Question

I wanted to have a method of adding functionality to pre-existing functions, so I made this:

Function.prototype.attachFunction = function(toAttach)
{
    var func = this; //This is the original function, which I want to attack toAttach function to.

    //Can I do this without wrapping it in a self-executing function? What's the difference?
    func = (function()
    {
        return function()
        {
            func();
            toAttach();
        }
    })();

    return func; //Return the newly made function, not really important.    
}

I paste this into the Google Chrome console, and there are no errors, however, it does not (or so it would seem) alter the original function at all.

f = function() {console.log("g");};
f.attachFunction(function(){console.log("New function!");});
f(); //Prints out just "g".

Answer 1

When attachFunction executes, it returns a function which executes func() and toAttach. However, if you change your code to be the following, it will attempt to execute both functions, where currently the old f is still called at the end.

f = function() {console.log("g");};
f = f.attachFunction(function(){console.log("New function!");});
f(); //Causes an infinite loop since calling func() is the same as calling this()

To merge two functions we need to wrap them the same way but not while extending Function

var func1 = function(){
    console.log('g');
};
var func2 = function(){
    console.log('New function!');
};

func1 = (function(f1,f2){
    return function(){            
        f1();
        f2();
    }
}(func1, func2));

func1(); //Writes out both g and New function!​​​​​​​​​​​​​​​​​​​​

The reason I pass in func1, and func2 as parameters is to prevent the same problem of getting into an infinite loop. I want to maintain a reference to these functions at that point in time.

A helper function would then be the following

function combineFunctions(f1, f2){
    return function(){
        f1();
        f2();
    }
}

and would be used like this

var f = function() {console.log("g");};
f = combineFunctions(f,function(){console.log("New function!");});
f();