[masklinn]

Except #forEach is a method of Array, and getElementsByTagName does not return an Array but a NodeList. Which doesn't have any of Array's methods (it has a length, it can be indexed, and it has an alternative indexation method - #item — but it's not an array at all)

[nailer]

You're right. I wasn't sure earlier, as I was typing on a phone with no JS console.

but never mind:

Array.forEach.call(paragraphs, function(paragraph){paragraph.innerHTML = 'Hello.';})

will work fine.