It is a online coding IDE, you could get most convenient abilities, including editing, running, debugging.
2) VisualStudio Code
You could download from https://code.visualstudio.com/. about how to debug, you could refer to the document https://code.visualstudio.com/Docs/debugging
3) FireFox developer tools
FireFox has one Scratchpad feature, but I don’t use it too much.