Question

Regarding a line of this script:

function Vehicle(hasEngine, hasWheels) {
    this.hasEngine = hasEngine || false;
    this.hasWheels = hasWheels || false;
}

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

Car.prototype = new Vehicle(true, true);
Car.prototype.constructor = Car; 
Car.prototype.displaySpecs = function () {
    console.log(this.make + ", " + this.model + ", " + this.hp + ", " + this.hasEngine + ", " + this.hasWheels);
}

var myAudi = new Car ("Audi", "A4", 150);
myAudi.displaySpecs(); // logs: Audi, A4, 150, true, true

My question is: what does

Car.prototype.constructor = Car;  

do? More importantly, what are the consequences of not doing this, and in which circumstances is it MOST useful?

Answer 1

It restores the .constructor property that was on the original prototype object that you overwrote. People restore it because it’s expected to be there.

Some people like to do…

if (my_obj.constructor === Car) { ... }

This isn’t required, since instanceof is a better test IMO.

if (my_obj instanceof Car) { ... }

if (my_obj instanceof Vehicle) { ... }