function foo() {
    this.tag = 'bar'
    this.bar = function(txt) {
        console.log('hello ' + txt);
    }
}

var bar = new foo();

function middleware(foo) {
    oldbar = foo.bar; /* save old function, don't make a reference! */
    foo.bar = function(txt) {
        /* ah, here's our chance to mutate txt argument! */
        txt = 'dozed off ' + txt;
        oldbar(txt);
    }

    foo.newtag = 'muhahaha';
}

middleware(bar); /* mutate! */
bar.bar('kyle');
console.log(bar.newtag);