At least Function.prototype.toString doesn't work right on callable objects. ES5 says that callable objects are of type Function (typeof RegExp == "function"), so this should probably made work. We should check for isCallable instead of isFunction.

"An implementation-dependent representation of the function is returned. This representation has the syntax of a FunctionDeclaration. Note in particular that the use and placement of white space, line terminators, and semicolons within the representation String is implementation-dependent. The toString function is not generic; it throws a TypeError exception if its this value is not a Function object. Therefore, it cannot be transferred to other kinds of objects for use as a method." Looks like ES5 doesn't want all callable objects to be allowed here, only explicitly functions. So I will fix this narrowly for proxies.

Please resummarize -- summary is wrong, ES1-5 are clear on this :-P. /be

Works for Proxies and Functions, Ecma doesn't want anything more.