I get undefined when it is written inside a function in javascript.Why is it

Question

var userbirth = prompt("Enter You Birthday. Please Enter in this format DDMMYYYY");
var bdate = userbirth.substring(0,2);
var bmonth = userbirth.substring(2,4);
var byear = userbirth.substring(4,8);
console.log(bdate);

function check2(){
 console.log(byear);
}
window.onload = check2();

In the first case when it is a outside function the code is executed correctly. But when it is inside a function it shows undefined.
Why is that like so? Can anybody help me with it.
Thanks in advance:)