reverse-iterable-map

reverse-iterable-map.js

A reverse-iterable map implementation based on the built-in Map object.

Table of Contents

• Install
• Examples
• Tests
• Why?
• But why a map?

Install

… for JavaScript

Downloads an ES module file.

curl -O https://github.com/kleinfreund/reverse-iterable-map.js/blob/master/src/reverse-iterable-map.js

import { ReverseIterableMap } from './src/reverse-iterable-map.js';

const map = new ReverseIterableMap();

… for Node.js (8.5+)

Installs the node package as a dependency. It doesn’t have any non-development dependencies itself.

npm install --save reverse-iterable-map

import { ReverseIterableMap } from 'reverse-iterable-map';

const map = new ReverseIterableMap();

Note, that Node.js version 8.5 or higher is required, as it comes with support for ES modules. If you don’t want to use it as an ES module, you will need to transpile the package yourself.

Examples

… on the website::

kleinfreund.github.io/reverse-iterable-map.js

Prints test results to the console.

… on a local HTTP server:

npm run example

Prints test results to the console.

Tests

… with Node’s experimental ES module feature:

npm test

Why?

Part of the additions to ECMAScript 2015 are the iteration protocols: Iterable and iterator. The former allows arbitrary objects to become iterable. Following the rules of the protocol gives one iteration capabilities via the following techniques:

• for...of statement
• Array.from() method
• Spread syntax

However, only the iteration in one direction is considered by the specification at the time. This means that we only get forward-iteration by default.

Now, with the iteration protocols, we could redefine the iteration behavior for our purpose and make an object backwards-iterable. At the same time, this means losing the ability to iterate forwards.

If you need both a forwards- and backwards-iterable object, this implementation might be for you.

But why a map?

That’s what I needed. To be precise, I needed to access an iterator at a specific location in my data structure and be able to iterate in both directions.

I tried to stick to the Map interface as close as possible.

Implementing a reverse-iterable array, for example, can be accomplished by using the same techniques of this implementation.