Negative Array Indexes

2013-11-15 00:00:00 +0000 by Alex R. Young

Sindre Sorhus sent in negative-array, a module for supporting negative array indexes. It's built using ES6's Proxy.

Proxies allow you to run methods when certain conditions are met, which means things like profilers become easier to implement. They're created with var proxy = Proxy(target, handler), where target is an object that will be wrapped with the proxy, and handler is an object that implements the proxy API.

The handler can include methods like has, defineProperty, getPrototypeOf, and more, for controlling access to an object. For more details on how this works, see the Direct Proxies page on the ECMAScript Harmony Wiki.

Sindre's module allows you to do this:

var negativeArray = require('negative-array');

// adds negative array index support to any passed array
var unicorn = negativeArray(['pony', 'cake', 'rainbow']);

// get the last item by using an negative index

It'll work in Node 0.8+ with the --harmony flag, and Chrome with Harmony enabled. Visit chrome://flags/#enable-javascript-harmony to set it up.

The implementation is what will probably become a classic pattern: Proxy is used to wrap the array instance with get and set methods that dynamically map the requested array index to something native JavaScript can handle.

Proxy(arr, {
  get: function (target, name) {
    var i = +name;
    return target[i < 0 ? target.length + i : i];
  set: function (target, name, val) {
    var i = +name;
    return target[i < 0 ? target.length + i : i] = val;

I like this example because it adds new functionality that feels like a language feature without changing built-in prototypes. It's clean and fairly easy to understand once you know what Proxy does. If you wanted to learn about proxies but couldn't find any good examples, then check out the source on GitHub.