Extending JavaScript Natives

Most built-in JavaScript types are constructors whose prototypes contain the methods and other properties that define their default behavior:

//(results will vary by browser)

Object.getOwnPropertyNames(Function.prototype)
//["bind", "arguments", "toString", "length", "call", "name", "apply", "caller", "constructor"]

You can’t delete or replace a native prototype, but you can edit the values of its properties, or create new ones:

//create a new array method that removes a member
Array.prototype.remove = function(member) {
  var index = this.indexOf(member);
  if (index > -1) {
    this.splice(index, 1);
  }
  return this;
}

['poppy', 'sesame', 'plain'].remove('poppy'); //["sesame", "plain"]
['ant', 'bee', 'fly'].remove('spider'); //["ant", "bee", "fly"]

Et voila! Our code gets a useful array extension for free. However if you brag about doing this in production code, expect to get pummeled by a wave of fierce disapproval. Some of it carries weight. Let’s sift the danger from the dogma and try to reach an honest conclusion:

Fixing the JavaScript typeof operator

Working with JavaScript’s typeof operator is a bit like operating a clapped-out old car (or an early model Dell Inspiron). It gets the job done (mostly) and you learn to work around the quirks – but you probably aspire to something better.

In this article I’ll give a brief overview of typeof before introducing a tiny new function which is a fully-loaded, more reliable alternative that works directly with the language internals.

Waldo: Search the JavaScript Object Model in under 1 KB

Here’s a tiny util that you can save as a bookmarklet and use to crawl the JavaScript object model of any web site. Waldo (thanks to @shinypb for the name!) lets you find and inspect instances by name, type or value, and it can be easily customized to add additional tests. It runs in the console on Chrome, Firefox, Safari and IE>8. It's sourced on github. Feel free to fork it if you want to add more search methods or a spiffy UI.

(Update: Check out this alternate version by @jdalton)

A fresh look at JavaScript Mixins

(Russian, Japanese)

In this article I’ll explore JavaScript mixins in detail, and introduce a less conventional, but to my mind more natural mixin strategy that I hope you’ll find useful. I’ll finish up with a profiler matrix summarizing the performance impact of each technique. [A big Thank You to the brilliant @kitcambridge for reviewing and improving the code on which this blog is based!]

JavaScript Strict Mode

The fifth edition of the ECMAScript specification introduced Strict Mode. Strict Mode imposes a layer of constraint on JavaScript – intended to protect you from the more perilous aspects of the language.

While researching this article I wrote 38 tests covering all the Strict Mode rules as defined in the ES5 specification. You can see how your favorite browser shapes up by clicking here.

The code for each test is reproduced at the end of the article as an aid to understanding the specification. You can also run the tests manually by copying and pasting the source into the console. The full source code is on my github repo.

Firefox 4 and IE10 (preview 1) already fully support Strict Mode, and Chrome 12 is nearly there. Strict Mode is here to stay – let’s dive in…

The JavaScript Comma Operator

(на русском, 日本)
 
Let’s begin with a funny tweet:

The ‘c’ at the end is for the lowly comma operator. Last in the line of operator precedence and rarely documented, the comma operator hides its light under a bushel. It may not be a JavaScript heavy-hitter but I like it anyway. Its simple, elegant and you should make it your friend. So, here we go – more than you’ll ever need to know about JavaScript’s bashful hero:

Rethinking JavaScript Object Enumeration

In JavaScript, enumeration across regular (non-Array) Objects is often more painful than it should be. Arrays are merrily dispatched through for and while loops using all manner of crazy, fun techniques; Objects are forever at the mercy of the pedestrian, one directional for-in loop, without which we can’t even learn the names and length of its own property set. Arrays have access to a plethora of elegant higher order functions (forEach, map, filter etc.); Objects don’t. Until now, that is.

Truth, Equality and JavaScript

 
You don’t have to be a JavaScript novice to get confused by this…

if ([0]) {
    console.log([0] == true); //false
    console.log(!![0]); //true
}

 
or this…

if ("potato") {
    console.log("potato" == false); //false
    console.log("potato" == true); //false
}

 
The good news is that there is a standard and all browsers follow it. Some authors will tell you to fear coercion and and code against it. I hope to persuade you that coercion is a feature to be leveraged (or at the very least understood), not avoided…

The JavaScript arguments object…and beyond

Spare a thought for JavaScript’s arguments object. It wants so desperately to be an array. It walks like an array, quacks like an array but flies like a turkey. During the early years of the language Brendan Eich came close to rewriting arguments as an array until ECMA came along and clipped its wings forever.

In spite of all this (or maybe because of it) we love the arguments object. In this article I’ll explore its niftyness and its quirkiness and I’ll finish up by looking at the likely successors: rest and spread…

Exploring JavaScript for-in loops

The for-in loop is the only cross-browser technique for iterating the properties of generic objects. There’s a bunch of literature about the dangers of using for-in to iterate arrays and when to apply the hasOwnProperty filter, but beyond that, documentation of this ubiquitous construct is surprisingly patchy. This article attempts to fill some gaps, I hope its useful.