Why do we need "var self = this" in classes in Javascript?

Question

Why can't we directly use this instead of self in the following example?

function SeatReservation(name, initialMeal) {
    var self = this;
    self.name = name;
    self.meal = ko.observable(initialMeal);
}

After responses, I've learned:

Yes, there is no need if there is no context switch in class.

But I will use this approach as "convention" although there is no need.

Answer 1

There's no reason why you can't use this directly there (and I would say it would be better for readability if you did).

However, the var self = this; is often needed in situations like the following (basically, any asynchronous action like event binding, AJAX handlers etc, where the resolution of this is deferred until it equals something else);

function SeatReservation(name, initialMeal) {
    var self = this;
    self.name = name;
    self.meal = ko.observable(initialMeal);

    setTimeout(function () {
        alert(self.name); // otherwise, this is window; use self to keep a reference to the "SeatReservation" instance.
    }, 100);
}

Answer 2

It is usually done in order to keep a reference to this when the context is changing. It is often used in event handlers or callback functions. But as mentioned before, there is no reason to use it in your specific example.

You will find more details in the following article: http://www.alistapart.com/articles/getoutbindingsituations