Question

If I have a dummy constructor object:

function Circle()
{
    this.radius = 3;
}

An instance of this object would have the single “radius” property. a) How do I query the constructor object for the number of properties it has?

b) How would I query Circle.prototype for the number of properties it has? Trying something like console.log(Object.getOwnPropertyNames(Circle.prototype)) doesn’t return anything

Answer 1

You have several terminology things wrong.

1. You don’t “query the constructor object”. You can enumerate the properties of an actual object, but not the constructor function.
2. The way you are creating properties in your code example, you are not using the object’s prototype so if you were to iterate the object’s prototype, you would not see the radius property.

Assuming what you really meant to say is: “How do I iterate over the properties of an instance of my Circle object?”, the answer would be like this:

function Circle()
{
    this.radius = 3;
    this.border = 1;
    this.color = "red";
}

var obj = new Circle();
for (var i in obj) {
    // hasOwnProperty makes sure we get properties only of Circle, 
    // not of ancestors like Object
    if (obj.hasOwnProperty(i)) {
        // i will be properties of obj on each iteration
        console.log(i);      // radius, border, color
    }
}

The prototype of an object is a different thing. You can think of it like a structure that every new instance of your object inherits automatically. You could use the prototype like this:

function Circle(r)
{
    this.radius = r;
    this.border = 1;
    this.color = "red";
}

Circle.prototype.calcArea = function() {
    return(Math.PI * this.radius * this.radius);
}

Circle.prototype.calcCircumference = function() {
    return(Math.PI * this.radius * 2);
}

This will automatically give every instance of Circle, two methods calcArea and calcCircumference.

var cir = new Circle(4);
console.log(cir.calcArea());    // 54.624

You can also add methods to the prototype of pre-existing objects that you do not have the code for such as Array (though you have to be careful when doing this). For example:

Array.prototype.isSorted = function() {
    for (var i = 1; i < this.length; i++) {
        if (this[i] < this[i-1]) {
            return(false);
        }
    }
    return(true);
}

var x = [1,3,6,8];
var y = [1,3,8,6];

console.log(x.isSorted());    // true
console.log(y.isSorted());    // false