How to handle 'undefined' in javascript | Learning Curve- Perfect Tutorial for the beginers

Variable ‘mydata’ is returning the value ‘undefined’, if you give this in an if condition its not working..

if(mydata==”undefined”) this condition should not work.so below code will solve the issue.

<!DOCTYPE html>
<html>
<head>
<script>
function test_type_of()
{
var mydata=document.getElementById('myhidden').val;
if (typeof mydata == 'undefined') {
//if(mydata=="undefined")
//{
alert('test');
}

}
</script>
</head>
<body>
<input type="hidden" value="" name="myhidden" id="myhidden">
<input type="button" name="click_button" id="click_button" value="Click" onclick="test_type_of();">
</body>
</html>

<!DOCTYPE html>

<html>

<head>

function test_type_of()

{

var mydata=document.getElementById('myhidden').val;

if (typeof mydata == 'undefined') {

//if(mydata=="undefined")

//{

alert('test');

}

</script>

</head>

<body>

</body>

</html>