In your first example, you are making a blank array, same as doing var x = []. The 2nd example makes an array of size 3 (with all elements undefined). The 3rd and 4th examples are the same, they both make arrays with those elements.
Be careful when using new Array().
var x = new Array(10); // array of size 10, all elements undefined
var y = new Array(10, 5); // array of size 2: [10, 5]
The preferred way is using the [] syntax.
var x = []; // array of size 0
var y = [10] // array of size 1: [1]
var z = []; // array of size 0
z[2] = 12; // z is now size 3: [undefined, undefined, 12]
In your first example, you are making a blank array, same as doing var x = []. The 2nd example makes an array of size 3 (with all elements undefined). The 3rd and 4th examples are the same, they both make arrays with those elements.
Be careful when using new Array().
var x = new Array(10); // array of size 10, all elements undefined
var y = new Array(10, 5); // array of size 2: [10, 5]
The preferred way is using the [] syntax.
var x = []; // array of size 0
var y = [10] // array of size 1: [1]
var z = []; // array of size 0
z[2] = 12; // z is now size 3: [undefined, undefined, 12]
The preferred way is to always use the literal syntax with square brackets; its behaviour is predictable for any number of items, unlike Array's. What's more, Array is not a keyword, and although it is not a realistic situation, someone could easily overwrite it:
function Array() { return []; }
alert(Array(1, 2, 3)); // An empty alert box
However, the larger issue is that of consistency. Someone refactoring code could come across this function:
function fetchValue(n) {
var arr = new Array(1, 2, 3);
return arr[n];
}
As it turns out, only fetchValue(0) is ever needed, so the programmer drops the other elements and breaks the code, because it now returns undefined:
var arr = new Array(1);
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.