Why doesn't the equal work?

Question

It says undefined every time. Can you help me?
https://code.sololearn.com/W916qpLEF36I/?ref=app

Answer 1

Don't use document.getElementById("res").value. That property is undefined.

Use either .innerHTML or .innerText

Answer 2

Thank you!

Answer 3

There's another question I made if you know the answer...