answers (1)

Jaromanda X
Jaromanda X

Reputation: 1

simply put

when a promise is resolved any current or future onFullfilled functions will be called with the parameter to that function being the value of the resolve function

similarly with reject/onRejected

Upvotes: 2

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