JS Object this.method() breaks via jQuery

This is not jQuery’s fault, it is integral to the way JavaScript handles objects.

Unlike in most other object-oriented languages, ‘this’ is not bound on a per-method level in JavaScript; instead, it’s determined purely by how the function is called:

Bob= {
    toString: function() { return 'Bob!'; },

    foo: function() { alert(this); }
};
Brian= {
    toString: function() { return 'Brian!'; },
};

Bob.foo(); // Bob!
Bob['foo'](); // Bob!

Brian.foo= Bob.foo;
Brian.foo(); // Brian! NOT Bob

var fn= Bob.foo;
fn(); // window NOT Bob

What you are doing in the case of:

$j('#MyButton').click( Bob.doSomething );

is like the last example with fn: you are pulling the function doSomething off Bob and passing it to jQuery’s event handler setter as a pure function: it no longer has any connection to Bob or any other object, so JavaScript passes in the global window object instead. (This is one of JavaScript’s worst design features, as you might not immediately notice that window isn’t Bob, and start accessing properties on it, causing weird and confusing interactions and errors.)

To remember Bob, you generally make a function as in nickyt’s answer, to keep a copy of ‘Bob’ in a closure so it can be remembered at callback time and used to call the real method. However there is now a standardised way of doing that in ECMAScript Fifth Edition:

$j('#MyButton').click( Bob.doSomething.bind(Bob) );

(You can also put extra arguments in the bind call to call doSomething back with them, which is handy.)

For browsers that don’t yet support Fifth Edition’s bind() method natively (which, at this point, is most of them), you can hack in your own implementation of bind (the Prototype library also does this), something like:

if (!Object.bind) {
    Function.prototype.bind= function(owner) {
        var that= this;
        var args= Array.prototype.slice.call(arguments, 1);
        return function() {
            return that.apply(owner,
                args.length===0? arguments : arguments.length===0? args :
                args.concat(Array.prototype.slice.call(arguments, 0))
            );
        };
    };
}