Chrome Dev Tools' editor has great autocompletion, and has done for a very long time. Also, you can 'map' assets to local files so live modifications can be saved without needing to import/export.
Dev Tools are what keep me using Chrome at work, they really are unparalleled.
https://developer.mozilla.org/en-US/docs/Tools/Scratchpad
That said, something missing from both is an easy way to import and export the code.