Entry

What are prototypes?

What is prototype inheritance?

Feb 23rd, 2000 21:47
Nathan Wallace, Martin Honnen, Martin Honnen

The client side javascript docs on developer.netscape.com have a page to
page comparison on how to define classes with java compared to object
prototypes with js.

Using a prototype is just the javascript way of definining a class/type
of objects with certain properties and methods.

Say you know you need to store data of persons aggregrated from a name
and an address you may have some declarational way in some language as
follows (pseudo code)

  class Person {
    String name;
    String address;
  }

In js you would define a constructor function for such objects

  function Person (name, address) {
    this.name = name || '';
    this.address = address || '';
  }

To create a person object you then use the expression

  new Person ('some name', 'some address')

To add methods to the objects of your class you define a javascript
function 

  function PersonToString () {
    return 'Name: ' + this.name + '\nAddress: ' + this.address;
  }

and instead of having some declarational way of including that function
in the class declaration you assign it as a property to the prototype

  Person.prototype.toString = PersonToString;

That just means that every object created with

  new Person (...)

will have a method 

  toString

defined.

Inheritance just means that you extend a certain class/type of object by
adding new properties and/or methods. For instance you could have a
subclass NetPerson of Person which additionally to the name and address
has a url property.

Now instead of having a declaration 

  class NetPerson extends Person {
    String url;
  }

in js you simply define a new constructor

  function NetPerson (name, address, url) {
    this.base = Person;
    this.base(name, address);
    this.url = url || '';
  }

for which you need to add the info that it inherits from Person. With js
you write

  NetPerson.prototype = new Person();

to do that.