Change input style if filled using a class name

I found fiddle to change input style if filled. But it's using ID. I want to use class name. I used document.getElementsByClassName(), but it's not working. Anyone can help? Thanks.

Here is the code

html

<input type="text" id="subEmail" onchange="checkFilled();"/>

Javascript

function checkFilled() {
var inputVal = document.getElementById("subEmail");
if (inputVal.value == "") {
    inputVal.style.backgroundColor = "yellow";
}
else{
    inputVal.style.backgroundColor = "";
}
}

checkFilled();

and here is the link to Fiddle