what does this mean: var _gaq = _gaq || [];

Answer 1

I would interpret this as:

If _gaq already stores a value that isn't convertible to false (e.g. a non-empty array), take that value; otherwise, make _gaq refer to an empty array."

Douglas Crockford writes the following about this in his book Javascript - The Good Parts:

The operator || can be used to define default values [...].

Some background info:

• In JavaScript, there's quite a few values that evaluate to false in a boolean expression; suspects are the number 0, the empty string, NaN, the undefined value (note that undefined is not a keyword!), and false itself.

• AFAIK, JavaScript's logical OR (||) operator uses short-circuiting, i.e. if the first term in an OR expression is true, the second one won't be evaluated.)

Answer 2

it means assign _gaq the value of _gaq unless it is undefined, in which case _gaq will be an empty list.

Answer 3

It's a short way to set _gaq to an empty array if _gaq is undefined. It's probably used to provide a default value for an argument to a function.

Answer 4

_gaq || []

Is an expression that will return _gaq if it's a non-false value ( I mean is not 0, nor false, nor '') or an empty array in the other case.

var _gaq = _gaq || [];

Always will set [] to _gaq. I tested it in this way from my firebug console:

_gaq = 'crazy value';
(function(){var _gaq = _gaq || []; 
            console.log(_gaq);
 })();

Having in mind that _gaq could be a variable defined in the global namespace. But is not the case.