Wierd behaviour of constructor property of an object

Question

In the following code, why is \'scotty.constructor\' set to \'Dog\' function ? How is the constructor property of an object set in general ? Is it set to the function that creat

Answer 1

Employee.prototype = Object.create(Person.prototype);
Employee.prototype.constructor = Employee; //If you don't set Object.prototype.constructor to Employee, 
                                           //it will take prototype.constructor of Person (parent). 
                                           //To avoid that, we set the prototype.constructor to Employee (child).

MDN Reference

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You need to set prototype's constructor to ShowDog manually again else it will see the parent constructor

ShowDog.prototype = new Dog();
ShowDog.prototype.constructor = ShowDog

// Dog Constructor
function Dog(name, breed, weight) {
  this.name = name;
  this.breed = breed;
  this.weight = weight;
}


// Dog Prototype
Dog.prototype.species = 'Canine';

Dog.prototype.bark = function() {
  if (this.weight > 25) {
    console.log(this.name + ' says Woof!');
  } else {
    console.log(this.name + ' says Yip!');
  }
};

Dog.prototype.run = function() {
  console.log('Run!');
};

// ShowDog Constructor
function ShowDog(name, breed, weight, handler) {
  this.name = name;
  this.breed = breed;
  this.weight = weight;
  this.handler = handler;
}

// ShowDog Prototype
ShowDog.prototype = new Dog();
ShowDog.prototype.constructor = ShowDog

ShowDog.prototype.league = 'Webville';

ShowDog.prototype.stack = function() {
  console.log('Stack');
};

// ShowDog Instance
var scotty = new ShowDog('Scotty', 'Scotish Terrier', 15, 'Cookie');
console.log('Scotty\'s constructor is ' + scotty.constructor);

Answer 2

Object don't have an own constructor property, it's taken from their protos. So, without this line:

ShowDog.prototype = new Dog();

the inheritance diagram would be

Once you assign new Dog to the prototype (thus discarding its current value), the picture becomes:

and the constructor value (Dog) is taken from scotty.__proto__.__proto__

To make inheritance work as intended, you have to assign the constructor manually:

ShowDog.prototype = new Dog()
ShowDog.prototype.constructor = ShowDog

which yields the following diagram: