Javascript prototypal inheritance and OOP

Javascript prototypal inheritance and OOP - javascript

I'm creating an application that allows a user to create widgets. There are several different types of widgets, and I have defined them using protypal inheritance. i.e.
//the base widget that all widgets inherit from
var Widget = function(){}
Widget.prototype.someFunction = function(){}
//widget type A
var A = function(){}
A.prototype = new Widget();
//widget type B
var B = function(){}
B.prototype = new Widget();
I have discovered that it will be convenient to add a method on the base class that can create a new widget instance of the same type. i.e.
//the base widget
var Widget = function(){};
Widget.prototype.clone = function(){
switch(this.type){
case 'A':
return new A();
break;
case 'B':
return new B();
break;
default:
break;
}
};
Which would allow me to get a new widget of the same type using the following code:
var widgetTypeA = new A();
var cloneOfWidgetTypeA = widgetTypeA.clone();
My concern is that the base widget now has to be explicitly aware of each of the types of widgets that inherit from it. Does this violate any principles of good OOP?

Widget.prototype.clone = function() {
var constructor = window[this.type];
return new constructor();
};
Assuming that all your subclasses are declared as globals of course.
But honestly I would out those sub classes in the Widget namespace, and access them through it rather than making everything global.
Widget.A = function(){};
Widget.A.prototype = new Widget();
Widget.prototype.clone = function() {
var constructor = Widget[this.type];
return new constructor();
};

Given that your constructors are globals, you could do something like:
var global = this;
Widget.prototype.clone = function() {
if (global[this.type])
return new global[this.type]();
};
Provided each instance has a type property whose value is the name of the constructor. Or you could fix the constructor property of constructor's prototype and do:
Widget.prototype.clone = function() {
return new this.constructor();
};
function A() { };
A.prototype = new Widget();
A.prototype.constructor = A;
var a = new A();
var aa = a.clone();
However, that assumes that you don't have any parameters to pass. If you do have parameters to pass, then you likely have to know which type you are making and so can call the correct constructor anyway.

If ECMA5 is supported:
use Object.create(Object.getPrototypeOf(this));
If ECMA5 is not supported:
create an anonymous function
set the prototype of the anonymous function to the non-standard attribute this.__proto__
Example:
var Widget = function() { };
Widget.prototype.clone = function() {
/*
Non-ECMA5:
var newClone = function() {};
newClone.prototype = this.__proto__;
return new newClone();
*/
// ECMA5
return Object.create(Object.getPrototypeOf(this));
}
var A = function() { };
A.prototype = new Widget();
A.prototype.name = "I'm an A";
var B = function() { };
B.prototype = new Widget();
B.prototype.name = "I'm a B";
var x1 = new A();
var y1 = x1.clone();
console.log("y1 should be A: %s", y1.name);
var x2 = new B();
var y2 = x2.clone();
console.log("y2 should be B: %s", y2.name);

The information you need is already available in the constructor property. However, overwriting prototype will lose it as I recently explained here.
Using my own class implementation for ECMAScript version 3 or version 5, your example would look like this:
var Widget = Class.extend({
someFunction : function() {
alert('someFunction executed');
},
clone : function() {
return new this.constructor;
}
});
var A = Widget.extend();
var B = Widget.extend({
constructor : function(arg) {
Widget.call(this); // call parent constructor
this.arg = arg;
},
// override someFunction()
someFunction : function() {
alert('someFunction executed, arg is ' + this.arg)
},
// clone() needs to be overriden as well:
// Widget's clone() doesn't know how to deal with constructor arguments
clone : function() {
return new this.constructor(this.arg);
}
});
var a = new A;
var a2 = a.clone();
a2.someFunction();
var b = new B(42);
var b2 = b.clone();
b2.someFunction();

Related

Javascript prototype method "Cannot set property"

I'm always getting Cannot set property 'saySomething' of undefined but why?
Am I making a mistake somewhere?
var Person = new Object();
Person.prototype.saySomething = function ()
{
console.log("hello");
};
Person.saySomething();

Debugging tip: You get this ..of undefined errors when you try to access some property of undefined.
When you do new Object(), it creates a new empty object which doesn't have a prototype property.
I am not sure what exactly are we trying to achieve here but you can access prototype of function and use it.
var Person = function() {};
Person.prototype.saySomething = function() {
console.log("hello");
};
var aperson = new Person();
aperson.saySomething();

The prototype property exists on functions, not on instantiated objects.
var Person = new Object();
console.log(Person.prototype); // undefined
var Person2 = function () {}
console.log(Person2.prototype); // {}
This is useful because things put on the prototype of a function will be shared by all object instances created with that function (by using new).
var Person = function() {};
Person.prototype.saySomething = function() {
console.log("hello");
};
console.log(
new Person().saySomething === Person.prototype.saySomething // true. they are the same function
);
If all you want is to add a method to the person object, there's no need for a prototype:
var Person = {};
Person.saySomething = function() {
console.log("hello");
};
Person.saySomething();
You can even use object literal syntax:
var Person = {
saySomething: function() {
console.log("hello");
}
};
Person.saySomething();

i was trying out some code thought of posting it, might help others.
<script>
var MODULE = {};
MODULE = (function (my) {
my.anotherMethod = function () {
console.log("hello ");
};
my.newMethod = function(){
console.log("hi new method ");
}
return my;
}(MODULE));
MODULE.anotherMethod();
MODULE.newMethod();
</script>
And please not var MODULE ={}, if this is not initialized with {} then it give cannot set property.

I know i am late to the party but as you see there is no satisfying answer available to the question so i am providing my own.
In your case when you write
var Person = new Object();
you are creating an instance of Object type.
You can add a property using prototype property to the Object, not to the instance of Object.which you can use by the instance laterly.
so you can define like
Object.prototype.saySomething = function ()
{
console.log("hello");
};
now you can call it like this.
Person.saySomething();

You can check here.
var Person = function(name) {
this.canTalk = true;
this.name = name;
};
Person.prototype.greet = function() {
if (this.canTalk) {
console.log('Hi, I am ' + this.name);
}
};
bob = new Person('bob');
bob.greet();

Javascript OOP Semi-circular inheritance

I try to resolve in a cleaner way that I made the following problem :
I've got two class A and B where A inherit from B.
What I want is when I call a method of the class B via the class A I get a new instance of B and then the method is executed, and keep the inheritance of instance A.
Note that the methods of B must return 'this' for chaining.
Example :
var classA = new class A();
classA.setName("John");
var result1 = classA.methodOfClassB().otherMethodOfClassB();
var result2 = classA.methodOfClassB().otherMethodOfClassB();
var result3 = result1.otherMethodOfClassB();
console.log(result1 === result2); //false
console.log(result1 === result3); //true
//Call a method of class A from class B
console.log(result1.methodOfClassAGetName()); //John
console.log(result1.methodOfClassAGetName()); //John
My implementation
function classA() {
var self = this;
self.methAclassA = function() {
/* ... */
console.log(this.methAclassB());
};
classB.call(self, true);
return (self);
}
function classB(isNew) {
var self = this;
var _getNewInstance = function() {
var instance = new classB(false);
instance.__proto__ = self;
return (instance);
};
self.methAclassB = function() {
if (isNew) {
return (_getNewInstance().methAclassB());
}
/* ... */
return (self);
};
}
Thank you in advance !
(Sorry for the English, this is not my first language)

Property shared between classes in javascript

function myclass(){
}
myclass.prototype = {
stuff: {},
getStuff: function(){
return Object.keys(this.stuff).length;
}
};
var m1 = new myclass();
m1.stuff[5] = 'test';
m1.stuff[6] = 'test';
var m2 = new myclass();
m2.stuff[12] = 'test';
m2.stuff[14] = 'test';
alert(m2.getStuff()); // 4 instead of 2 ?!
Can someone plz explain why does the alert print 4?
If I created a new instance of my function, the stuff property should be empty.

What you are doing here is generally considered bad practice. You don't want to overwrite the prototype, but you want to add to it.
Now to core of your problem. You are defining stuff as an attribute to the prototype. That means any object that is created using that prototype will share that object. If you want each instance to have it's own stuff, it needs to be an instance variable:
function myclass(){
this.stuff = {};
}
myclass.prototype = {
getStuff: function(){
return Object.keys(this.stuff).length;
}
};
But like I said, don't redefine the prototype, add onto it:
function myclass(){
this.stuff = {};
}
myclass.prototype.getStuff = function(){
return Object.keys(this.stuff).length;
}
Now, anything added to stuff will only be added to that particular instance:
var foo = new myclass();
foo.stuff.thing = "Hey, I'm foo";
var bar = new myclass();
bar.stuff.thing = "Hey, I'm bar";
console.log(bar.stuff); //{thing: "Hey, I'm bar"}

Static variables with John Resig's simple class pattern?

I'm referring to this article.
In it, he defines a function that looks something like this:
function makeClass() {
return function _class() {
if(this instanceof _class) {
if(typeof this.init === 'function') {
this.init.apply(this, arguments);
}
} else {
throw new Error('Constructor called as a function');
}
};
}
And then you can use it with something like this:
var MyClass = makeClass();
MyClass.prototype = {
init: function(width, height) { ... },
clear: function(ctx) {... },
draw: function(ctx) { ... }
}
But now I want to initialize some static variables that should be shared across all instances. How do I do that?

Well, the easiest approach is to define a static variable as a prototype property:
MyClass.prototype.xxx: 3, // ...
var t1 = new MyClass();
console.log(t1.xxx); // 3
... but it won't behave as static properties in other languages usually do:
var t2 = new MyClass();
t2.xxx = 5;
console.log(t1.xxx); // still 3 :(
The other way around is to use the fact that properties might be attached to functions as well:
MyClass.xxx = 3;
... but that narrows the ways we can use this property (it can't be called by t1.xxx from the previous examples).
There's another way, though. One can define static properties as variables, local to init method, accessible by methods, defined... in this init method as well. ) Like this.
init: function() {
var xxx = 3;
MyClass.prototype.getXXX = function() {
return xxx;
};
MyClass.prototype.setXXX = function(newXXX) {
xxx = newXXX;
}
}
Then all one can use this property simply by this:
var t1 = new MyClass();
var t2 = new MyClass();
console.log(t1.getXXX()); // 3
console.log(t2.getXXX()); // 3
t1.setXXX(5);
console.log(t1.getXXX()); // 5 now
console.log(t2.getXXX()); // 5 as well, behold the power of closures!
And here's a fiddle used.
UPDATE: this approach is better be used, I suppose, when we need to work with a (sort of) container of the static class data, that is to be shared by all objects - but we don't know exactly what can actually be stored in this container. Then we use just two functions - getStatic and setStatic - to store and retrieve data by string keys or some other identifiers. It may seem a bit confusing, and it is, but I think it may be worth an effort. )

Just add it to MyClass itself.
MyClass.myVariable = 42;
It's not really static in the Java/C# sense, but gives you the same effect.

I "solved" this problem by using a naming convention.
I wanted the convenience of the Class.extend({ }) syntax, but also a way to declare "static" properties within it.
I opted for a leading underscore to declare a static property, although you could do whatever you liked.
Usage:
var myClass = Class.extend({
_staticProperty: 1337
, instanceProperty: 'foo'
, instanceMethod: function() { }
, ctor: function() {
this.base();
}
});
note I've renamed init and this._super() from the original code
And the code:
/* Simple JavaScript Inheritance
* Modified by Andrew Bullock http://blog.muonlab.com to add static properties
* By John Resig http://ejohn.org/
* MIT Licensed.
*/
// Inspired by base2 and Prototype
(function () {
var initializing = false, fnTest = /xyz/.test(function () { xyz; }) ? /\bbase\b/ : /.*/;
// The base Class implementation (does nothing)
this.Class = function () { };
// Create a new Class that inherits from this class
Class.extend = function (prop) {
var base = this.prototype;
// Instantiate a base class (but only create the instance,
// don't run the init constructor)
initializing = true;
var prototype = new this();
initializing = false;
// The dummy class constructor
function Class() {
// All construction is actually done in the ctor method
if (!initializing && this.ctor)
this.ctor.apply(this, arguments);
}
// Copy static properties from base
for (var name in this) {
if (name.substr(0, 1) == '_')
Class[name] = this[name];
}
// Copy the properties over onto the new prototype
for (name in prop) {
// Check if we're overwriting an existing function
if (typeof prop[name] == "function" && typeof base[name] == "function" && fnTest.test(prop[name])) {
prototype[name] = (function(name, fn) {
return function() {
var tmp = this.base;
// Add a new .base() method that is the same method
// but on the super-class
this.base = base[name];
// The method only need to be bound temporarily, so we
// remove it when we're done executing
var ret = fn.apply(this, arguments);
this.base = tmp;
return ret;
};
})(name, prop[name]);
} else if (name.substr(0, 1) == '_') {
Class[name] = prop[name];
} else {
prototype[name] = prop[name];
}
}
// Populate our constructed prototype object
Class.prototype = prototype;
// Enforce the constructor to be what we expect
Class.prototype.constructor = Class;
// And make this class extendable
Class.extend = arguments.callee;
return Class;
};
})();

If you don't care about browser support, you could also use a WeakMap of constructor/static properties pairs. Here's the idea: http://jsfiddle.net/DfNNU/2/. This requires MyClass.prototype.constructor, which you should not discard. So, you'd need to add back constructor: MyClass to the prototype.
var statics = (function() {
var map = new WeakMap;
return function(inst) {
var ctor = inst.constructor;
return map.get(ctor) || map.set(ctor, {});
};
})();
Use it like:
var a = function() {};
var b = function() {};
var inst1 = new a;
var inst2 = new a;
var inst3 = new b;
statics(inst1).foo = 123;
statics(inst3).foo = 456;
console.log( statics(inst1).foo ); // 123
console.log( statics(inst2).foo ); // 123
console.log( statics(inst3).foo ); // 456

I've modified John Resig's class to provide copy over the parent's static members to the new class, which adds this:
for (var name in this) {
if (!Class[name]) {
Class[name] = this[name];
}
}
Here's a fiddle.
// This is a modified version of John Resig's simple inheritence class to add copying of static methods
// The new code is the for loop commented with "add in the static members"
/* Simple JavaScript Inheritance
* By John Resig http://ejohn.org/
* MIT Licensed.
*/
// Inspired by base2 and Prototype
(function(){
var initializing = false, fnTest = /xyz/.test(function(){xyz;}) ? /\b_super\b/ : /.*/;
// The base Class implementation (does nothing)
this.Class = function(){};
// Create a new Class that inherits from this class
Class.extend = function(prop) {
var _super = this.prototype;
// Instantiate a base class (but only create the instance,
// don't run the init constructor)
initializing = true;
var prototype = new this();
initializing = false;
// Copy the properties over onto the new prototype
for (var name in prop) {
// Check if we're overwriting an existing function
prototype[name] = typeof prop[name] == "function" &&
typeof _super[name] == "function" && fnTest.test(prop[name]) ?
(function(name, fn){
return function() {
var tmp = this._super;
// Add a new ._super() method that is the same method
// but on the super-class
this._super = _super[name];
// The method only need to be bound temporarily, so we
// remove it when we're done executing
var ret = fn.apply(this, arguments);
this._super = tmp;
return ret;
};
})(name, prop[name]) :
prop[name];
}
// The dummy class constructor
function Class() {
// All construction is actually done in the init method
if ( !initializing && this.init )
this.init.apply(this, arguments);
}
// Populate our constructed prototype object
Class.prototype = prototype;
// Enforce the constructor to be what we expect
Class.prototype.constructor = Class;
//add in the static members
for (var name in this) {
if (!Class[name]) {
Class[name] = this[name];
}
}
// And make this class extendable
Class.extend = arguments.callee;
return Class;
};
})();
function addText(text) {
document.getElementById('greetings').innerHTML = document.getElementById("greetings").innerHTML + '<br>' + text;
}
//parent class with a prototype method and two static methods
var Parent = Class.extend({
hello: function () {
addText('parent.hello');
}
});
Parent.static = function() {
addText('Parent.static');
}
Parent.overrideStatic = function() {
addText('Parent.overrideStatic');
}
//child class that overrides one of the parent's static methods
var Child = Parent.extend();
Child.overrideStatic = function() {
addText('Child.overrideStatic');
}
var parent = new Parent();
parent.hello();
Parent.static();
var child = new Child();
child.hello(); //should output parent.hello
Child.static(); //should output Parent.static
Child.overrideStatic();
<div id="greetings"></div>

Pass in an optional list of static members in the call to 'extend'. This method adds the static properties (if any) to a 'statics' attribute on the constructor function.
Code Changes
Changes as follows. These lines added just after the 'dummy class constructor' code:
if(staticProp) {
Class.statics = [];
for (var name in staticProp) {
!Class.statics[name] && (Class.statics[name] = staticProp[name]);
}
}
An additional argument 'staticProp' added when type is declared in order to allow introduction of static members at this stage:
Class.extend = function(prop,staticProp) {
A fiddle can be found here, includes some tests.
Usage Examples
Can define statics at type declaration time like so using the second optional constructor argument:
var A = Class.extend({},{myStatic:1});
Can access/define statics inside an instance method:
var B = Class.extend({test:function(){B.statics.myStatic=2;}});
Or from outside an instance:
A.statics.myStatic=3;
Example with requirejs:
Put Class.js in the baseUrl folder. Example new class definition. Not mandatory to name the file of the new class the same as the 'var C' (i.e. C.js) but probably better for readability so references to the C name within the class's methods are aligned to any external references to its static members:
define(['Class'],function($) {
var C = Class.extend({
init: function(params){
C.statics.myStatic++; // access static data
}
},{
myStatic: 123
});
return C;
});
Another class in D.js refers to static data in class C:
define(['Class', 'C'],function($,C) {
var D = Class.extend({
init: function(params){
C.statics.myStatic++; // static data of another class
}
},{});
return D;
});

javascript "polymorphic callable objects"

I saw this article on polymorphic callable objects and was trying to get it to work, however it seems that they are not really polymorphic, or at least they do not respect the prototype chain.
This code prints undefined, not "hello there".
Does this method not work with prototypes, or am I doing something wrong?
var callableType = function (constructor) {
return function () {
var callableInstance = function () {
return callableInstance.callOverload.apply(callableInstance, arguments);
};
constructor.apply(callableInstance, arguments);
return callableInstance;
};
};
var X = callableType(function() {
this.callOverload = function(){console.log('called!')};
});
X.prototype.hello = "hello there";
var x_i = new X();
console.log(x_i.hello);

You'd need to change this:
var X = callableType(function() {
this.callOverload = function(){console.log('called!')};
});
to this:
var X = new (callableType(function() {
this.callOverload = function(){console.log('called!')};
}));
Notice the new as well as the parentheses around the callableType invocation.
The parentheses allows callableType to be invoked and return the function, which is used as the constructor for new.
EDIT:
var X = callableType(function() {
this.callOverload = function() {
console.log('called!')
};
});
var someType = X(); // the returned constructor is referenced
var anotherType = X(); // the returned constructor is referenced
someType.prototype.hello = "hello there"; // modify the prototype of
anotherType.prototype.hello = "howdy"; // both constructors
var some_i = new someType(); // create a new "someType" object
console.log(some_i.hello, some_i);
var another_i = new anotherType(); // create a new "anotherType" object
console.log(another_i.hello, another_i);
someType(); // or just invoke the callOverload
anotherType();
I really don't know how/where/why you'd use this pattern, but I suppose there's some good reason.

Develop Reference

JavaScript is the programming language of the Web.

Javascript prototypal inheritance and OOP - javascript

Related

Javascript prototype method "Cannot set property"

Javascript OOP Semi-circular inheritance

Property shared between classes in javascript

Static variables with John Resig's simple class pattern?

javascript "polymorphic callable objects"

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