Promises in JS.md

Promises in JS

The Promise object represents the eventual completion (or failure) of an asynchronous operation and its resulting value.

A promise is a special JavaScript object that links the “producing code” and the “consuming code” together.

The “producing code” takes whatever time it needs to produce the promised result, and the “promise” makes that result available to all of the subscribed code when it’s ready.

The constructor syntax for a promise object is:

let promise = new Promise(function(resolve, reject) {
  	// executor (the producing code)
    // do something
    const result = 'some result'
  	if(success) {
        resolve(result)
    } else {
        reject(new Error('error'))
    }
});

Its arguments resolve and reject are callbacks provided by JavaScript itself. Our code is only inside the executor.

When the executor obtains the result, be it soon or late, doesn’t matter, it should call one of these callbacks:

• resolve(value) — if the job is finished successfully, with result value.
• reject(error) — if an error has occurred, error is the error object.

So to summarize: the executor runs automatically and attempts to perform a job. When it is finished with the attempt, it calls resolve if it was successful or reject if there was an error.

The promise object returned by the new Promise constructor has these internal properties:

• state — initially "pending", then changes to either "fulfilled" when resolve is called or "rejected" when reject is called.

  A Promise is in one of these states:

  • pending: initial state, neither fulfilled nor rejected.
  • fulfilled: meaning that the operation was completed successfully.
  • rejected: meaning that the operation failed.

• result — initially undefined, then changes to value when resolve(value) called or error when reject(error) is called.