One thing I found irritating when using SharePoint was the built-in editor. Other than color coding the code, it didn’t really have any other features. I was working by copy/pasting the file into Atom, and then editing the file, and copy/pasting it back.
However, if you right-click on the file, and select “Open With”…
…you can actually set up separate text editors to open the file.
After you open it you will be able to make changes and when you save the file in the text editor, it will update in SharePoint as well.