Now and again, especially when working with different files and dependencies in Javascript, it's useful to be able to check whether a variable is defined without risk of getting a 'x is not defined' error. There is a neat trick to do it, which makes use of the javascript window object.

var foo = "bar";

if ( typeof( window[ 'foo' ] ) != "undefined" ) {
   alert("foo is defined")
} 
else {
   alert("foo is undefined")
}