Q:

Struggling with classList.add and getElementsByClassName

I'm trying to add a extra class to some elements with a specific class(input-fieldset).
<fieldset id="pay" class="input-fieldset"></fieldset>
<fieldset id="address" class="input-fieldset"></fieldset>

So I did some searches and found this:
var element = document.getElementsByClassName('input-fieldset');
element.classList.add(' input-fieldset-awesome');

I'm trying to add the class input-fieldset-awesome.
But it doesn't work, I get the error:

Uncaught TypeError: Cannot read property 'add' of undefined(anonymous function)

What am I doing wrong? 

A:

.getElementsByClassName() returns an HTMLCollection (array of objects) that must be iterated.
The code below will accomplish what you're after.
// Get desired elements
var element = document.getElementsByClassName('input-fieldset');

// Iterate through the retrieved elements and add the necessary class names.
for(var i = 0; i < element.length; i++)
{
    element[i].classList.add('input-fieldset-awesome');
    console.log(element[i].className);
}

Here's a working fiddle to play with.

