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

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.