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

4 posts in this topic

In [b]Javascript[/b], if I know a variable type, but not necessarily the value, is it better on memory or run-time performance to instantiate it with a 'dummy' value, or is it better to just leave it undefined, and then only instantiate it when it is first needed?

For example...

var array1;
var number1;
var string1;
/* vs */
var array2 = [];
var number2 = 0.0;
var string2 = "";

// later in code...
function onInit(){
number1 = 3.142;
number2 = 2.718;
string1 = "Hello";
string2 = "World";

Is it best to fill the variable with a dummy 'type' or just leave it 'undefined' ?

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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.

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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 [url="http://www.adequatelygood.com/2010/2/JavaScript-Scoping-and-Hoisting"]variable hoisting[/url]. 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...
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 ([url="http://bonsaiden.github.com/JavaScript-Garden/#core.undefined"]more info here[/url]). Everyone's needs are different but I would reccomend using if(typeof array !== 'undefined') if you must check for variable initialization.

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