javascript hasOwnProperty returns true instead of false?

Question

I don't understand why alert(John.hasOwnProperty('firstName')); returns true whereas firstName is defined in Person prototype not in instance John ?

https://jsfiddle.net/xfdnsg2w/

  Person = function(firstName, lastName) {

      this.firstName = firstName;
      this.lastName = lastName;

  }

  var John = new Person("John");
  alert(John.hasOwnProperty('toString'));
  alert(John.hasOwnProperty('firstName'));

Answer 1

The "firstName" property in your code is not defined in the Person prototype. It's initialized in the constructor as an "own" property.

Even if there were "firstName" and "lastName" properties on the prototype, as soon as you assign values to them in the constructor they'd become "own" properties anyway. Prototype properties are generally used as properties to access, and usually they have functions as values.