I've spent the last couple of hours trying to find a solution to my problem but it seems to be hopeless.
Basically I need to know how to call a parent method from a child class.
All the stuff that I've tried so far ends up in either not working or over-writing the parent method.
I am using the following code to set up OOP in javascript:
// SET UP OOP
// surrogate constructor (empty function)
function surrogateCtor() {}
function extend(base, sub) {
// copy the prototype from the base to setup inheritance
surrogateCtor.prototype = base.prototype;
sub.prototype = new surrogateCtor();
sub.prototype.constructor = sub;
}
// parent class
function ParentObject(name) {
this.name = name;
}
// parent's methods
ParentObject.prototype = {
myMethod: function(arg) {
this.name = arg;
}
}
// child
function ChildObject(name) {
// call the parent's constructor
ParentObject.call(this, name);
this.myMethod = function(arg) {
// HOW DO I CALL THE PARENT METHOD HERE?
// do stuff
}
}
// setup the prototype chain
extend(ParentObject, ChildObject);
I need to call the parent's method first and then add some more stuff to it in the child class.
In most OOP languages that would be as simple as calling parent.myMethod()
But I really cant grasp how its done in javascript.
Any help is much appreciated, thank you!
ES6 style allows you to use new features, such as super keyword. super keyword it's all about parent class context, when you are using ES6 classes syntax. As a very simple example, checkout:
Remember: We cannot invoke parent static methods via super keyword inside an instance method. Calling method should also be static.
Invocation of static method via instance method - TypeError !
class Foo {
static classMethod() {
return 'hello';
}
}
class Bar extends Foo {
classMethod() {
return super.classMethod() + ', too';
}
}
console.log(Bar.classMethod()); // 'hello' - Invokes inherited static method
console.log((new Bar()).classMethod()); // 'Uncaught TypeError' - Invokes on instance method
Invocation of static method via super - This works!
class Foo {
static classMethod() {
return 'hello';
}
}
class Bar extends Foo {
static classMethod() {
return super.classMethod() + ', too';
}
}
console.log(Bar.classMethod()); // 'hello, too'
Now super context changes based on invocation - Voila!
class Foo {
static classMethod() {
return 'hello i am static only';
}
classMethod() {
return 'hello there i am an instance ';
}
}
class Bar extends Foo {
classMethod() {
return super.classMethod() + ', too';
}
}
console.log((new Bar()).classMethod()); // "hello there i am an instance , too"
console.log(Bar.classMethod()); // "hello i am static only"
Also, you can use super to call parent constructor:
class Foo {}
class Bar extends Foo {
constructor(num) {
let tmp = num * 2; // OK
this.num = num; // ReferenceError
super();
this.num = num; // OK
}
}
And of course you can use it to access parent class properties super.prop.
So, use ES6 and be happy.
Here's how its done: ParentClass.prototype.myMethod();
Or if you want to call it in the context of the current instance, you can do:
ParentClass.prototype.myMethod.call(this)
Same goes for calling a parent method from child class with arguments:
ParentClass.prototype.myMethod.call(this, arg1, arg2, ..) * Hint: use apply() instead of call() to pass arguments as an array.
Well in order to do this, you are not limited with the Class abstraction of ES6. Accessing the parent constructor's prototype methods is possible through the __proto__ property (I am pretty sure there will be fellow JS coders to complain that it's depreciated) which is depreciated but at the same time discovered that it is actually an essential tool for sub-classing needs (especially for the Array sub-classing needs though). So while the __proto__ property is still available in all major JS engines that i know, ES6 introduced the Object.getPrototypeOf() functionality on top of it. The super() tool in the Class abstraction is a syntactical sugar of this.
So in case you don't have access to the parent constructor's name and don't want to use the Class abstraction you may still do as follows;
function ChildObject(name) {
// call the parent's constructor
ParentObject.call(this, name);
this.myMethod = function(arg) {
//this.__proto__.__proto__.myMethod.call(this,arg);
Object.getPrototypeOf(Object.getPrototypeOf(this)).myMethod.call(this,arg);
}
}
Here's a nice way for child objects to have access to parent properties and methods using JavaScript's prototype chain, and it's compatible with Internet Explorer. JavaScript searches the prototype chain for methods and we want the child’s prototype chain to looks like this:
Child instance -> Child’s prototype (with Child methods) -> Parent’s prototype (with Parent methods) -> Object prototype -> null
The child methods can also call shadowed parent methods, as shown at the three asterisks *** below.
Here’s how:
//Parent constructor
function ParentConstructor(firstName){
//add parent properties:
this.parentProperty = firstName;
}
//add 2 Parent methods:
ParentConstructor.prototype.parentMethod = function(argument){
console.log(
"Parent says: argument=" + argument +
", parentProperty=" + this.parentProperty +
", childProperty=" + this.childProperty
);
};
ParentConstructor.prototype.commonMethod = function(argument){
console.log("Hello from Parent! argument=" + argument);
};
//Child constructor
function ChildConstructor(firstName, lastName){
//first add parent's properties
ParentConstructor.call(this, firstName);
//now add child's properties:
this.childProperty = lastName;
}
//insert Parent's methods into Child's prototype chain
var rCopyParentProto = Object.create(ParentConstructor.prototype);
rCopyParentProto.constructor = ChildConstructor;
ChildConstructor.prototype = rCopyParentProto;
//add 2 Child methods:
ChildConstructor.prototype.childMethod = function(argument){
console.log(
"Child says: argument=" + argument +
", parentProperty=" + this.parentProperty +
", childProperty=" + this.childProperty
);
};
ChildConstructor.prototype.commonMethod = function(argument){
console.log("Hello from Child! argument=" + argument);
// *** call Parent's version of common method
ParentConstructor.prototype.commonMethod(argument);
};
//create an instance of Child
var child_1 = new ChildConstructor('Albert', 'Einstein');
//call Child method
child_1.childMethod('do child method');
//call Parent method
child_1.parentMethod('do parent method');
//call common method
child_1.commonMethod('do common method');
In case of multiple inheritance level, this function can be used as a super() method in other languages. Here is a demo fiddle, with some tests, you can use it like this, inside your method use : call_base(this, 'method_name', arguments);
It make use of quite recent ES functions, an compatibility with older browsers is not guarantee. Tested in IE11, FF29, CH35.
/**
* Call super method of the given object and method.
* This function create a temporary variable called "_call_base_reference",
* to inspect whole inheritance linage. It will be deleted at the end of inspection.
*
* Usage : Inside your method use call_base(this, 'method_name', arguments);
*
* #param {object} object The owner object of the method and inheritance linage
* #param {string} method The name of the super method to find.
* #param {array} args The calls arguments, basically use the "arguments" special variable.
* #returns {*} The data returned from the super method.
*/
function call_base(object, method, args) {
// We get base object, first time it will be passed object,
// but in case of multiple inheritance, it will be instance of parent objects.
var base = object.hasOwnProperty('_call_base_reference') ? object._call_base_reference : object,
// We get matching method, from current object,
// this is a reference to define super method.
object_current_method = base[method],
// Temp object wo receive method definition.
descriptor = null,
// We define super function after founding current position.
is_super = false,
// Contain output data.
output = null;
while (base !== undefined) {
// Get method info
descriptor = Object.getOwnPropertyDescriptor(base, method);
if (descriptor !== undefined) {
// We search for current object method to define inherited part of chain.
if (descriptor.value === object_current_method) {
// Further loops will be considered as inherited function.
is_super = true;
}
// We already have found current object method.
else if (is_super === true) {
// We need to pass original object to apply() as first argument,
// this allow to keep original instance definition along all method
// inheritance. But we also need to save reference to "base" who
// contain parent class, it will be used into this function startup
// to begin at the right chain position.
object._call_base_reference = base;
// Apply super method.
output = descriptor.value.apply(object, args);
// Property have been used into super function if another
// call_base() is launched. Reference is not useful anymore.
delete object._call_base_reference;
// Job is done.
return output;
}
}
// Iterate to the next parent inherited.
base = Object.getPrototypeOf(base);
}
}
How about something based on Douglas Crockford idea:
function Shape(){}
Shape.prototype.name = 'Shape';
Shape.prototype.toString = function(){
return this.constructor.parent
? this.constructor.parent.toString() + ',' + this.name
: this.name;
};
function TwoDShape(){}
var F = function(){};
F.prototype = Shape.prototype;
TwoDShape.prototype = new F();
TwoDShape.prototype.constructor = TwoDShape;
TwoDShape.parent = Shape.prototype;
TwoDShape.prototype.name = '2D Shape';
var my = new TwoDShape();
console.log(my.toString()); ===> Shape,2D Shape
There is a much easier and more compact solution for multilevel prototype lookup, but it requires Proxy support. Usage: SUPER(<instance>).<method>(<args>), for example, assuming two classes A and B extends A with method m: SUPER(new B).m().
function SUPER(instance) {
return new Proxy(instance, {
get(target, prop) {
return Object.getPrototypeOf(Object.getPrototypeOf(target))[prop].bind(target);
}
});
}
more flexible answer with classic js.
You define "_parent = A.prototype;" in the child class, then you can call parent's methods with apply:
class A{
_msg='A';
_msgOnlyA=' great from A';
constructor(){
}
hello(){
console.log('hello '+this._msg+', '+this._msgOnlyA);
}
};
class B extends A{
_parent = A.prototype;
_msg='B';
constructor(){
super();
}
hello(){
this._parent.hello.apply(this, arguments);
console.log('hello '+this._msg);
}
};
var b = new B();
b.hello();
While you can call the parent method by the prototype of the parent, you will need to pass the current child instance for using call, apply, or bind method. The bind method will create a new function so I doesn't recommend that if you care for performance except it only called once.
As an alternative you can replace the child method and put the parent method on the instance while calling the original child method.
function proxy(context, parent){
var proto = parent.prototype;
var list = Object.getOwnPropertyNames(proto);
for(var i=0; i < list.length; i++){
var key = list[i];
// Create only when child have similar method name
if(context[key] !== proto[key]){
let currentMethod = context[key];
let parentMethod = proto[key];
context[key] = function(){
context.super = parentMethod;
return currentMethod.apply(context, arguments);
}
}
}
}
// ========= The usage would be like this ==========
class Parent {
first = "Home";
constructor(){
console.log('Parent created');
}
add(arg){
return this.first + ", Parent "+arg;
}
}
class Child extends Parent{
constructor(b){
super();
proxy(this, Parent);
console.log('Child created');
}
// Comment this to call method from parent only
add(arg){
return super.add(arg) + ", Child "+arg;
}
}
var family = new Child();
console.log(family.add('B'));
Related
I am confused should I declare methods inside or outside the constructor method in a class definition?
How does it help to declare to methods inside the constructor function in a class definition?
What is the disadvantage of declaring methods outside the constructor function?
class animal {
constructor(name, type, color){
this.name = name;
this.type = type;
this.color = color;
this.sound = () => {console.log(this.name,this.type,this.color)}
}
}
class animal {
constructor(name, type, color){
this.name = name;
this.type = type;
this.color = color;
}
sound = () => {console.log(this.name,this.type,this.color)}
}
Creating objects in either way, results in the similar objects. What am I missing?
The definition of functions inside the constructor was mostly used to store and access private variables in calls like this:
function MyClass() {
let privateVar = { something: "usefull" };
this.getPrivateVar = function getPrivateVar() { return privateVar; };
}
new MyClass().getPrivateVar(); // { something: "usefull" }
Just because calls produce a similar result, doesn't mean that they have an equal behavior. They can act completly differently depending on how they get called or how they where defined.
This counts especially for arrow functions as they don't have a this binding. They access the closest this providing environment, when defined.
class MyClass {}
MyClass.prototype.arrow = () => this;
console.log(new MyClass().arrow() == globalThis); // true
The answers from How does JavaScript .prototype work? explain pretty well how prototype is working and for classes every property and function defined inside it, will be stored on constructor.prototype, with some exceptions like the static and #private properties.
Private properties are only available and accessible inside the class and not in an inherited or an extended class and can't be accessed by a classic prototype.
class MyClass {
#private = "example";
doSomethingWithMyPrivate() { return this.#private }
}
MyClass.prototype.getThePrivate = function() { return this.#private };
new MyClass().doSomethingWithMyPrivate(); // example
// Both produce the same SyntaxError:
// Private field '#private' must be declared in an enclosing class
new MyClass().#private;
new MyClass().getThePrivate();
Static properties will be placed on the constructor directly.
class MyClass {
static doSomething() { console.log("Okay!") }
}
MyClass.doSomething(); // Okay!
new MyClass().doSomething(); // TypeError: doSomething is not a function
Every instance of a class will access the prototype references or the inheriting objects prototype until any object has a match with the property-name, if not it returns undefined or throw a more specific error depending on the situation.
From this answer:
Javascript has a mechanism when looking up properties on Objects which is called 'prototypal inheritance', here is what is basically does:
First is checked if the property is located on the Object itself. If so this property is returned.
If the property is not located on the object itself it will 'climb up the protochain'. It basically looks at the object referred to by the proto property. There it checks if the property is available on the object referred to by proto
If the property isn't located on the proto object it will climb up the proto chain all the way up to Object object.
If it cannot find the property nowhere on the object and its prototype chain it will return undefined.
So if you define the functions inside the constructor, each instance gets a new function assigned (they won't have the same reference). Which means, depending on the class (new animal).sound == (new animal).sound, the statement will return true (your 2nd example) or false (your 1st example) and the prototypal inheritance will take place or not.
// the old way
function Foo() {
if (this instanceof Foo) {
this.fx = () => {};
} else throw new TypeError(`Constructor Foo cannot be invoked without 'new'`);
}
Foo.prototype.pfx = function() {};
console.log((new Foo).fx, (new Foo).fx == (new Foo).fx); // arrow ƒ, false
console.log((new Foo).pfx, (new Foo).pfx == (new Foo).pfx); // ƒ, true
// the class way
class Bar {
constructor() { // similar too "ƒ Foo() {}" with the addition of super ƒ
this.fx = () => {};
}
// this is almost like Bar.prototype.pfx = ...
pfx() {}
}
// works like before
Bar.prototype.moo = function moo() {};
console.log((new Bar).fx, (new Bar).fx == (new Bar).fx); // arrow ƒ, false
console.log((new Bar).pfx, (new Bar).pfx == (new Bar).pfx); // ƒ, true
console.log((new Bar).moo, (new Bar).moo == (new Bar).moo); // ƒ, true
If you define properties as mentioned in your example #1, you can "override" other methods and this may cause trouble with private properties as mentioned above.
class Foo {
constructor() {
// just for the example
this.fx = Foo.prototype.fx;
}
fx() { console.log("I am broken"); };
}
class Bar extends Foo {
fx() { console.log("I fix something"); }
}
class Moo extends Bar {
constructor() {
super();
delete this.fx;
}
}
/*
* If instance has no own property called fx the default
* behavior will take place:
* Test the construtor.prototype if it has one and execute this.
* If not, look at the inheriting classes in descending order
* (Moo > Bar > Foo > Object)
* But bar.fx() => has an own property called fx which takes place
*/
let bar = new Bar();
bar.fx(); // I am broken
// none of the inheriting classes has a toJSON ƒ,
// so Object.prototype.toJSON will be called.
console.log(bar.toJSON == Object.prototype.toJSON) // true
// the Moo constructor deletes the own property after the Foo constructor was evaluated, so Bar.prototype.fx takes place
new Moo().fx() // I fix something
Searching over the internet I'm always bumping on this approach of Javascript classes extension
function extend(Child, Parent) {
var F = function() { }
F.prototype = Parent.prototype
Child.prototype = new F()
Child.prototype.constructor = Child
Child.superclass = Parent.prototype
}
But how is that different from this one?
function extend(Child, Parent) {
var p = new Parent()
Child.prototype = p
Child.prototype.constructor = Child
Child.superclass = p
}
This last one also works perfect. So why should I use this extra var F = function() { } move then?
Invoking the original constructor directly can have undesirable side effects, like not working properly if certain expected arguments are not passed.
That's why they use a "proxy" function, which lets you get a new object that inherits from Parent() without actually invoking Parent().
Here's a simple example:
function Person(name, age) {
if (name === undefined)
throw "A name is required";
this.name = name + "";
this.age = age;
}
If Person is the parent, it'll throw an error because there was no name passed.
The first example is (as cookie monster mentioned in the comment) a shim for the following piece of code which might be easier to understand.:
function extend(Child, Parent) {
Child.prototype = Object.create(Parent.prototype);
Child.prototype.constructor = Child;
Child.superclass = Parent.prototype;
}
Basically, this implementation makes the object that all Child instances inherit from (Child.prototype) inherit from the object that all Parent instances inherit from (Parent.prototype). Intuitively this is the most accurate representation of class inheritance JavaScript provides.
The second implementation of extends is flawed, because all Child instances will inherit from a specific Parent instance. Should there be significant differences between Parent instances (due to the parameters passed to the constructor for example), the Child instances can not accurate represent that, because they all inherit from a Parent instance created by calling the Parent constructor with no arguments.
Here is an example of what the first implementation can do and the second one can not:
function Parent(name, age) {
this.name = name;
this.age = age;
}
Parent.prototype.greet = function() { return 'I am parent ' + this.name; }
function Child(name){
Parent.call(this, name, 20); // notice the call to the superclass
}
extend(Child, Parent);
Parent.prototype.greet = function() { return 'I am child ' + this.name + ' and i\'m ' + this.age; }
var c = new Child('Tom');
console.log(c.greet()); // I am child Tom and i'm 20
As a sidenote, in the Child constructor i have called the Parent constructor. This is actually quite common when dealing with classical inheritance, so that's another point for the first implementation. It isn't actually required, the Child constructor can safely ignore calling the Parent constructor, but keep in mind that that call basically ensures that the new object created is a valid Parent instance before being a child Instance. In my example if you were to not call the Parent constructor, the name and age properties would not be set on the Child instance, so the greet method would return I am child undefined and i'm undefined, far from what you would expect.
It's worthwhile exploring the different ways you can extend and add bits to an object in JavaScript.
Using constructors
(function (global) {
function SuperClass() { }
var p = SuperClass.prototype;
p.doSomething = function() {
console.log('do something (super)')
};
function OtherClass() {
SuperClass.call(this);
}
OtherClass.prototype = new SuperClass();
Window.OtherClass = OtherClass;
}(window));
var o = new OtherClass();
Using object.create (no double instantiation) - Not supported on all browsers.
(function (global) {
// SuperClass - superclass
function SuperClass() {
}
var p = SuperClass.prototype;
p.doSomething = function() {
console.log('do something (super)')
};
function OtherClass() {
SuperClass.call(this);
}
OtherClass.prototype = Object.create(SuperClass.prototype);
Window.OtherClass = OtherClass;
}(window));
Functional Mixins:
When you want to mixin a generic set of methods/properties into an object.
var mixin = function () {
this.methodA = function () {
};
this.methodA = function () {
};
return this;
}
var object = function () {
this.methodB = function () {
}
}
mixin.call(object.prototype);
A very good details explanation of all the methods:
http://javascriptweblog.wordpress.com/2011/05/31/a-fresh-look-at-javascript-mixins/
MyGlobalObject;
function TheFunctionICanUseRightAwaySingleForAllInstansesAndWithoutInstanse() {
function() {
alert('NO CONSTRUCTOR WAS CALLED');
}
};
The Long-named function must be callable from MyGlobalObject, which in turn must be available as a global (to window) variable in all times after script was loaded. It should support extensibility in accordance with latest standards.
I'm at architectural dilemma of how to built JS base for an application (almost 100% JS).
We need an object i.e. window.MyObject (like a module, like jQuery) so
It can be created with
VAR1
var MyGlobalObjConstructor = function(){
this.GlobalFunctionInObject = function(){
alert('called with MyGlobalObj.GlobalFunctionInObject()');
}
};
window.MyGlobalObj = new MyGlobalObjConstructor();
Is MyGlobalObj extensible? Can I create child objects, which will inherit current state of MyGlobalObj (extended functions/properties MyGlobalObj.NewFunc e.g.)? What is the main difference between using prototype (VAR3)?
By GlobaldFunction I mean single instance for all initialized/instantiated (possibly instantializable) instances..
Or with
VAR2
var MyGlobalObj = {
GlobalFunctionInObject: function...
GlobalFunctionInObject2: function...
};
MyGlobalObj.GlobalFunctionInObject();
// here I lose all hierarchy elements, no prototype,
// can I use GlobalFunctionInObject2 in GlobalFunctionInObject?
Or with
VAR3
var MyGlobalConstuctor = function(){} // already 'well-formed' object
MyGlobalConstuctor.prototype.GlobalFunctionInObject = function...
};
var MyGlobalObj = new MyGlobalConstuctor();
// so I'm sceptical to NEW, because I have ALREADY wrote my functions
// which I expect to be in memory, single instance of each of them,
// so creating MyObject2,3,4 with NEW MyGC() makes no sense to me.
// DO I REALLY HAVE TO USE "MyGlobalConstuctor.prototype." FOR EACH FUNCTION?!!!!
What's the difference defining MyGlobalObj as a function and as an object (result of func or VAR2)?
OR VAR4?
I see in Chrome Debugger both prototype and __proto__ special fields. I've read that that's OK, but why are they not saved in a single prototype?
So, what is the correct/optimal way to implement window.MyObject, so one could MyObject.MyFunction(); What are the differences (pro/contra) of variants 1 2 and 3?
Variation 1 - Mixin
function SomeType() {
var priv = "I'm private";
this.publ = "I'm public";
this.action = function() {
return priv + this.publ;
};
}
var obj = new SomeType();
With this method you are creating a new object every time you call new SomeType(), creating all its methods and adding all this method to the new object. Every time you create an object.
Pros
It looks like classical inheritance so it's easy to understand to Java-C#-C++-etc people.
It can have private variables per instance since you have one function closure per each object you create
It allows multiple inheritance, also known as Twitter-mixins or functional mixins
obj instanceof SomeType will return true
Cons
It consumes more memory as more objects you create because with each object you are creating a new closure and creating each of it's methods again.
Private properties are private, not protected, subtypes can't access them
No easy way to know if a object has some Type as superclass.
Inheritance
function SubType() {
SomeType.call(this);
this.newMethod = function() {
// can't access priv
return this.publ;
};
}
var child = new SubType();
child instanceof SomeType will return false there is no other way to know if child has SomeType methods than look if it has them one by one.
Variation 2 - Object literal with prototyping
var obj = {
publ: "I'm public",
_convention: "I'm public too, but please don't touch me!",
someMethod: function() {
return this.publ + this._convention;
}
};
In this case you are creating a single object. If you are going to need only one instance of this type it can be the best solution.
Pros
It's quick and easy to understand.
Performant
Cons
No privacy, every property is public.
Inheritance
You can inherit a object prototyping it.
var child = Object.create(obj);
child.otherMethod = function() {
return this._convention + this.publ;
};
If you are on a old browser you will need to garantee Object.create works:
if (!Object.create) {
Object.create = function(obj) {
function tmp() { }
tmp.prototype = obj;
return new tmp;
};
}
To know if a object is a prototype of another you can use
obj.isPrototypeOf(child); // true
Variation 3 - Constructor pattern
UPDATE: This is the pattern ES6 classes are sugar syntax of. If you use ES6 classes you are following this pattern under the hood.
class SomeType {
constructor() {
// REALLY important to declare every non-function property here
this.publ = "I'm public";
this._convention = "I'm public too, but please don't touch me!";
}
someMethod() {
return this.publ + this._convention;
}
}
class SubType extends SomeType {
constructor() {
super(/* parent constructor parameters here */);
this.otherValue = 'Hi';
}
otherMethod() {
return this._convention + this.publ + this.otherValue;
}
}
function SomeType() {
// REALLY important to declare every non-function property here
this.publ = "I'm public";
this._convention = "I'm public too, but please don't touch me!";
}
SomeType.prototype.someMethod = function() {
return this.publ + this._convention;
};
var obj = new SomeType();
You can re-assign the prototype insteadd of adding each method if you are not inheriting and remember to re-assign the constructor property:
SomeType.prototype = {
constructor: SomeType,
someMethod = function() {
return this.publ + this._convention;
}
};
Or use _.extend or $.extend if you have underscore or jquery in your page
_.extend(SomeType.prototype, {
someMethod = function() {
return this.publ + this._convention;
}
};
The new keyword under the hood simply does this:
function doNew(Constructor) {
var instance = Object.create(Constructor.prototype);
instance.constructor();
return instance;
}
var obj = doNew(SomeType);
What you have is a function than has no methods; it just has a prototype property with a list of functions, the new operator means to create a new object and use this function's prototype (Object.create) and constructor property as initializer.
Pros
Performant
Prototype chain will allow you to know if a object inherits from some type
Cons
Two-step inheritance
Inheritance
function SubType() {
// Step 1, exactly as Variation 1
// This inherits the non-function properties
SomeType.call(this);
this.otherValue = 'Hi';
}
// Step 2, this inherits the methods
SubType.prototype = Object.create(SomeType.prototype);
SubType.prototype.otherMethod = function() {
return this._convention + this.publ + this.otherValue;
};
var child = new SubType();
You may think it looks like a super-set of Variation 2... and you'll be right. It's like variation 2 but with a initializer function (the constructor);
child instanceof SubType and child instanceof SomeType will return both true
Curiosity: Under the hood instanceof operator does is
function isInstanceOf(obj, Type) {
return Type.prototype.isPrototypeOf(obj);
}
Variation 4 - Overwrite __proto__
When you do Object.create(obj) under the hood it does
function fakeCreate(obj) {
var child = {};
child.__proto__ = obj;
return child;
}
var child = fakeCreate(obj);
The __proto__ property modifies directly the object's hidden [Prototype] property. As this can break JavaScript behaviour, it's not standard. And the standard way is preferred (Object.create).
Pros
Quick and performant
Cons
Non-standard
Dangerous; you can't have a hashmap since the __proto__ key can change the object's prototype
Inheritance
var child = { __proto__: obj };
obj.isPrototypeOf(child); // true
Comment questions
1. var1: what happens in SomeType.call(this)? Is 'call' special function?
Oh, yes, functions are objects so they have methods, I will mention three: .call(), .apply() and .bind()
When you use .call() on a function, you can pass one extra argument, the context, the value of this inside the function, for example:
var obj = {
test: function(arg1, arg2) {
console.log(this);
console.log(arg1);
console.log(arg2);
}
};
// These two ways to invoke the function are equivalent
obj.test('hi', 'lol');
// If we call fn('hi', 'lol') it will receive "window" as "this" so we have to use call.
var fn = obj.test;
fn.call(obj, 'hi', 'lol');
So when we do SomeType.call(this) we are passing the object this to function SomeCall, as you remember this function will add methods to object this.
2. var3: With your "REALLY define properties" do you mean if I use them in functions? Is it a convention? Because getting this.newProperty without it being defined at the same level with other member functions is not a problem.
I mean any property your object will have that is not a function must be defined on the constructor, not on the prototype, otherwise you will face one of the more confusing JS problems. You can see it here, but it's outside of the focus of this question.
3. Var3: what happens if I don't re-assign constructor?
Actually you might not see the difference and this is what makes it a dangerous bug. Every function's prototype object has a constructor property so you can access the constructor from an instance.
function A() { }
// When you create a function automatically, JS does this:
// A.prototype = { constructor: A };
A.prototype.someMethod = function() {
console.log(this.constructor === A); // true
this.constructor.staticMethod();
return new this.constructor();
};
A.staticMethod = function() { };
It's not a best practice because not everybody knows about it, but sometimes it helps. But if you reassign the prototype...
A.prototype = {
someMethod = function() {
console.log(this.constructor === A); // false
console.log(this.constructor === Object); // true
this.constructor.staticMethod();
return new this.constructor();
}
};
A.prototype is a new object, a instance of Object than prototypes Object.prototype and Object.prototype.constructor is Object. Confusing, right? :P
So if you overwrite the prototype and don't reset the "constructor" property, it will refer to Object instead of A, and if you try to use the "constructor" property to access some static method you may get crazy.
I usually settle with returning an object with functions as properties:
var newCat = function (name) {
return {name: name, purr: function () {alert(name + ' purrs')}};
};
var myCat = newCat('Felix');
myCat.name; // 'Felix'
myCat.purr(); // alert fires
You can have inheritance by calling the newCat function and extend the object you get:
var newLion = function (name) {
var lion = newCat(name);
lion.roar = function () {
alert(name + ' roar loudly');
}
return lion;
}
If you want a global cats object:
var cats = (function () {
var newCat = function (name) {
return {
name: name,
purr: function () {
alert(name + ' is purring')
}
};
};
return {
newCat: newCat
};
}());
Now you can call:
var mySecondCat = cats.newCat('Alice');
I have a base class:
function Monster() {
this.health = 100;
}
Monster.prototype.growl = function() {
console.log("Grr!");
}
That I want to extend and create another class with:
function Monkey extends Monster() {
this.bananaCount = 5;
}
Monkey.prototype.eatBanana {
this.bananaCount--;
this.health++; //Accessing variable from parent class monster
this.growl(); //Accessing function from parent class monster
}
I've done quite a bit of research and there appears to be many convoluted solutions for doing this in JavaScript. What would be the simplest and most reliable way of accomplishing this in JS?
Updated below for ES6
March 2013 and ES5
This MDN document describes extending classes well:
https://developer.mozilla.org/en-US/docs/JavaScript/Introduction_to_Object-Oriented_JavaScript
In particular, here is now they handle it:
// define the Person Class
function Person() {}
Person.prototype.walk = function(){
alert ('I am walking!');
};
Person.prototype.sayHello = function(){
alert ('hello');
};
// define the Student class
function Student() {
// Call the parent constructor
Person.call(this);
}
// inherit Person
Student.prototype = Object.create(Person.prototype);
// correct the constructor pointer because it points to Person
Student.prototype.constructor = Student;
// replace the sayHello method
Student.prototype.sayHello = function(){
alert('hi, I am a student');
}
// add sayGoodBye method
Student.prototype.sayGoodBye = function(){
alert('goodBye');
}
var student1 = new Student();
student1.sayHello();
student1.walk();
student1.sayGoodBye();
// check inheritance
alert(student1 instanceof Person); // true
alert(student1 instanceof Student); // true
Note that Object.create() is unsupported in some older browsers, including IE8:
If you are in the position of needing to support these, the linked MDN document suggests using a polyfill, or the following approximation:
function createObject(proto) {
function ctor() { }
ctor.prototype = proto;
return new ctor();
}
Using this like Student.prototype = createObject(Person.prototype) is preferable to using new Person() in that it avoids calling the parent's constructor function when inheriting the prototype, and only calls the parent constructor when the inheritor's constructor is being called.
May 2017 and ES6
Thankfully, the JavaScript designers have heard our pleas for help and have adopted a more suitable way of approaching this issue.
MDN has another great example on ES6 class inheritance, but I'll show the exact same set of classes as above reproduced in ES6:
class Person {
sayHello() {
alert('hello');
}
walk() {
alert('I am walking!');
}
}
class Student extends Person {
sayGoodBye() {
alert('goodBye');
}
sayHello() {
alert('hi, I am a student');
}
}
var student1 = new Student();
student1.sayHello();
student1.walk();
student1.sayGoodBye();
// check inheritance
alert(student1 instanceof Person); // true
alert(student1 instanceof Student); // true
Clean and understandable, just like we all want. Keep in mind, that while ES6 is pretty common, it's not supported everywhere:
ES6 gives you now the opportunity to use class & extends keywords :
Then , your code will be :
You have a base class:
class Monster{
constructor(){
this.health = 100;
}
growl() {
console.log("Grr!");
}
}
That You want to extend and create another class with:
class Monkey extends Monster {
constructor(){
super(); //don't forget "super"
this.bananaCount = 5;
}
eatBanana() {
this.bananaCount--;
this.health++; //Accessing variable from parent class monster
this.growl(); //Accessing function from parent class monster
}
}
Try this:
Function.prototype.extends = function(parent) {
this.prototype = Object.create(parent.prototype);
};
Monkey.extends(Monster);
function Monkey() {
Monster.apply(this, arguments); // call super
}
Edit: I put a quick demo here http://jsbin.com/anekew/1/edit. Note that extends is a reserved word in JS and you may get warnings when linting your code, you can simply name it inherits, that's what I usually do.
With this helper in place and using an object props as only parameter, inheritance in JS becomes a bit simpler:
Function.prototype.inherits = function(parent) {
this.prototype = Object.create(parent.prototype);
};
function Monster(props) {
this.health = props.health || 100;
}
Monster.prototype = {
growl: function() {
return 'Grrrrr';
}
};
Monkey.inherits(Monster);
function Monkey() {
Monster.apply(this, arguments);
}
var monkey = new Monkey({ health: 200 });
console.log(monkey.health); //=> 200
console.log(monkey.growl()); //=> "Grrrr"
If you don't like the prototype approach, because it doesn't really behave in a nice OOP-way, you could try this:
var BaseClass = function()
{
this.some_var = "foobar";
/**
* #return string
*/
this.someMethod = function() {
return this.some_var;
}
};
var MyClass = new Class({ extends: BaseClass }, function()
{
/**
* #param string value
*/
this.__construct = function(value)
{
this.some_var = value;
}
})
Using lightweight library (2k minified): https://github.com/haroldiedema/joii
I can propose one variant, just have read in book, it seems the simplest:
function Parent() {
this.name = 'default name';
};
function Child() {
this.address = '11 street';
};
Child.prototype = new Parent(); // child class inherits from Parent
Child.prototype.constructor = Child; // constructor alignment
var a = new Child();
console.log(a.name); // "default name" trying to reach property of inherited class
This is an extension (excuse the pun) of elclanrs' solution to include detail on instance methods, as well as taking an extensible approach to that aspect of the question; I fully acknowledge that this is put together thanks to David Flanagan's "JavaScript: The Definitive Guide" (partially adjusted for this context). Note that this is clearly more verbose than other solutions, but would probably benefit in the long-term.
First we use David's simple "extend" function, which copies properties to a specified object:
function extend(o,p) {
for (var prop in p) {
o[prop] = p[prop];
}
return o;
}
Then we implement his Subclass definition utility:
function defineSubclass(superclass, // Constructor of our superclass
constructor, // Constructor of our new subclass
methods, // Instance methods
statics) { // Class properties
// Set up the prototype object of the subclass
constructor.prototype = Object.create(superclass.prototype);
constructor.prototype.constructor = constructor;
if (methods) extend(constructor.prototype, methods);
if (statics) extend(constructor, statics);
return constructor;
}
For the last bit of preparation we enhance our Function prototype with David's new jiggery-pokery:
Function.prototype.extend = function(constructor, methods, statics) {
return defineSubclass(this, constructor, methods, statics);
};
After defining our Monster class, we do the following (which is re-usable for any new Classes we want to extend/inherit):
var Monkey = Monster.extend(
// constructor
function Monkey() {
this.bananaCount = 5;
Monster.apply(this, arguments); // Superclass()
},
// methods added to prototype
{
eatBanana: function () {
this.bananaCount--;
this.health++;
this.growl();
}
}
);
For traditional extending you can simply write superclass as constructor function,
and then apply this constructor for your inherited class.
function AbstractClass() {
this.superclass_method = function(message) {
// do something
};
}
function Child() {
AbstractClass.apply(this);
// Now Child will have superclass_method()
}
Example on angularjs:
http://plnkr.co/edit/eFixlsgF3nJ1LeWUJKsd?p=preview
app.service('noisyThing',
['notify',function(notify){
this._constructor = function() {
this.scream = function(message) {
message = message + " by " + this.get_mouth();
notify(message);
console.log(message);
};
this.get_mouth = function(){
return 'abstract mouth';
}
}
}])
.service('cat',
['noisyThing', function(noisyThing){
noisyThing._constructor.apply(this)
this.meow = function() {
this.scream('meooooow');
}
this.get_mouth = function(){
return 'fluffy mouth';
}
}])
.service('bird',
['noisyThing', function(noisyThing){
noisyThing._constructor.apply(this)
this.twit = function() {
this.scream('fuuuuuuck');
}
}])
For Autodidacts:
function BaseClass(toBePrivate){
var morePrivates;
this.isNotPrivate = 'I know';
// add your stuff
}
var o = BaseClass.prototype;
// add your prototype stuff
o.stuff_is_never_private = 'whatever_except_getter_and_setter';
// MiddleClass extends BaseClass
function MiddleClass(toBePrivate){
BaseClass.call(this);
// add your stuff
var morePrivates;
this.isNotPrivate = 'I know';
}
var o = MiddleClass.prototype = Object.create(BaseClass.prototype);
MiddleClass.prototype.constructor = MiddleClass;
// add your prototype stuff
o.stuff_is_never_private = 'whatever_except_getter_and_setter';
// TopClass extends MiddleClass
function TopClass(toBePrivate){
MiddleClass.call(this);
// add your stuff
var morePrivates;
this.isNotPrivate = 'I know';
}
var o = TopClass.prototype = Object.create(MiddleClass.prototype);
TopClass.prototype.constructor = TopClass;
// add your prototype stuff
o.stuff_is_never_private = 'whatever_except_getter_and_setter';
// to be continued...
Create "instance" with getter and setter:
function doNotExtendMe(toBePrivate){
var morePrivates;
return {
// add getters, setters and any stuff you want
}
}
Summary:
There are multiple ways which can solve the problem of extending a constructor function with a prototype in Javascript. Which of these methods is the 'best' solution is opinion based. However, here are two frequently used methods in order to extend a constructor's function prototype.
ES 2015 Classes:
class Monster {
constructor(health) {
this.health = health
}
growl () {
console.log("Grr!");
}
}
class Monkey extends Monster {
constructor (health) {
super(health) // call super to execute the constructor function of Monster
this.bananaCount = 5;
}
}
const monkey = new Monkey(50);
console.log(typeof Monster);
console.log(monkey);
The above approach of using ES 2015 classes is nothing more than syntactic sugar over the prototypal inheritance pattern in javascript. Here the first log where we evaluate typeof Monster we can observe that this is function. This is because classes are just constructor functions under the hood. Nonetheless you may like this way of implementing prototypal inheritance and definitively should learn it. It is used in major frameworks such as ReactJS and Angular2+.
Factory function using Object.create():
function makeMonkey (bananaCount) {
// here we define the prototype
const Monster = {
health: 100,
growl: function() {
console.log("Grr!");}
}
const monkey = Object.create(Monster);
monkey.bananaCount = bananaCount;
return monkey;
}
const chimp = makeMonkey(30);
chimp.growl();
console.log(chimp.bananaCount);
This method uses the Object.create() method which takes an object which will be the prototype of the newly created object it returns. Therefore we first create the prototype object in this function and then call Object.create() which returns an empty object with the __proto__ property set to the Monster object. After this we can initialize all the properties of the object, in this example we assign the bananacount to the newly created object.
the absolutely minimal (and correct, unlike many of the answers above) version is:
function Monkey(param){
this.someProperty = param;
}
Monkey.prototype = Object.create(Monster.prototype);
Monkey.prototype.eatBanana = function(banana){ banana.eat() }
That's all. You can read here the longer explanation
I have been trying to learn OOP with JavaScript before I start attempting to learn backbone.js.
I want to be able to data bind but I can't seem to get it to work.
I've just made a simple protoype of a budget website that you can put in a budget and input how much you've spent, and it will show if you've gone over.
function BudgetItem(spent, budget){
this.setSpent = function(spent){
this.spent = spent;
}
this.setBudget = function(budget){
this.budget = budget;
}
this.getSpent = function(){
return this.spent;
}
this.getBudget = function(){
return this.budget;
}
}
function BudgetType(type){
this.getType = function(){
return type;
}
}
BudgetType.prototype = new BudgetItem();
$(document).ready(function(){
var food = new BudgetType('food');
$('.budget').html(food.getBudget());
$('.editbudget').change(function(){
food.setBudget($('.editbudget').data())
});
})
That's my code thus far. I'm not sure if I'm doing it right. Am I supposed to extend things? Also, can someone explain how to dynamically data bind without a library?
First I'll give you some theory. A Javascript function is a dynamic object, just like Object is, and a new instance can be created using the new keyword much like you are doing in your listener. When this happens, the function itself will run as a constructor while the this keyword will be bound to the newly created object. What you're doing above then is in fact adding new properties on the fly as you're passing in their values for the first time... which is fine, but not very clear to another reader.
Now for the tricky part. Every function has a link to a "hidden" Prototype object. This is an anonymous (not accessible by name) object created by the JavaScript runtime and passed as a reference to the user object through the prototype property. This Prototype object also has a reference to the function through its constructor property. To test what I'm saying for yourself, try the following:
BudgetItem.prototype.constructor === BudgetItem // true
Putting it all together, you can now think of functions as constructors to (hidden) classes that are created for you behind the scenes, accessible through the function's prototype property. So, you could add the fields to the Prototype object directly as so:
function BudgetItem(spent) {
this.spent = spent
}
BudgetItem.prototype.setSpent = function(spent) { this.spent = spent };
BudgetItem.prototype.getSpent = function() { return this.spent };
Another problem is inheritance and passing parameters to the constructor. Again, your version is valid but you lose the ability to pass the spent and budget values when initializing a BudgetType. What I would do is forget prototypes and go:
function BudgetType(type, spent) {
var instance = new BudgetItem(spent);
instance.type = type;
return instance;
}
This is close to what Scott Sauyet suggested above but more powerful. Now you can pass both parameters (and more) and have a more complicated inheritance tree.
Finally, what you can do is create private (or pseudo-private, more accurately) properties by providing a getter to an otherwise automatic variable (one passed as an argument or initialised inside the function). This is a special feature of the language and it works like so:
function BudgetType(type, spent) {
var instance = new BudgetItem(spent);
instance.getType = function() {
return type;
}
return instance;
}
Now you can access the 'type' passed in the constructor by obj.getType() but cannot override the initial value. Even if you define obj.type = 'New Value' the getType() will return the initial parameter passed because it has a reference to another context which was created when the object was initialised and never got released due to the closure.
Hope that helps...
if you want all instances of objects to reference the same members/values you can use a closure:
// create a constrctor for you object wrapped in a closure
myCon = (function() {
// define shared members up here
var mySharedObj = new function () {
this.member = "a";
}();
// return the actual constructor
return function () {
this.mySharedObj = mySharedObj;
}
}());
// create two instances of the object
var a = new myCon();
var b = new myCon();
// Altering the shared object from one
a.mySharedObj.member = "b";
// Alters it for all
console.log(b.mySharedObj.member);
If you want to build objects from other objects(sort of like other languages' class whatever extends baseClass), but do not want them to share values via reference(instead a clone of values), you can use something like the following:
Object.prototype.extendsUpon = (function (_prop, _args) {
return function (base) {
for (var key in base) {
if (_prop.call(base, key)) {
this[key] = base[key];
}
}
function con(child){
this.constructor = child;
}
con.prototype = base.prototype;
this.prototype = new con(this);
this.__base__ = base.prototype;
var args = _args.call(arguments);
args.shift();
base.constructor.apply(this, args);
}
}(Object.prototype.hasOwnProperty, Array.prototype.slice));
Then to build objects ontop of objects:
// Base Object Constructor
function Fruit(name) {
this.fruitname = name;
}
Fruit.prototype.yum = function() {
return "I had an " + this.fruitname;
}
// Object constructor that derives from the Base Object
function Favorite() {
// Derive this object from a specified base object:
// #arg0 -> Object Constructor to use as base
// #arg1+ -> arguments passed to the BaseObject's constructor
this.extendsUpon(Fruit, "apple");
// From here proceed as usual
// To access members from the base object that have been over-written,
// use "this.__base__.MEMBER.apply(this, arguments)"
}
Favorite.prototype.yum = function() {
return this.__base__.yum.apply(this) + " and it was my favorite";
}
var mmm = new Favorite();
// Outputs: "I had an apple and it was my favorite"
mmm.yum();