Custom Promise

Implement a custom Promise in JavaScript

Reference Shubham Khatri

Usage

Installation

pnpm add @laqudee/custom-promise

examples

{
  "name": "custom-promise-test",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  //   "type": "module",  // if add type: module; test:cjs can't work
  "scripts": {
    "test": "node src/test.es.js",
    "test:es": "node src/test.es.mjs",
    "test:cjs": "node src/test.cjs.js"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "dependencies": {
    "@laqudee/custom-promise": "^1.0.0"
  }
}

// for cjs test.cjs.js
const CustomPromise = require('custom-promise')

// for es test.es.js with type: module(in package.json) or test.es.mjs
import CustomPromise from 'custom-promise'

Local Development

# build
pnpm build

# link fro global
pnpm link --global

# when use custom-promise
pnpm link --global @laqudee/custom-promise

# test
pnpm test:es
pnpm test:cjs

Promises/A+ Compliance Test Suite

This suite tests compliance of a promise implementation with the Promises/A+ specification.

Promises/A+ CTS

• add promise-aplus-tests

pnpm add promises-aplus-tests -D

• add script cammand

"scripts": {
  "test": "promises-aplus-tests promise-a-plus-test.js"
}

• run test

pnpm test

This lib is temporarily uninstalled because it uses an older version of mocha that is causing some security issues. You can use this lib locally to test custom promises, but make sure it is uninstalled when you publish your own repository or npm package.

• NPM or Github complains about security issues

pnpm dev Instructions

• pnpm dev will use node and esm execute the src/index.js file.

Implement Process

States, Handlers and value

• States:

  • Pending
  • Fulfilled
  • Rejected

• Handlers

  • then()
  • catch()
  • finally()

• value

  • resolve() return value
  • reject() return reason

Start Coding

A Promise is executed as soon as it is created, so callback function will be called inside the constructor with reject and resolve methods passed as paramenters to it.

refer to: Building Promises from scratch