Question

In the following code snippet I declare a global variable and then check for its presence inside a function.

<script>
x = 5;
$(function() {
   var x = x || 3;
   console.log(x); // prints 3
});
</script>

This behaves differently:

<script>
x = 5;
$(function() {
   var y = x || 3;
   console.log(y); // prints 5
});
</script>

I expect that in the first example, the variable declaration in the inner scope will detect that x already exists in the global scope, and take its value. Why does the first example 3?

Specifically I recently wrote some code that checks var _gaq = _gaq || [] in a jQuery ready scope and was confused when nothing was getting pubbed to Analytics.

Answer 1

You’re looking for x in the wrong scope. Due to variable hoisting, var x has actually defined a local x variable with a value of undefined before your x || 3 check happens:

var x = x || 3;

is actually:

var x = undefined;
x = x || 3;

Simply change it to look for x on the window object:

var x = window.x || 3;