Pure Javascript listen to input value change

As a basic example...

HTML:

<input type="text" name="Thing" value="" />

Script:

/* event listener */
document.getElementsByName("Thing")[0].addEventListener('change', doThing);

/* function */
function doThing(){
   alert('Horray! Someone wrote "' + this.value + '"!');
}

Here's a fiddle: http://jsfiddle.net/Niffler/514gg4tk/

This is what events are for.

HTMLInputElementObject.addEventListener('input', function (evt) {
    something(this.value);
});