Finished the drum machine. All opinions are welcome. I just wanted to note that codesandbox.io is far superior to codepen…especially for react projects! Drum Machine
When you turn off machine it still handles keydown presses. 
Getting elements by id and adding event listeners it not a very Reacty.
Read this: https://reactjs.org/docs/handling-events.html
oh…I totally missed that. Thanks a lot… I fixed it.