Access and Office add-in checkbox checked status

In my Office add-in I have a checkbox like the following:

<div class="ms-CheckBox">
    <input id="inputId" type="checkbox" class="ms-CheckBox-input" />
    <label id="labelId" role="checkbox" class="ms-CheckBox-field" aria-checked="false" name="checkboxA" for="inputId>
        <span class="ms-Label">Text</span>
    </label
</div>

I want to retrieve through JavaScript its checked status (or its aria-ckecked status, I'm still not getting the differences between them), which I thought was through document.getElementById( 'labelId' ).checked, since it's specified in the documentation that they have an optional checked member, but I only get an undefined with it.

I'm very new to these technologies and have a couple concerns:

Does "optional member" mean that I have to explicitly create it so that it exists? If so, how can I do that?

However the checked member may come to existance, do I have to manually handle its value every time it's clicked on by the user or is it already internally managed and I simply haven't found the way to access it yet?

Maybe I just can't see a mistake I've made on the html code for the checkbox?

Thank you in advance!