Terse JavaScript 101

Terse JavaScript 101 – part 1

by James

at 2011-10-19 05:23:00

original http://james.padolsey.com/javascript/terse-javascript-101-part-1/

While some folk will argue that verbose code aids readability, I think almost the opposite, and in this post I’m going to list some basic tips for minimising redundant clutter in your code. JavaScript is a fun language at its core — it’s worth learning the tiny details.

I hope this post is useful to someone. I wrote it not for beginners (although it will be helpful to them), but for all those people writing JavaScript on a daily basis yet haven’t been afforded the time/motivation to get to know the language properly.

new Object()

Nope, don’t do this:

var o = new Object();

Or any of this:

var a = new Array();
var b = new Object();
var c = new RegExp("1(.)3");

Do this instead:

var a = [];
var b = {};
var c = /1(.)3/;

Array literal, object literal, regular expression literal. They rock. There’s usually no need to use the constructor functions when literals are available (please see the note below though).

Note: There are situations when calling Array or RegExp directly can be useful. In such situations, you needn’t bother with the new operator. Array(1,2,3) will produce the same as new Array(1,2,3). Calling RegExp directly can be useful for dynamically building a regular expression from various strings.

Calling a constructor

If you’re not passing any arguments then the parenthesis are not required. E.g.

var a = new Animal();

… and this becomes:

var a = new Animal;

Petty, perhaps. Still worth knowing though.

var;var;var

Quick and easy. Less of this:

var fun = 123;
var tun = 456;
var run = 789;

And more of this:

var fun = 123,
    tun = 456,
    run = 789;

This is why you should use a tab width of FOUR (or 4 spaces).

Undefined & Null

The following is universally true in JavaScript (assuming undefined hasn’t been mutilated):

undefined == null

“So what?”, you say? Well, it means you can replace this:

if (foo === undefined || foo === null) {
    //...
}

… With this:

if (foo == null) {
    //...
}

If something is == to null then it is either null or undefined. Those are the only possible values.

Returning undefined

Not returning something from a function is the same as returning undefined, so you may as well not bother explicitly returning undefined:

function a() {}
function b() { return; }
function c() { return undefined; }
 
a() === b(); // true
b() === c(); // true

Note: you should be careful with the undefined value as it can be changed. E.g. You can do undefined = true beforehand. The universe would shatter if you did that though, so please don’t. Many libraries declare an empty (undefined) variable in an enclosed scope, which provides a real undefined:

(function(){
 
    // My private scope. They can't see me in here.
 
    var undef;
 
    undef; // => definitely undefined
 
}());

Or, more succinctly:

(function(undef){
 
    undef; // => definitely undefined
 
}()); // <- not passing anything in!

Empty array values

This is entirely legal in JavaScript:

var a = [,,,'foo'];
a[3]; // => "foo"

One potential use-case of this was discussed in my recent post, `match.()` trick.

Less blocks, more expressions

I’m sure we’ve all seen something like this before:

function validateFoo(foo) {
    var regexResult = /^foo+$/.test(foo);
    if (regexResult) {
        return true;
    }
    return false;
}

A cleaner and more sensible solution is this:

function validateFoo(foo) {
    return /^foo+$/.test(foo);
}

No fussing with if statements and multiple returns. You can do everything within a single return statement.

Another example. We want to create a function that logs a name, specified by the arguments forename and surname, but if these arguments aren’t passed then the function should use the default values, “Bob” and “Smith”, respectively:

function printName(forename, surname) {
 
    if (forename == null || forename == "") {
        forename = 'Bob';
    }
 
    if (surname == null || surname == "") {
        surname = 'Smith';
    }
 
    console.log(forename + ' ' + surname);
 
}

This can be condensed to:

function printName(forename, surname) {
 
    forename = forename || 'Bob';
    surname = surname || 'Smith';
 
    console.log(forename + ' ' + surname);
 
}

Here, we’re using the logical OR operator (||) to provide a default value (right hand side) if the left hand side evaluates to false. Values that evaluate to false (i.e. “falsey”) are: An empty string (""), null, undefined, 0 and false.

So, this:

forename = forename || 'Bob'

… is essentially saying: if forename is a truthy value, then assign forename to forename (in other words: don’t do anything), but if it is a falsey value (e.g. an empty string), then assign the string "Bob" to forename.

So, our new printName function will behave like so:

//                            -- Logged --
printName();                  // Bob Smith
printName(0, 0);              // Bob Smith
printName(false, null);       // Bob Smith
printName('James');           // James Smith
printName('', 'Jones');       // Bob Jones
printName('Bill', 'Jones');   // Bill Jones

Part 1 [OVER]

That’s it for today. There may be a part two soon enough, depending on how useful this is to readers. I know many of you have surpassed this stage of learning, so sorry for boring you.

Recommended viewing/reading

Seriously, if you’re writing JavaScript as part of your job then you definitely owe it to future maintainers of your code to learning JS properly.

EDIT: Crockford links don’t work. The entire video can be seen on youtube instead.

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