Question about prototype declarations in JS and "standard practice"

js_question.md

There's a couple ways to define an "object" in JavaScript.

The following uses a standard function to declare a class and some attributes and an accessor:

function Car(make, model, year) {
    this.make  = make;
    this.model = model;
    this.year  = year;

    this.display = function() {
        return '' + this.year + ' ' + this.make + ' ' + this.model;
    }
}

car = new Car('2006', 'Hyundai', 'Elantra');
console.log('I drive a ' + car.display());

According to things I've read, and logic, this is actually slightly inefficient if you want to instantiate a whole fleet of cars. That's because the code declaring the accessor runs each and every time, which is both slowing you down (processing time) and bloating you up (taking up memory for each object with an anonymous function). The Better Way™ is to use prototype:

function Car(make, model, year) {
    this.make  = make;
    this.model = model;
    this.year  = year;
}

Car.prototype.display = function() {
    return '' + this.year + ' ' + this.make + ' ' + this.model;
}

car = new Car('2006', 'Hyundai', 'Elantra');
console.log('I drive a ' + car.display());

This is all well and good, except that I tend to prefer my methods to be all scoped inside of the "class" to which they belong. The question I have is, is this a common way to solve the prototyping problem, or is it frowned upon? It's not as inefficient as redeclaring the functions each time, but it's slightly less efficient than definitively declaring the functions only once:

function Car(make, model, year) {
    this.make  = make;
    this.model = model;
    this.year  = year;

    if (this.prototype.display) {
        this.prototype.display = function() {
            return '' + this.year + ' ' + this.make + ' ' + this.model;
        }
    }
}

car = new Car('2006', 'Hyundai', 'Elantra');
console.log('I drive a ' + car.display());

I'm actually not entirely sure if that'll work, as I'm unsure if this.prototype inside the declaration refers to the same thing as Car.prototype (I'd generally prefer to keep the class name written in as few places as possible) but I'm about to Try It And See.

my_final_answer.js

function Car(make, model, year) {
    var proto = Object.getPrototypeOf(this);

    this.make  = make;
    this.model = model;
    this.year  = year;

    proto.display = proto.display ||
    function() {
        return '' + this.year + ' ' + this.make +  ' ' + this.model;
    };
}

car = new Car('Hyundai', 'Elantra', '2006');
console.log('I drive a ' + car.display());

problemz.js

function Vehicle() {}

function Car(make, model, year) {
    var proto = Object.getPrototypeOf(this);

    this.make  = make;
    this.model = model;
    this.year  = year;

    proto.display = proto.display ||
    function() {
        return '' + this.year + ' ' + this.make +  ' ' + this.model;
    };
}
Car.prototype = new Vehicle();

function FlyingCar(make, model, year) {}
FlyingCar.prototype = new Car();

truck    = new Vehicle();
car      = new Car('Hyundai', 'Elantra', '2006');
spacecar = new FlyingCar('Toyama', 'Astro', '2050');

if (truck.display) {
    console.log('The truck is a ' + truck.display());
}
else {
    console.log('Truck? There is no truck.');
    try {
        console.log('But if it were, it would be a ' + truck.display());
    }
    catch (e) {
        console.log('Honest.');
        console.log(e);
    }
}

console.log('I drive a ' + car.display());
console.log('But I fly a ' + spacecar.display());


/* OUTPUT:
 *
 * Truck? There is no truck.
 * Honest.
 * > TypeError
 * I drive a 2006 Hyundai Elantra
 * But I fly a undefined undefined undefined
 */

problemz_fixed?.js

function Vehicle() {}

function Car(make, model, year) {
    var proto = Object.getPrototypeOf(this);

    this.make  = make;
    this.model = model;
    this.year  = year;

    proto.display = proto.display ||
    function() {
        return '' + this.year + ' ' + this.make +  ' ' + this.model;
    };
}
Car.prototype = Vehicle;

function FlyingCar(make, model, year) {
    var proto = Object.getPrototypeOf(this);
    proto.apply(this, arguments);
}
FlyingCar.prototype = Car;

truck    = new Vehicle();
car      = new Car('Hyundai', 'Elantra', '2006');
spacecar = new FlyingCar('Toyama', 'Astro', '2050');

if (truck.display) {
    console.log('The truck is a ' + truck.display());
}
else {
    console.log('Truck? There is no truck.');
    try {
        console.log('But if it were, it would be a ' + truck.display());
    }
    catch (e) {
        console.log('Honest.');
        console.log(e.message);
    }
}

console.log('I drive a '   + car.display());
console.log('But I fly a ' + spacecar.display());


/* OUTPUT:
 *
 * Truck? There is no truck.
 * Honest.
 * Object #<Vehicle> has no method 'display'
 * I drive a 2006 Hyundai Elantra
 * But I fly a 2050 Toyama Astro
 */

Best I can seem to come up with:

function Car(make, model, year) {
    var klass = Car;

    this.make  = make;
    this.model = model;
    this.year  = year;

    console.log(this);
    if (!klass.prototype.display) {
        klass.prototype.display = function() {
            return '' + this.year + ' ' + this.make +  ' ' +
                   this.model;
        };
    }
}

car = new Car('2006', 'Hyundai', 'Elantra');
console.log('I drive a ' + car.display());
``​`

Best I can seem to come up with:

function Car(make, model, year) {
    var klass = Car;

    this.make  = make;
    this.model = model;
    this.year  = year;

    console.log(this);
    if (!klass.prototype.display) {
        klass.prototype.display = function() {
            return '' + this.year + ' ' + this.make +  ' ' +
                   this.model;
        };
    }
}

car = new Car('2006', 'Hyundai', 'Elantra');
console.log('I drive a ' + car.display());​

function Car(make, model, year) {
    this.make  = make;
    this.model = model;
    this.year  = year;

    if (!Object.getPrototypeOf(this).display) {
        Object.getPrototypeOf(this).display = function() {
            return '' + this.year + ' ' + this.make + ' ' + this.model;
        };
        console.log('CREATED DISPLAY');
    } else {
        console.log("DIDN'T CREATE DISPLAY");
    }
}

function Car(make, model, year) {
    var proto = Object.getPrototypeOf(this);

    this.make  = make;
    this.model = model;
    this.year  = year;

    console.log(this);
    if (!proto.display) {
        proto.display = function() {
            return '' + this.year + ' ' + this.make +  ' ' + this.model;
        };
    }
}

My next question is whether there's anything similar to ||= in JS, cause the following did not work:

    proto.display ||= function() {
        return '' + this.year + ' ' + this.make +  ' ' + this.model;
    };

This works for me:

function Vehicle() {}

function Car(make, model, year) {
    var proto = Object.getPrototypeOf(this);

    this.make  = make;
    this.model = model;
    this.year  = year;

    proto.display = proto.display ||
    function() {
        return '' + this.year + ' ' + this.make +  ' ' + this.model;
    };
}
Car.prototype = new Vehicle();

function FlyingCar(make, model, year) {}
FlyingCar.prototype = new Car();

var a = new FlyingCar('a','b','c');
a.display()

It outputs "undefined undefined undefined"

Hey, lookit that - all my comments posted, days later. Weird.