Covenant NPM | npm.io

covenant

Covenant is a 64-line Promises/A+ implementation written in Coffeescript. Covenant passes the Promises/A+ Test Suite. It compiles to under 200 lines of javascript, less than 500 bytes without compression.

Installation

Download it, clone it, or npm install wizardwerdna/covenant

Covenant has no dependencies, but does use process.nextTick, found in modern Browsers. If process.nextTick is not a function, Covenant falls back to setImmediate, and then to setTimeout. If you are using ancient browsers, it is highly recommended that you use a shim to implement (fake) nextTick and/or setImmediate.

The API

// load the coffeescript compiler (unless running precompiled version)
// and then follow up with obvious variations of the coffeescript below
require('coffee-script');

# alternatively, simply transpile a direct javascript file and use that
# instead
coffee -c node_modules/covenant/covenant.coffee

# get the Promise class
Covenant = require('covenant').Covenant

# create a new pending promise
covenant = new Covenant

# fulfill it
covenant.fulfill(value)

# reject it
covenant.reject(reason)

# schedule asynchronous handers
covenant.then onFulfilled, onRejected

Why another promise implementation?

I set out to write Covenant, just for myself, so to achieve a better understanding of the nuances of the Promise pattern. The Promises/A+ specification seemed elegant, but I couldn't seem to grok it without more. Reading the code of various compliant mplementations was helpful, but I still didn't seem to own it. I began the experiment by "cowboy" coding a first set, using the test suite to verify that things were working. Finally, I discarded that code as one to throw away, and rebuilt it in pure BDD red-green-refactor style.

Having a well-understood testbed for promises, I will probably extend covenant to a more full-featured implementation.

Credits

I am indebted, in particular, to the work of Brian Cavalier, whose when.js, and avow.js libraries illuminate what can be done both in a full-featured and minimalist implementation.