Develop Reference

Trying to emulate 'new' operator

How can I emulate 'new' operator with function myNew? Tried everything but I can't get it work.
function Person(name) {
this.name = name;
}
Person.prototype.getName = function() {
return this.name;
}
function myNew(){
var obj = Object.create(Object.prototype);
var instance = this.apply(obj, arguments);
return instance;
}
var person = myNew(Person, 'Test');
console.log(person instanceof Person); // true
person.getName(); // Test

If you want an emulation without Reflect.construct, try this:
function myNew(constructor) {
var instance = Object.create(constructor.prototype)
var args = Array.from(arguments)
args.shift()
constructor.apply(instance, args)
return instance
}
If you support ES6, you can write it shorter as:
function myNew(constructor, ...args) {
var instance = Object.create(constructor.prototype)
constructor.apply(instance, args)
return instance
}
Edit: Just noticed Array.from is available in ES6 but if you want to support ES5, you can easily simulate it
Edit 2:
Looking to the MDN page of the new operator, I noticed that I forgot an important step in object creation from a constructor. The algorithm I used is almost identical to the one explained on the page except for the crucial part that, there are constructors that actually return something (not all constructors return undefined). In this case, the returned object is the evaluation of the new expression. So the final form (I hope) of the function will be
function myNew(constructor, ...args) {
var instance = Object.create(constructor.prototype)
var obj = constructor.apply(instance, args)
if(obj instanceof Object) return obj
return instance
}

You can use Reflect.construct.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Reflect/construct
function Person(name) {
this.name = name;
}
Person.prototype.getName = function() {
return this.name;
}
var person = Reflect.construct(Person, ['Test']);
console.log(person instanceof Person); // true
console.log(person.getName()); // Test

JavaScript inheritance and super constructor

I have found and adapted a JavaScript "class" extend function from coffeescript:
var extend = (function() {
var hasProp = Object.prototype.hasOwnProperty;
function ctor(child) {
this.constructor = child;
}
return function(child, parent) {
for (var key in parent) {
if (hasProp.call(parent, key)) {
child[key] = parent[key];
}
}
ctor.prototype = parent.prototype;
child.prototype = new ctor(child);
child.__super__ = parent.prototype;
// child.prototype.__super__ = parent.prototype; // better?
return child;
};
})();
I am wondering, if there is a reason why they used child.__super__ instead of child.prototype.__super__ (see out-commented code line).
I like the out-commented version more because:
You can access super properties via this.__super__.propertyName instead of ClassName.__super__.propertyName. So you have no redundancy in the class naming.
This makes even more sense for nested inheritance since you can use this.__super__.__super__.propertyName instead of ClassName.__super__.constructor.__super__.propertyName
I do not see any reason for it, but you could even still call "static" functions in a "static" way like that:
ClassName.prototype.__super__.constructor.staticMethod()
Are there any drawbacks with my version that I might have overlooked?
EDIT: I corrected the line to var hasProp = Object.prototype.hasOwnProperty;

Because you're not supposed to use __super__ in your code at all.
It's a compiler artifact, every use of the super macro/keyword/whatever it is will compile to
ClassName.__super__.methodName.call(this, …) // or
ClassName.__super__.methodName.apply(this, …)
// or, in static class functions even
ClassName.__super___.constructor.functionName.call(this, …)
They don't trust the dynamic this binding that you have proposed to use (this.__super__), they rather went for a static reference of the parent. Actually it might have been a good idea not to use a property at all, but just a local super variable in their module scope.
Also, this.__super__ will not work in an inherited method:
function A() { }
A.prototype.method = function() { console.log("works") };
function B() { A.call(this); }
B.prototype = Object.create(A.prototype);
B.prototype.__super__ = A.prototype;
B.prototype.method = function() { this.__super__.method.call(this); }
function C() { B.call(this); }
C.prototype = Object.create(B.prototype);
C.prototype.__super__ = B.prototype;
var b = new B(), c = new C();
b.method() // "works"
c.method() // Maximum recursion depth exceeded
Stack Overflow because you did not get the .__super__ that you expected!

Simple JavaScript Inheritence plugin error

I read John Resig's blog article on a Simple JavaScript Inheritance plugin that he wrote based on base2 and prototype.
Example code is here: http://jsfiddle.net/rFfX9/
//'use strict';
/* 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;
// And make this class extendable
Class.extend = arguments.callee;
return Class;
};
})();
This works. However when I uncomment 'use strict'; at the top it throws an exception, but can't figure out the solution. Any ideas?

In this case when you call anonymous function this will be undefined you don't call the function as a method of an object. In not strict mode this will be window object.
See also: Why is "this" in an anonymous function undefined when using strict?

Object constructors building off other Object constructors

I'm currently in the process of converting a quite large actionscript library to work in a nodejs project of mine. While doing so I stumbled upon something that could be an issue: Building classes from classes.
Is there a way to use an object as the base for another object(IE: inherits all members from the base object, then overwrites same name members from the extending object)?
Right now this is what I'm doing, though it's getting a bit difficult to manage now that there are 3+ classes built one on top of another:
// The base object which others may extend
function A() {
this.a = "pie";
}
A.prototype.yum = function() {
return this.a + " is AWESOME!";
}
// The "extends A" object.
// Instead of creating an instance of "B", I current just create an instance of "A",
// then adding the members from "B" to it at which point I return the "A" instance.
function B() {
var a = new A();
a.b = "pie";
// Notice how I have to declare the overwriting function here instead of being able
// to drop it into B's prototype. The reason this bothers me is instead of just
// having one copy of the function(s) stored, each time a "new B" is created the
// function is duplicated... for 100s of "B" objects created, that seems like poor
// memory management
a.yum = function () {
return "I like " + this.a + " and " + this.b;
};
return a;
}
console.log((B()).yum());
Is it possible to do something along the following?
I know this isn't valid, but it gives the idea.
function A(){
this.a = "pie"
}
A.prototype.yum = function () {
return this.a + " is AWESOME!";
}
function B(){
// Throws an "illegal left hand assignment" Exception due to overwriting `this`;
this = new A();
this.b = "cake"
}
B.prototype.yum = function () {
return "I like "+this.a+" and "+this.b;
}
console.log((new B()).yum());
Notes:
1: I know javascript doesn't have classes; it uses objects and prototypes. Otherwise I wouldn't be asking.
2: This isn't the actual code im (trying) to convert; it's a generalized example
3: Please do not suggest a library. I know at times they are valuable, but I'd rather not have to maintain, depend on and include an entire library for the project.
ANSWER:
I know it's bad form to alter native member prototypes, but I think this merits it, due to the lack of possible functionality, and the size of it.
Object.prototype.extendsUpon = function (p) {
var h = Object.prototype.hasOwnProperty;
for(var k in p)if(h.call(p,k))this[k]=p[k];
function c(c){this.constructor=c;}
c.prototype = p.prototype;
this.prototype = new c(this);
this.__base__ = p.prototype;
}
function object_Constructor_built_ontop_of_another_constructor() {
this.extendsUpon(base_Object_to_built_atop_off);
this.__base__.constructor.apply(this, arguments);
// From here proceed as usual
/* To access members from the base object that have been over written,
* use "this.__base__.MEMBER.apply(this, arguments)" */
}

Very much possible. You can do it in multiple ways, the more complete is used in coffeescript:
var ClassBase, ClassTop,
__hasProp = {}.hasOwnProperty,
__extends = function(child, parent) { for (var key in parent) { if (__hasProp.call(parent, key)) child[key] = parent[key]; } function ctor() { this.constructor = child; } ctor.prototype = parent.prototype; child.prototype = new ctor(); child.__super__ = parent.prototype; return child; };
ClassBase = (function() {
function ClassBase() {}
return ClassBase;
})();
ClassTop = (function(_super) {
__extends(ClassTop, _super);
function ClassTop() {
return ClassTop.__super__.constructor.apply(this, arguments);
}
return ClassTop;
})(ClassBase);
There is going to be some boilerplate code. ClassTop is inheriting everything from ClassBase. The classes don't have much inside them other then an __extend, a (function(_super... and some constructor boilerplate but it's fairly simple.
The inheritance is mostly managed by the __extends boilerplate that does some magic. The full __extends method is beautified here:
__extends = function (child, parent) {
for (var key in parent) {
if (__hasProp.call(parent, key)) child[key] = parent[key];
}
function ctor() {
this.constructor = child;
}
ctor.prototype = parent.prototype;
child.prototype = new ctor();
child.__super__ = parent.prototype;
return child;
};
Again, much less scary then before. You're basically checking properties that the parent has and applying them to the child. More information can be found here: http://www.jimmycuadra.com/posts/coffeescript-classes-under-the-hood

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;
});