In Smalltalk, you have an "image" that's both your source code, the editor, and the app you're running. So there's no "compile time" or "runtime" - you create a function, the IDE already understands it, the runtime already knows about it, etc.
So maybe in ClojureScript, it would be possible to be developing a code on the browser, then somehow "drop in" an editor in the browser, evaluate things, see how it works, and when you save, everything is already updated