Why doesn't getElementByClassName work?

Question

atom

Hello. And now it's time to face a ridiculous problem. Can't get element by getElementByClassName. I just have no idea why... I tried all the selectors, nothing helps. Apparently it's not so easy to get an element from the DOM?
Gives this error:
document.getElementByClassName is not a function
https://jsfiddle.net/90tq6awh/5/

Answer 1

In vain you are so, this is space, not a function

Answer 2

getElementsByClassName