Wrapping functions and function.length

Question

Let's consider I have the following code

/*...*/
var _fun = fun;
fun = function() {
  /*...*/
  _fun.apply(this, arguments);
}

I have just lost the .length data on _fun because I tried to wrap it with some interception logic.

The following doesn't work

var f = function(a,b) { };
console.log(f.length); // 2
f.length = 4;
console.log(f.length); // 2

The annotated ES5.1 specification states that .length is defined as follows

Object.defineProperty(fun, "length", {
  value: /*...*/,
  writable: false,
  configurable: false,
  enumerable: false
}

Given that the logic inside fun requires .length to be accurate, how can I intercept and overwrite this function without destroying the .length data?

I have a feeling I will need to use eval and the dodgy Function.prototype.toString to construct a new string with the same number of arguments. I want to avoid this.

Answer 1

I know you'd prefer some other way, but all I can think of is to hack together something with the Function constructor. Messy, to say the least, but it seems to work:

var replaceFn = (function(){
    var args = 'abcdefghijklmnopqrstuvwxyz'.split('');
    return function replaceFn(oldFn, newFn) {
        var argSig = args.slice(0, oldFn.length).join(',');
        return Function(
            'argSig, newFn',
            'return function('
                + argSig +
            '){return newFn.apply(this, arguments)}'
        )(argSig, newFn);
    };
}());

// Usage:
var _fun = fun;

fun = replaceFn(fun, function() {
  /* ... */
  _fun.apply(this, arguments);
});