Terse JavaScript 101 – Part 2

Posted in ‘JavaScript‘ by James Padolsey on October 29th, 2011

Part one, posted last week, explored a few different ways of de-cluttering your JavaScript, and, in the process, making it more readable.

Some developers don’t like the idea of using a language’s more idiosyncratic features because it does, potentially, make your code less readable in the eyes of those who haven’t learned the language properly. I think it’s still up for debate. While you’re pondering that, part II awaits…

If you haven’t checked out the “Truthy & Falsey” introduction then please do so before continuing.


It’s something we barely think about but JavaScript’s expressive diversity allows us to invent new patterns for looping. First, there’s the standard:

for (var i = 0; i < array.length; i++) {}

This is conventional and will be known to developers coming from any language. There’s nothing inherently wrong with it. That said, for me, it’s not ideal.

The first improvement I usually make is to cache the length property of the array:

for (var i = 0, l = array.length; i < l; i++) {}

This doesn’t make it shorter, but it does make it more efficient, although marginally so…

Another change we can make is to combine the iteration expression (i++) and the conditional expression (i < l), like so:

for (var i = -1, l = array.length; ++i < l;) {}

Notice that we’re starting with an index of -1, but won’t worry because before the loop’s body runs, the conditional expression is run, and in the above code, the conditional expression is iterating the index and testing it against the length all in one.

It’s important to note that the prefix increment/decrement operators (++i/--i) return the changed value, while the postfix increment/decrement operators (i++/i--) return the old value. To make this clearer:

var x = 5;
alert(x++); // alerts 5 (old value)
alert(x); // alerts 6

var y = 5;
alert(++y); // alerts 6 (new value)
alert(y); // alerts 6

Another looping “pattern” in JavaScript is to assign and rely on what’s returned from that assignment for the conditional expression:

var elements = document.getElementsByTagName('a');

for (var i = -1, el; el = elements[++i];) {
  el; // => a single element

Yesterday’s post on truthy & falsey bears some relevance here. The expression el=elements[++i] will return the value of elements[++i]. If this value is falsey the loop will not continue. An element list is full of truthy values (objects). We don’t need to be worried about falsey values. Note that in this approach, you may be sacrificing performance for terseness.

In many situations, we do want to iterate over falsey values too, e.g.

var myArray = [1, 2, 3, null, 8, 54, null, 0, 32];

With something like this it would be best to use the traditional i<length conditional expression, but if you’re so inclined feel free to mix the conditional and iteration expressions together like shown further up (++i<l).

If we were feeling a bit crazy, we could combine everything — iteration, conditional test, and assignment of indexed value — in a single expression:

var array = ['a','b','c'];

for (var i = -1, l = array.length, item; item = array[++i], i < l;) {

// alerts, a, b, c

Here we used the useful comma operator which always returns its right-hand operand. In this example we used it to make sure that i < l is what the condition expression returns. Before the comma we’re assigning the value at the iterated index within the array.

I don’t really suggest doing all of this in one line, and to be honest, there’s nothing wrong with conventional looping structures. What I am trying to put across is how much expressive diversity JavaScript provides.


Assignments are expressions too, so you can slot them in wherever you want:

var a = 1;
alert(a = 2); // alerts 2
alert(a); // alerts 2

An assignment operator will always return whatever is on its right-hand side. Be careful with slotting in assignment expressions wherever you want though, because they can be misconstrued for equality operators. For example:

var match;

if (match = '1/2/3'.match(/\d/)) {

To some, upon initial inspection, it may appear that we’re testing match for equality against '1/2/3'.match(/\d/) when in fact the latter is being assigned to the former, and the if statement will run if the assignment expression returns a truthy value (i.e. if its right-hand side operand is truthy).

Casting to a number

Quick and easy:

var str = '222';
var num = Number(str); // => 222

typeof num; // => 'number'

A shortcut is the unary plus (+) operator, used like so:

var str = '222';
var num = +str; // => 222

typeof num; // => 'number'

It works in exactly the same way.

Casting to a string

Once again, quick and easy:

var arr = [1,2,3];
var str = String(arr); // => '1,2,3'

typeof str; // => 'string'

The shortcut, this time, is to simply concatenate an empty string to the value that you’re casting:

var arr = [1,2,3];
var str = '' + arr; // => '1,2,3'

typeof str; // => 'string'

Saving references

Property lookup can be verbose and inefficient. To save space and to benefit performance it’s common to save references and access those properties via the new reference instead of having to evaluate the entire expression every time you want the nested value:

// Original:

// "Caching"
var style = document.body.style;

// Whenever I need to access the `color` value:

It’s often referred to as caching. Essentially, all that’s happening is that a new reference is being created for an object and being assigned to an identifier that you specify. Let’s start with this:

var data = {
    config: {
        doSomething: function(item){

for (var i = -1, l = someArray.length; ++i < l;) {

It would make sense to minimise the amount of property access that needs to occur within the loop, and thus, potentially, anywhere else in our application that references data.config too:

var config = data.config;

Now the loop can simply reference config instead of data.config:

for (var i = -1, l = someArray.length; ++i < l;) {

We could even “cache” the function itself:

var doSomething = data.config.doSomething;

And then call it directly within the loop:

for (var i = -1, l = someArray.length; ++i < l;) {

IMPORTANT: It’s important to note that, when you assign a member function (that is, a function that is a property of an object) you are de-binding its this value, i.e. the context in which it runs. Here’s an example:

var obj = {
    value: 234,
    fn: function(){ return this.value; }

obj.fn(); // => 234

var fn = obj.fn;
fn(); // => undefined

De-binding the fn method will make its context the global (window) object instead of the obj which we defined, so when our fn function looks for its `this.value` it won’t find it (because this is the window, which doesn’t have `value` defined).

End of part 2


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