Notes:

• You can type special characters with a backslash prefix, as shown by hovering over the character bar above. Documentation links for primitives are here.
• Any modifier with enter will execute, not just shift. The code window is vertically resizeable.
• Support for function headers and multiple bodies is missing.
• It's slow, but not that slow: usually most of the time is spent compiling your code, and sometimes from formatting the output. Chrome is faster than Firefox here, and speeds up over a few runs as JIT compilation kicks in.