Question

Or, in other words, how to make this work:

function foo(){}
//do something that modifies foo as if it was defined with "function foo(a,b,c){};"
console.log(foo.length); 
//output: 3

Answer 1

It is possible, but maybe not very nice:

function lengthDecorator(fun) {
    function update(len) {
        var args = []; // array of parameter names
        for (var i = 0; i < len; ++i) {
            args.push('a' + i);
        }

        var result = new Function('fun',
            'return function(' + args.join(',') + ') {' +
                'var args = Array.prototype.slice.call(arguments);' +
                'return fun.apply(this, args);' + // call supplied function
            '}'
        ); // create a function that will return a function

        result = result(fun); // make the fun param known to the inner function
        result.update = update;
        return result;
    }
    return update(fun.length);
}

Example usage:

var foo = lengthDecorator(function(a,b) {
    return a+b;
});

print('foo.length: ' + foo.length);
print('foo(2, 3): ' + foo(2, 3));

print('');
foo = foo.update(42);
print('foo.length: ' + foo.length);
print('foo(2, 3): ' + foo(2, 3));

Output:

foo.length: 2
foo(2, 3): 5

foo.length: 42
foo(2, 3): 5

(Live demo: Ideone.com, jsFiddle)

lengthDecorator wraps the supplied function with a function that takes the same amount of parameters as the supplied function. The parameter count can be changed with update.

C.f.

• new Function(...): Dynamically create a new function.
• fun.apply(...): “Calls a function with a given this value and arguments provided as an array.”