Also, you should avoid placing your curlybraces on the line after your parenthesis(). Some say its a matter of preference but it can matter in certain circumstances. A bad habit to start early on. Proper syntax is :
function myFunction() {
var myVariable = "Welcome";
alert(myVariable);
}
I would also avoid the use of non-descriptive variables such as "aa" or "bb". Use something that describes what the variable holds without being too lengthy. There is nothing wrong with username or password as variable names. Again, being picky but its a bad habit to start early on.