onload « scope « Javascript Data Type Q&A

1. JavaScript: How is "function onload() {}" different from "onload = function() {}"?    stackoverflow.com

In the answers to this question, we read that function f() {} defines the name locally, while [var] f = function() {} defines it globally. That makes perfect sense to ...

2. javascript window.onload scope    stackoverflow.com

Can someone explain why the alert returns "undefined" instead of "hello"?

window.onload = function() {  
    var a = 'hello';  
    alert(window.a);  
}