Jimmy Breck-McKye

Developing opinions

Using jsPerf: a how-to guide

If you’re interested in testing the performance of a particular piece of JavaScript, you might be interested in jsPerf.

jsPerf allows you to run and compare the speed of arbitrary snippets of JavaScript against a blob of HTML of your choice. This means you can test not just pure JavaScript, but DOM manipulation and jQuery calls too.

Protected members in JavaScript

If you’ve done much OOP in JavaScript, you can probably already simulate private member variables, by putting variables in a constructor’s closure (if not, go read this summary by Douglas Crockford). But you may not know that you can also emulate the protected status of C++ and Java — variables shared between parent and child classes, yet not exposed as public. This is occasionally useful, but is a little obscure to implement in JS if you don’t know how.

The basic idea is use the ‘parasitic constructor’ pattern — where a child constructor directly calls the parent constructor — and passing in an object defined in the context of the child constructor. The parent constructor decorates this object, and because JavaScript object arguments are effectively passed by reference, the members of this object are visible within the child constructor. As such, your child and parent constructors have a ‘shared secret’ object that effectively acts as a map of all the protected members.

JavaScript function variables don't have to be anonymous

If you’ve used JavaScript much, you’ll probably know one of its very convenient features — the ability to assign functions to variables, send them as arguments, and return them as values. This lets you write code like this, where functions themselves return other functions:

Anonymous function bonanza
This code creates a basic 'curry' function
(see my earlier post for an explanation)
It relies heavily on function variables.

var add = function(x, y) { // <- A function variable
return x + y;

var curryUtil = {
curry : function(functionToCurry, x) { // <- A function object property
return function(y) { // <- A function return value
return functionToCurry(x,y);

var add7 = curryUtil.curry(add, 7);
add7(3); // returns 10

var double = curryUtil.curry(function(x,y){ // <- A function literal argument
return x * y;
}, 2);
double(8); // returns 16

The problem with this, however, is that all the functions in the above example are anonymous — they aren’t named. Beyond making your code more obscure than it needs to be, this also makes debugging them a problem

Why I like parasitic inheritance

When you’re starting out with JavaScript, it doesn’t take long for the question of object oriented programming to raise its head. As soon as you want to build anything non-trivial you’re going to want some means of structuring your solution and imposing some kind of abstraction on your code. Most tutorials these days emphasise use of the prototype chain - either through function.prototype or the ECMAScript 4 Object.create() interface. I, however, am still fond of the more basic parasitic inheritance approach - where inheritance is basically a special case of composition - and I wanted to outline my reasons why.

JavaScript curry

If you’ve had much exposure to functional programming, you might have heard of ‘currying’. This funny term refers to a kind of functional composition: taking a function that takes two (or more) parameters, and turning it into a function that takes less parameters, and automatically applies the other arguments, which have been ‘baked’ into it.

For example, let’s say we have an ‘add’ function - function add(x,y) { return x + y }. I might have an instance where I want to add various numbers to a constant - say, 7. Rather than constantly calling add(7, x), currying lets me create an ‘add7’ function. Whatever I pass, add7 always adds seven to it.

Document loading and DOM lifecycle events

Document loading and load events can be a bit confusing for newcomers. Multiple names for the same basic things, incompletely documented, ambiguously explained and those ever-present browser inconsistencies don’t exactly help. I want to try and remedy this by providing a rundown of document loading lifecycle events.

Disabling Firefox Safe Mode

A couple of weeks ago I faced a problem where Firefox’s “Enter Safe Mode” dialog was causing problems for a Selenium test suite I was trying to run.

Introducing Web Accessibility

A couple of weeks ago I gave a presentation, “Introducing web accessibility”, and I thought it might be worth sharing with the wider world. It provides a broad overview for developers who’ve never encountered accessibility before.

Styling form elements

Form elements are notoriously difficult to style. Even apparently simple tasks like horizontally aligning different types of input can prove a headache for experienced developers. Only by understanding how form elements are laid out can we master them.