Defining prototype methods inside the constructor

Question

Today, I saw a JavaScript pattern I have never seen in my whole life. I cannot tell the purpose of using this pattern. It seems wrong to me, but I want to be a little conservative. It might be some awesome pattern I never saw before.

function Dog() {
    Dog.prototype.bark = function () {
        alert('woof!');
    }

    this.bark = function () {
        Dog.prototype.bark();
    }

    this.bark();
}

First, I'm not a fan for making methods (as privileged members) inside the constructor for no reason. It would cause creating functions every time when an instance is created. Second, in this code snippet, it also calls the prototype name "Dog", instead of "this". This makes me super confused.

Anyone knows what good about it?

Thanks! Grace

Answer 1

This is a very bad idea, for a great number of reasons. A few of which are:

1. Adding methods to the prototype in the constructor will cause the prototype method to be replaced for all instances, everytime you instantiate a new Dog.
2. Calling Dog.prototype.bark() means that this will be Dog.prototype and not your instance of Dog, which can cause serious issues.
3. this.bark = function () { Dog.prototype.bark(); } is some serious WTF. Because this.bark will already evaluate to the prototype method making this unnecessary. And calling it like this actually destroys the natural this value, as mentioned in #2.

Here is what is should be:

function Dog() {
  this.makeSound();
};

Dog.prototype.bark = function() {
  alert('woof');
};

Dog.prototype.makeSound = function() {
  this.bark();
};

Or alternatively, without the prototype at all:

function Dog() {
  this.bark = function() {
    alert('woof');
  };

  this.makeSound = function() {
    this.bark();
  };

  this.makeSound();
};

I would not trust this snippet of yours at all.