CODEWITHSUNDEEP
javascriptpromise
SasiKumar Chidambaram
SasiKumar Chidambaram

Reputation: 33

How Promise works?

I am learning Promise in JS and I have few queries related to it.

This is a code snippet I have,

var promise = new Promise(
  (resolve, reject) => {
    var data = 'resolve';
    resolve(data);
    console.log('promise executor');
  });

setTimeout(
  () => {
    promise.then(
      (data) => {
        console.log(data);
      });
    promise.then(
      (data) => {
        console.log(data);
      });
  }, 2000);

The output comes as,

promise executor
resolve
resolve

From the above output, I understand that body of executor has executed immediately during construction of Promise. Now, I have few doubts/queries as follows,

  1. During executor's execution, resolve was actually undefined. Still, the browser does not throw error and discards the call, how?
  2. The resolve function is set after a delay of 2 seconds. Still browser calls the function, though the executor has already executed and data variable has lost the life. How?
  3. Second time setting of resolve also gives an output. Doesn't it have limitation in number of calls to .then() ?

Upvotes: 3

Views: 1545

Answers (2)

Neha Tawar
Neha Tawar

Reputation: 705

1.A promise may be in one of 3 possible states: fulfilled, rejected, or pending. Promise users can attach callbacks to handle the fulfilled value or the reason for rejection.

The promise constructor takes one argument, a callback with two parameters, resolve and reject. Do something within the callback, perhaps async, then call resolve if everything worked, otherwise call reject.

A promise is an object which can be returned synchronously from an asynchronous function. It will be in one of 3 possible states:

Fulfilled: onFulfilled() will be called (e.g., resolve() was called)

Rejected: onRejected() will be called (e.g., reject() was called)

Pending: not yet fulfilled or rejected

A promise is settled if it’s not pending (it has been resolved or rejected). Sometimes people use resolved and settled to mean the same thing: not pending. Once settled, a promise can not be resettled. Calling resolve() or reject() again will have no effect. The immutability of a settled promise is an important feature.

2 .The fact promises do not resolve immediately sometimes and defer sometimes means that the API is consistent. Otherwise, you get undefined behavior in the order of execution.

3.Once the promise gets resolved, .then function gets called immediately so if for one promise you write n number of .then functions then all will get executed at same time with same data sent in resolve.

Upvotes: 1

CertainPerformance
CertainPerformance

Reputation: 370619

  1. resolve there is a natural parameter of the promise constructor - it does exist. (Feel free to add in console.log(typeof resolve);)
  2. At this point, the promise has run, and is now a resolved promise with the value of 'resolve'. The data variable doesn't really exist at that point, but the promise still has been resolved with that 'resolve' value. So, further .thens added to the promise will immediately run with that value.
  3. No, no limitation - that's one of the great things about promises, you can pass them around without being limited to only one .then. Check out the docs if you want: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Using_promises

Upvotes: 1

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