defining array in javascript

They do the same thing. Advantages to the [] notation are:

• It's shorter.
• If someone does something silly like redefine the Array symbol, it still works.
• There's no ambiguity when you only define a single entry, whereas when you write new Array(3), if you're used to seeing entries listed in the constructor, you could easily misread that to mean [3], when in fact it creates a new array with a length of 3 and no entries.
• It may be a tiny little bit faster (depending on JavaScript implementation), because when you say new Array, the interpreter has to go look up the Array symbol, which means traversing all entries in the scope chain until it gets to the global object and finds it, whereas with [] it doesn't need to do that. The odds of that having any tangible real-world impact in normal use cases are low. Still, though...

So there are several good reasons to use [].

Advantages to new Array:

• You can set the initial length of the array, e.g., var a = new Array(3);

I haven't had any reason to do that in several years (not since learning that arrays aren't really arrays and there's no point trying to pre-allocate them). And if you really want to, you can always do this:

var a = [];
a.length = 3;

This benchmark on JSPerf shows the array literal form to be generally faster than the constructor on some browsers (and not slower on any).

This behavior is, of course, totally implementation dependent, so you'll need to run your own test on your own target platforms.

There's no difference in your usage.

The only real usage difference is passing an integer parameter to new Array() which will set an initial array length (which you can't do with the [] array-literal notation). But they create identical objects either way in your use case.