I need to trigger function bar() whenever function foo() fires. I have no control over function foo or whether it will change in the future. I have this situation regularly (and I hate it).

I wrote this function to add my code to the end of function foo:

function appendToFunction(fn,code){ 

if(typeof fn == 'function'){

    var fnCode = fn.toString() ;

    fnCode = fnCode.replace(/}$/,code+n}) ;

    window.eval(fnCode);                    // Global scope

    return true ;

}

else{

    return false ;

}

}

eg:

appendToFunction(foo,bar(););

This strikes me as a terrible idea - but it works. Can somebody point me in a better (safer) direction please.

EDIT: foo is not a specific function, but many functions that I wind up dealing with. They don't change dynamically in the page. But they may be changed (eg. form validation demands) from time to time.

Solution:
I settled on a modified version of Adam's Answer. It's not perfect, but it's better than what I had:

var oldFoo = foo ;

foo = function(){

        var result = oldFoo.apply(this, arguments);

        bar();

        return result ;

}

NB. Watch out for some native functions in IE6/7 that don't have an .apply() method.