Promizen

Promizen is an implementation of the Promises/A+ specification, passing the promises-aplus/promises-tests test.

Prerequisites

I will not delve into the features of Promizen here, as this repository is intended primarily for JavaScript knowledge and technology sharing session, and Promizen itself is merely a compliant implementation of the Promise/A+ specification, with nothing particularly surprising. Below, I will list some of the resources that were crucial to the my development of Promizen.

• Refer to the Promises/A+ specification for detailed guidelines, If you want to implement your own Promise.

  The Promises/A+ specification focuses on the behavior of the then method, which is pivotal for handling asynchronous tasks and chaining operations in an elegant coding way.

  Beyond the core then functionality, the Promise constructor along with theresolve, reject and additional methods, play a vital role in implementing a robust and acceptable Promise (I mean, based on compliance with the Promise/A+specification, the custom Promise we implemented can exhibit behavior and usage patterns that are virtually indistinguishable from those found in modern web browsers).

  The ECMAScript specification is the most authoritative reference for obtaining the comprehensive information. However, if you find the extensive and formal documentation overwhelming, the GitHub repository Promises Unwrapping, which closely mirrors the specification, offers a more accessible alternative. Additionally, the MDN documentation on Asynchronous JavaScript provides an in-depth look at asynchronous JavaScript and Promise, making it a user-friendly resource that is easier to understand.

• To verify whether our implementation adheres to the Promise/A+ specification, we can utilize the test suite available from the Github repository promises-aplus/promises-tests. Successfully passing this test suite is the gold standard for ensuring that our Promise conforms to the Promise/A+ specification.

Install

Install with npm:

npm i promizen

Usage

const Promizen = require('promizen')

// create a new Promizen instance with an executor.
const promizen = new Promizen((resolve, reject) => {
  console.log("Hello!")
  setTimeout(() => { resolve('How are u?') }, 0)
})

// Promizen supports then chaining
// The then method returns a new Promizen instance
const newPromizen = promizen.then((value) => {
    console.log('First then:', value)
    return "I'm fine, thanks, and u?"
  }
).then((value) => {
  console.log('Second then:', value)
  throw "I'm not fine."
}).catch((reason) => {
  console.error('Catch:', reason)
})

// check the status of promizens by logging
console.log('promizen:', promizen)
setTimeout(() => { console.log('promizen:', promizen) }, 0)
console.log('newPromizen', newPromizen)
setTimeout(() => { console.log('newPromizen:', newPromizen) }, 0)

/**
 * output:
 * Hello!
 * promizen: Promizen {
 *   __fate: 'unresolved',
 *   state: 'pending',
 *   value: null,
 *   reason: null,
 *   __pendingHandlers: [
 *     {
 *       onFulfilled: [Function: fulfilledAction],
 *       onRejected: [Function: rejectedAction]
 *     }
 *   ]
 * }
 * newPromizen Promizen {
 *   __fate: 'unresolved',
 *   state: 'pending',
 *   value: null,
 *   reason: null,
 *   __pendingHandlers: []
 * }
 * First then: How are u?
 * Second then: I'm fine, thanks, and u?
 * Catch: I'm not fine.
 * promizen: Promizen {
 *   __fate: 'resolved',
 *   state: 'fulfilled',
 *   value: 'How are u?',
 *   reason: null,
 *   __pendingHandlers: [
 *     {
 *       onFulfilled: [Function: fulfilledAction],
 *       onRejected: [Function: rejectedAction]
 *     }
 *   ]
 * }
 * newPromizen: Promizen {
 *   __fate: 'resolved',
 *   state: 'fulfilled',
 *   value: undefined,
 *   reason: null,
 *   __pendingHandlers: []
 * }
 */