This is delightful! (and almost perfect)
One obvious miss would be some sort of drag-drop or snapping to make things interact --- say a line to link a number slider w/ a number field (as opposed to typing the JavaScript code).
Or, perhaps this could borrow from Scratch/Blockly and have outline shapes when selected which would allow things to snap together and interact (this would also help with visual appearance) --- maybe there could be two outlines, an outer one to guide layout and an inner one to guide functionality?
