gistfile1.js

var foo = 'bar';
var a = function(){ 
  console.log(foo); 
}
var b = function(){ 
  console.log(foo); 
  var foo = 'blarg';
}

a()
// 'bar' is logged. no surprises.

b()
// undefined is logged! defining a var *later* in the function
// prevents foo from being looked up in the closure scope

See this explanation.

http://www.adequatelygood.com/2010/2/JavaScript-Scoping-and-Hoisting