Redeclaring a javascript variable

It's nothing more than a reminder that if you do this:

var x=5;
var x;
alert(x);

Result will be 5.

If you re-declare variable in some other languages for example - result will be undefined, or NaN, but not in javascript.

An example of redeclaring a variable can be found in Google Analytics. When the JavaScript tracking code is initiated by the Google Analytics script, it declares or redeclares _gaq in this way:

var _gaq = _gaq || [];

In other words, if _gaq is already defined, _gaq is "redeclared" as itself. If it is not defined, it will be declared for the first time as an empty array.

This allows the Google Analytics tracking code to support other scripts which may need to use the variable before Google Analytics code has initiated. As @xralf pointed out, JavaScript allows for this.

Redeclaring a variable is useful in situations where it cannot be known if the variable has already been defined.

By redeclaring a variable conditionally, as Google Analytics tracking code does, it allows for a variable to safely originate from more than one place.

In this example it could be safe for other code using the _gaq variable to likewise check for a predefined _gaq variable. If it exists, it knows it can use it. If it doesn't exist, it knows that it should define it before trying to use it.