Javascript - Better to instantiate variables, or leave them undefined?

it is better to initialize to a "null" value, so this way you can do error checking against this null value, to make sure you don't incorrectly try to access invalid data.

memory-wise, you are not saving anything, the variable still takes up physical memory, regardless of it's content.

performance-wise, it's doubtful you would see any return whatsoever, any performance gain(and this would be the upmost minimal), would almost never translate to being a bottleneck point for an application, as such, it's pretty much a moot point to discuss.

Thanks slicer4ever, that is a good idea about setting them to null for error checking.

Slicer pretty much covered it, but I will reinforce the importance of never intentionally leaving your data in an invalid state. That's just asking for trouble. Set it to something that won't throw an error upon an attempt to access it, even if it is a "dummy" value.

It's perfectly valid to leave your variables 'undefined' at the top of your scope. In fact, it's what the JavaScript interpreter does anyway though a mechanism called variable hoisting. Devs often do this explicitly at the top of their scope in a single, comma-seperated list like this:

[source lang="jscript"]function(){
var array, number, string;

// or
var array = [],
number = 0,
string = '';

// function body...
}[/source]
I prefer the latter method, because it helps the routine document itself.

As for testing against null: I'd be careful. It's one of JavaScript's many quirks that null and undefined are strage cats and can behave unexpectedly (more info here). Everyone's needs are different but I would reccomend using if(typeof array !== 'undefined') if you must check for variable initialization.