Inheriting objects

Question

I have read that Javascript's inheritance is prototypal.What does it mean?How can an object defined by the programer inherit the properties of a predefined object such as window ? For example I need the function eval() in my own class . How can it be achieved?

Answer 1

Setting aside the question of whether you should inherit from window, here's a simple example that demonstrates how to do it:

function Test() {
    // constructor code
}

Test.prototype = window;

var t = new Test();
t.eval();

When invoked using the new operator, the Test function creates a new instance of Test whose prototype is linked to the window object. A function's prototype can be any object.

Answer 2

Protip: Don't use new for declaring objects. Object.create is the proper way to create JavaScript objects. It's supported in all modern browsers. For other browsers, a nice shim is at the bottom of this article:

http://javascript.crockford.com/prototypal.html

One problem with constructors that use 'new' is that if people accidentally call it like a regular function, it will make modifications on the global variable (i.e. window if in the browser) using the variable this, so this should only be reserved for prototype functions, and I prefer to use that instead of this

I personally use a style that supports new obj(), obj(), and obj.init(), which may be one extra function pointer, but I think it adds semantic meaning

function obj(){
    var that = Object.create(obj.prototype)
    //any constructor logic goes here
    return that
}
obj.prototype = Object.create(Superclass.prototype)

obj.init = obj //completely optional

//other function declarations

obj.prototype.someFunction = function(){
    //logic for someFunction
}

Answer 3

What do you want exactly echieve to? Here is the simplest way (not the best) of prototype inheritance:

var obj1=new Object1Constructor();
var obj2=new Object2Constructor();

obj2.prototype=obj1;

It means that obj1 inherits all properties of obj2

I've forgot the main thing: eval==evil;

UPDATE: I've made mistakes in the code above. That is not inheritance. Here is updated code:

var Object1Constructor=function(){ // one object constuctor
    //properties definition goes here
}

var Object2Constructor=function(){ // another object constuctor
    //properties definition goes here
}

Object2Constructor.prototype=new Object1Constructor();

var obj=new Object2Constructor();

And that is inharitance. Now obj has properties defined in Object2Constructor and Object1Constructor - parent 'class'. See CMS's comment below. He is totally right.