Javascript constructor "this reference"

Hello,
I just came to the object part in JavaScript, which is the instance of the object can be iterated by the for-loop (or with-loop):

function Car(seat_sth, engine_sth, radio_sth) {
    this.seats = seat_sth;
    this.engine = engine_sth;
    this.radio = radio_sth;
}
var work_car = new Car("Leather", "V-6", "Cassette/Disc");
for (var propname in work_car) {
    document.write(propname + ":    " + work_car[propname] + "<br>")
}

Instead of iterating an instance of the object, is there a way to have object constructor to traverse all object members similar to "this reference" in C++/PERL?

function Car(seat_sth, engine_sth, radio_sth) {
    this.seats = seat_sth;
    this.engine = engine_sth;
    this.radio = radio_sth;
    this.describe = describe_car();
    }
function describe_car() {
    for (var propname in this) {
    document.write(propname + ":    " + this[propname] + "<br>")
    }
}

Just started JS, and not sure I am asking a right question. Thanks!