Make You a Promise for Great Good!

"It is easy to make promises - it is hard work to keep them."

-- Boris Johnson

Workshop

This workshop is a starter-project with tests for implementing a fully-compliant Promise constructor.

implement your Promise constructor in ./src/adapter.js and run:

npm install
npm test

To see fully implemented examples go to ./src/examples/ and/or run:

npm run test:native
npm run test:bluebird

Quick Refresher

A promise is an object or function with a then method whose behavior conforms to the specification
A promise must be in one of three states: pending, fulfilled, or rejected.
A promise must provide a then method which accepts two arguments: promise.then(onFulfilled, onRejected)
For details, go to https://promisesaplus.com

Notes, Tips and Recommendations

The tests cover a lot of edge cases, don't feel obligated to make them pass in the order they appear
It might be easier to implement the resolve logic first, and defer implementing the reject logic. They have a lot of similarities
If promise2 = promise1.then(onFulfilled, onRejected), implementations may allow promise2 === promise1, provided all requirements are met. I personally found it more convenient to implement promise2 !== promise1
npm install lodash
Don't accidentally use the native Promise in your implementation

Background

A promise represents the eventual result of an asynchronous operation.

The Promise constructor was introduced in ECMAScript 6 (June 2015) and gave a native implementation to the Promise pattern. Prior to this, a number of JS libraries popped up, including the popular bluebird.js (link)

Original proposal: http://wiki.commonjs.org/wiki/Promises/A
Extended Spec: https://promisesaplus.com / https://github.com/promises-aplus/promises-spec
Promises/A+ Compliance Test Suite: https://github.com/promises-aplus/promises-tests

FP Anecdote