Question

I have a Class that has a private variable with a public setter/getter function:

function Circle(rad) {
    var r = rad;

    this.radius = function(rad) {
        if(!arguments.length) return r;
        r = rad;
        return this;
    }
}

var shape = new Circle(10);
console.log( shape.radius() ); // 10
shape.r = 50;
console.log( shape.radius() ); // 10

How can I replicate this using Object.prototype? Or, when would I want to use a closure instead of Object.prototype? This is the closest I could come up with, but as you can see, you can change the property directly.

function Circle(r) {
    this.r = r;
}

Circle.prototype.radius = function(r) {
    if(!arguments.length) return this.r;
    this.r = r;
    return this;
};

var shape = new Circle(10);
console.log( shape.radius() ); // 10
shape.r = 50;
console.log( shape.radius() ); // 50

Answer 1

If you’re going to use the prototype to store an object’s properties, they are accessible from any code that has a reference to the object. It’s impossible to do what you want.

What many JS devs do is just name private properties with a leading underscore so that others know not to mess with it, but it doesn’t give you any real protection beyond a suggestion

Reasons to use closure based approach

• True private variables, be confident that no one will mess with your privates

Reasons to use prototype

• Less memory used (no closures for every instance)
• Easier to debug (properties are visible on the object itself)
• Allows monkey patching

Readers: Please edit the answer with reasons for whatever you think is the best solution.