What are the best practices to follow when declaring an array in Javascript?

Question

Mostly, people use var a = [] because Douglas Crockford says so.

His reasons include the non-intuitive and inconsistent behaviour of new Array():

var a = new Array(5);     // an array pre-sized to 5 elements long
var b = new Array(5, 10); // an array with two elements in it

Note that there's no way with new Array() to create an array with just one pre-specified number element in it!

Using [] is actually more efficient, and safer too! It's possible to overwrite the Array constructor and make it do odd things, but you can't overwrite the behaviour of [].

Personally, I always use the [] syntax, and similarly always use {} syntax in place of new Object().

One significant difference is that [] will always instantiate a new Array, whereas new Array could be hijacked to create a different object.

(function () {
    "use strict";
    var foo,
        bar;
    //don't do this, it's a bad idea
    function Array() {
        alert('foo');
    }
    foo = new Array();
    bar = [];
}());​

In my example code, I've kept the Array function hidden from the rest of the document scope, however it's more likely that if you ever run into this sort of issue that the code won't have been left in a nice closure, and will likely be difficult to locate.

_{Disclaimer: It's not a good idea to hijack the Array constructor.}

for maintainability, use []

The array literal is more predictable, as most developers use it. Most array usage out there will be using the literal, and there is value in having your code match up with what other developers use.

for empty arrays, use []

var ns = [];
var names = [ 'john', 'brian' ];

As shown here, using the literal for empty and a couple of known elements is fatster than the Array constructor.

for an array of known size, use new Array(size)

If the size is known, then using the Array constructor significantly improves performance. However it also means you have to deal with an array which already has 'undefined' filling all it's values.

As shown here

// fill an array with 100 numbers
var ns = new Array( 100 );
for ( var i = 0; i < 100; i++ ) {
    ns[i] = i;
}

This also works for very small arrays too.

No, there is actually no reason to use one notation over the other one for empty Arrays.

However, most browsers show a slightly better performance using x = []; than calling the Constructor.

If you need to create an Array with a specific size, you kind of need to use x = new Array(10); for instance, which would create an Array with 10 undefined slots.

• var arr=[] uses the array/object literal
• var arr = new Array() use the array/object constructor

The speediest way to define an array or object is literal way, because you don't need to call the constructor

var arr1 = new Array(1, 2, 3, 4);
var arr2 = [1, 2, 3, 4];

alert(arr1[0]); // 1
alert(arr2[0]); // 1

var arr3 = new Array(200);
var arr4 = [200];

alert(arr3[0]); // 'undefined'
alert(arr4[0]); // 200