README.md

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Promise For ES

✨ A ES2021 Promise implementation based on ES3 has high compatibility, and comply with ECMA-262 and Promises/A+

The best way to learn Promise is to implement it.

Feature

1. Base on ES3, almost all browsers are supported;
2. Comply with ECMA-262 and Promises/A+, pass the Promises/A+ compliance test, and other related tests;
3. Implement the new features about Promise of ES2018、ES2020、ES2021;

Support

Ability	Version	Support
new Promise(executor)	ES2015	✔
Promise.prototype.then(onFulfilled, onRejected)	ES2015	✔
Promise.prototype.catch(onRejected)	ES2015	✔
Promise.prototype.finally(onFinally)	ES2018	✔
Promise.resolve(value)	ES2015	✔
Promise.reject(reason)	ES2015	✔
Promise.all(iterable)	ES2015	✔
Promise.race(iterable)	ES2015	✔
Promise.allSettled(iterable)	ES2020	✔
Promise.any(iterable)	ES2021	✔

Install

npm i -S promise-for-es

Usage

1. As a polyfill

// ES Module
import 'promise-for-es/polyfill';

// CommonJS
require('promise-for-es/polyfill');

2. As a ponyfill

// ES Module
import Promise from 'promise-for-es';

// CommonJS
const Promise = require('promise-for-es');

Core logic

Using the example below:

const executor = (resolutionFunc, rejectionFunc) => {
    // business logic
};
const p1 = new Promise(executor);
p1.then(onFulfilled, onRejected);

p1.then(onFulfilled, onRejected)

1. Create a new Promise object p2 ;
2. Check the state of p1 :
   1. If "pending", push onFulfilled into the fulfill list of p1, and push onRejected into the reject list;
   2. If "fulfilled", create a micro task with onFulfilled, p2 and the result of p1 ;
   3. If "rejected", create a micro task with onRejected, p2 and the result of p1 ;
3. return p2 ;

new Promise(executor)

1. Create the resolving functions: resolutionFunc, rejectionFunc ;
2. Call executor with resolutionFunc and rejectionFunc as the arguments;

resolutionFunc(value)

1. If any resolving function has been called, return;
2. If value is thenable, create a micro task with value, return;
3. Change the state of p1 to "fulfilled";
4. Create a micro task for each element of fulfill list;

rejectionFunc(reason)

1. If any resolving function has been called, return;
2. Change the state of p1 to "rejected";
3. Create a micro task for each element of reject list;

Test

1. npm run test:aplus to run Promises/A+ compliance test;
2. npm run test:es6 to run promises-es6-tests;
3. npm run test:core-js to run the core-js tests about Promise;

Reference

1. ECMA-262
2. Promises/A+