SomeClass.prototype = {
    someMethod: function() {
        var somevar = 1;
        var self = this;

        someOtherFuncThatWantsACallback(123, function callback() {
            // in this scope we can access somevar even though it's outside
            alert(somevar);
            // but if we want to access "this" from the class, we need to use the "self" alias
        });

    }
};