global scope and local scope in javascript [duplicate]

Answer 1

It is called "hoisting" in JavaScript. Your function is automatically transformed into this one:

var a = 1;
function test() {
  var a;
  alert(a);
  a = 2;
  alert(a);
}
test();

Nice read about that: http://net.tutsplus.com/tutorials/javascript-ajax/quick-tip-javascript-hoisting-explained/

Answer 2

What you're seeing is variable hoisting in action.

This is how the code is being interpreted:

function test(){
    var a; // a === undefined
    alert(a);
    a = 2;
    alert(a);
}