global scope and local scope in javascript

Consider the following snippet in javascript.
The output of the following snippet is :
The first alert shows "undefined" whereas the second alert shows "2"

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

Why does the first alert not show the value of the global variable a which is 1?

Answer

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/