How about this? (I am not a 100% about this one) f = Object.defineProperties(x => x, {toString:{value:()=>'a'}, [Symbol.toStringTag]:{value:'a'}}) and we

How about this? (I am not a 100% about this one) f = Object.defineProperties(x => x, {toString:{value:()=>'a'}, [Symbol.toStringTag]:{value:'a'}}) and we will change it to: f = Object.defineProperties(y => y, {toString:{value:()=>'a'}, [Symbol.toStringTag]:{value:'a'}}) Edit: ah you -can- break it actually. A challenge for others to figure out how to break this one.