How to handle 'undefined' in JavaScript

As is often the case with JavaScript, there are multiple ways to do this:

typeof foo !== 'undefined'
window.foo !== undefined
'foo' in window

The first two should be equivalent (as long as foo isn't shadowed by a local variable), whereas the last one will return true if the global varible is defined, but not initialized (or explicitly set to undefined).

You can check the fact with

if (typeof jsVar == 'undefined') {
  ...
}