What is “var _gaq = _gaq || []; ” for?

Question

The Async Tracking code in Google Analytics looks like this:

var _gaq = _gaq || []; 
_gaq.push([\'_setAccount\', \'UA-XXXXX-X\']); 
_gaq.push([\'_trackPagev         


        

Answer 1

Using || in assignment is a common programming trick which takes advantage of the evaluation direction of the operator, which is left to right. That means that it evaluates the left side first. Then, and only if that is false (or a false equivalent), does it evaluate the right side.

You can also take advantage of the || or && operators in a simple if statement, so that

if (a > 5) {
  do_a();
}

if (!some_boolean) {
  do_b();
}

become

a > 5 && do_a();
some_boolean || do_b(); // Note that the negation operator `!` is gone!

which are both way nicer to look at.

The reason languages allow this, is because it is a waste of time evaluating the right side if the left side will make the entire line fail anyways. So it just ignores it unless it's needed.