Why does "Undefined" occur even defining the exact element to be hit?

Question

I’m applying a injection in the web browser address bar, on the page of a video on youtube, in which I want to extract the date of the post of the same.

If we open this link we see a div similar to this:

When selecting the date of posting and then use the "View Selection Source" tool available in the web browser itself, we can see the source code of the selected line:

In possession of the class name watch-time-text, I tried to do the following:

javascript:postado=document.getElementsByClassName("watch-time-text").innerHTML; alert(postado);

And the following occurred:

Why is this undefined even the element existing on the page?

Because a gif is worth a thousand words

Answer 1

The method getElementsByClassName returns a collection of elements and not an element only.

If you are sure there is only one element with this class you can use index 0 to get it.

postado = document.getElementsByClassName("watch-time-text")[0].innerHTML;
alert(postado);