In the previous JavaScript article tutorial - Some Essential JavaScript Prototype Functions to Enhance your JavaScript Applications - jsB@nk presented to you one JavaScript prototype function to copy/clone a JavaScript object, but in this post, there's only JavaScript source code to make it without detailed instructions and comments.

Therefore, in this JavaScript article tutorial, jsB@nk will show you a full detailed instructions and guides in copying JavaScript objects with many JavaScript example codes and many ways to use.

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Everyone knows Javascript doesn�t have classes. Thankfully most Javascript programmers are very good at playing pretend.

Because so many people attempt to introduce a classical structure to their Javscript code, I figure I�ll share some of my findings. Below is one way of allowing a base class function to copy an instance of a subclass while maintaining proper type.

Let�s say we have an Animal class and we subclass it to create a Cat class. We also want the Animal class to have a clone() method that will return work with its subclasses. That is, if I clone a Cat, I want �instanceof Cat� and �instanceof Animal� to both be true.

Let�s make the classes real quick. Here�s an Animal:

function Animal() {

  // animal stuff
}
 
// All of Animal's methods will go into its prototype
Animal.prototype = {
  saySomething: function() {

    return "I am an animal!";
  },
 
  copy: function() {

    var newcopy;
    if (this.subConstructor) {

      newcopy = new this.subConstructor;
    } else {

      newcopy = new Animal();
    }
 
    // maybe copy over other values

 
    return newcopy;
  }
};

And then the Cat. Typically the easiest way to make a Javascript function be a �subclass� of another function is to set the prototype of the subclass equal to �new baseclass.� With cat we�ll go a little further and also save the constructor.

Cat:

function Cat() {
  this.constructor(); // call animal's constructor

  // cat stuff
}
 
// Save the cat's constructor, replace Cat's prototype and constructor
// Then restore cat's constructor as subConstructor
var con = Cat.prototype.constructor;

Cat.prototype = new Animal();
Cat.prototype.constructor = Animal;

Cat.prototype.subConstructor = con
 
// override the saySomething function
Cat.prototype.saySomething = function() {

  return "Meow!";
};

There you have it. Cats created this way will be �instanceof Cat� and �instanceof Animal�, and so will their clones. We should of course make tests to see, and lucky for us all of them check out OK:

// some tests and their results in the comment
 
var c = new Cat();

console.log(c instanceof Animal);   // true
console.log(c instanceof Cat);      // true

console.log(c.saySomething());      // "Meow!"
 
var a = new Animal();

console.log(a instanceof Animal);   // true
console.log(a instanceof Cat);      // false

console.log(a.saySomething());      // "I am an animal!"
 
var copyCat = c.copy();

console.log(copyCat instanceof Animal); // true
console.log(copyCat instanceof Cat);    // true

console.log(copyCat.saySomething());    // "Meow!"
 
var copyAnimal = a.copy();

console.log(copyAnimal instanceof Animal); // true
console.log(copyAnimal instanceof Cat);    // false

console.log(copyAnimal.saySomething());    // "I am an animal!"

You might want to use the four lines needed for subclassing elsewhere so its a good idea to stuff them into a function that you�d put in a utility class/var somewhere. So we could make:

function derive(base, sub) {

  var con = sub.prototype.constructor;
  sub.prototype = new base();

  sub.prototype.constructor = base;
  sub.prototype.subConstructor = con

}

And then making a clone-able subclass would be as easy as calling derive after each constructor:

function Cat() {
  this.constructor(); // call animal's constructor

  // cat stuff
}
 
derive(Animal, Cat);
 
// override the saySomething function

Cat.prototype.saySomething = function() {
  return "Meow!";

};

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by nikigor, 14/07/2011 06:07 PM
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Also started learning java scripts, I think yuudu create their own

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