Form Validation With Javascript - Field Value Must Be Between 2 Specific Numbers

I have a form on a page. On the form there is a field named Birthyear. I need to figure out the following in javascript: When a value between 1900-2012 is entered into the field, t

Solution 1:

function validateForm(){
var x=document.forms["myForm"]["username"].value;

if (x==""){
alert("Username Required!");
document.getElementById("username").focus();
}


var dob = document.forms["myForm"]["birthYear"];
var y = dob.value;

if(!isNaN(y)){

if(y>=1900 && y<=2012) {
    //Correct
 } else {
     //Wrong
     document.getElementById("birthYear").style.background= "yellow";
     document.getElementById("birthYear").focus();
     return false;
 }
} else {
  alert('must be a number');
}
}