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Chocolat is an editor for the mac - very similar looks-wise to TextMate - that uses javascript for it's scripting api, though I've never used it.

there are editors fully written in js c9.io Brackets lightTable

Pentadactyl lets you browse the web from the keyboard in a way that mimics vim, and does so using Javascript. So ... kind of.

FWIW the Zeus editor is scriptable in Lua, Python, JavaScript and TCL just to name a few.