The outcome of those two is the same. Likewise the outcomes of
var a = new Array(3, 4);
and
var a = [3, 4];
are the same.
However,
var a = new Array(3);
is not the same as
var a = [3];
The former creates an array with 3 empty elements, the latter an array with a single element initialised to 3. So the constructor syntax is dangerous to use with just one argument. Plus, the literal is just easier to type and spot visually.
There is a slight performance penalty for using named constructors: the runtime needs to look up the "Array" entry in the scope; nothing (except common sense) stops you from redefining it or shadowing it with a local variable. The literal syntax can be resolved at parse/compile time.
Fun JS array fact: the implicit arguments argument in each function looks like an array but its prototype is actually not Array. None of the Array methods work on it, although the special length property behaves as you'd expect.
new is a keyword and has various other gotchas. The most egregious is probably is the silent but deadly failure when you forget it altogether:
function MyURL(loc) {
this.location = loc;
}
// benign:
var url1 = new MyURL("http://news.ycombinator.com");
// redirects the user to Hacker News:
var url2 = MyURL("http://news.ycombinator.com");
Calling a free function binds the global object (window in the browser) to the implicit this parameter; assigning a URL to window.location is clearly quite an obvious bug, most cases of forgetting new when calling a constructor are much more subtle. Luckily, you can defend against it in the constructor's code:
function MyURL(loc) {
if (this === (function(){return this;})())
{
// might want to log this during development, too...
return new MyURL(loc);
}
this.location = loc;
}
I strongly recommend reading what Douglas Crockford has written on the topic of JavaScript, or watching his excellent videos. (this from someone who detests video as a medium of conveying this sort of info)
pmjordon has a great reply, but I'd like to add that I'm a little surprised by your mention of jQuery... jQuery is just a framework built on top of JavaScript; it's not like it's a different language or even a language extension. It's just a standard piece of JavaScript code, an object which you can use.
I am unfamiliar with the details of JavaScript's semantics, so I won't be able to tell at a glance what is part of the language and what is part of a library.
var a = new Array(); and var a = [];
Would someone shed some light on this?