UT Editor is a simple text editor for Google Drive, just like Windows Notepad.
If you install UT Editor, you can open plain text files such as text/plain, text/html, text/css, application/json, etc. from Google Drive by double-clicking them. You can save the opened file by choosing [File]>[Save] (or Ctrl+S or Command+S). Unsupported mime-type files can be opened by selecting [File]>[Open] after starting the UT Editor.
For JSON and program code, it is possible to check the formatting and syntax.