Question

var Dog = function(name) {
this.name = name;
this.sayName();
}

Dog.prototype.sayName = function() {
  alert(this.name);
}

I’m creating new instance of Dog object Dog('Bowwow'), but method sayName() is undefined. Why?

Or maybe I should do something like (but I can’t see difference)…

var Dog = function(name) {

  this.name = name;

  this.sayName();

  this.prototype.sayName = function() {
    alert(this.name);
  }
}

Thank you.

Answer 1

JavaScript is a little dodgy in this area, your code works as long as you call Dog using the new constructor.

new Dog("Hello world")

The new constructor makes this behave like you want it to. Otherwise it is totally different.