`this` is magic

this_is_magic.js

function Foo() {
    var self = this;
    this.hello = function(fn) {
        fn.apply(self, Array.prototype.splice.call(arguments, 1));
    };
    this.bar = function(what) {
        console.log('ok:' + what);
    };
}

var foo = new Foo();

foo.hello(function(plop) {
    this.bar(plop); // "ok:booh"
}, 'booh');