How do I create an abstract base class in JavaScript

Question

Is it possible to simulate abstract base class in JavaScript  What is the most elegant way to do it   Say  I want to do something like  -  var cat   new Animal  cat    var dog   new Animal  dog     cat say    dog say      It should output   bark    meow

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Another thing you might want to enforce is making sure your abstract class is not instantiated  You can do that by defining a function that acts as FLAG ones set as the Abstract class constructor  This will then try to construct the FLAG which will call its constructor containing exception to be thrown  Example below    function         var FLAG ABSTRACT   function   class            throw  Error  Trying to instantiate an abstract class     class            var Class   function              Class prototype constructor   new FLAG ABSTRACT  Class                        will throw exception     var  foo   new Class

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Question is quite old  but I created some possible solution how to create abstract  class  and block creation of object that type    x000D   x000D    our Abstract class x000D  var Animal function    x000D     x000D      this name  Animal   x000D      this fullname this name  x000D       x000D        check if we have abstract paramater in prototype x000D      if  Object getPrototypeOf this  hasOwnProperty  abstract     x000D       x000D      throw new Error  Can t instantiate abstract class     x000D       x000D       x000D        x000D       x000D   x000D     x000D   x000D    very important - Animal prototype has property abstract x000D  Animal prototype abstract true  x000D   x000D  Animal prototype hello function    x000D   x000D     console log  Hello from   this name   x000D     x000D   x000D  Animal prototype fullHello function    x000D   x000D     console log  Hello from   this fullname   x000D     x000D   x000D    first inheritans x000D  var Cat function    x000D   x000D     Animal call this    run constructor of animal x000D       x000D      this name  Cat   x000D       x000D      this fullname this fullname   -   this name  x000D   x000D     x000D   x000D  Cat prototype Object create Animal prototype   x000D   x000D    second inheritans x000D  var Tiger function    x000D   x000D      Cat call this    run constructor of animal x000D       x000D      this name  Tiger   x000D       x000D      this fullname this fullname   -   this name  x000D       x000D     x000D   x000D  Tiger prototype Object create Cat prototype   x000D   x000D    cat can be used x000D  console log  WE CREATE CAT     x000D  var cat new Cat    x000D  cat hello    x000D  cat fullHello    x000D   x000D    tiger can be used x000D   x000D  console log  WE CREATE TIGER     x000D  var tiger new Tiger    x000D  tiger hello    x000D  tiger fullHello    x000D   x000D   x000D  console log  WE CREATE ANIMAL   IT IS ABSTRACT       x000D    animal is abstract  cannot be used - see error in console x000D  var animal new Animal    x000D  animal animal fullHello    x000D   x000D   x000D    As You can see last object give us error  it is because Animal in prototype has property abstract  To be sure it is Animal not something which has Animal prototype in prototype chain I do   Object getPrototypeOf this  hasOwnProperty  abstract     So I check that my closest prototype object has abstract property  only object created directly from Animal prototype will have this condition on true  Function hasOwnProperty checks only properties of current object not his prototypes  so this gives us 100  sure that property is declared here not in prototype chain      Every object descended from Object inherits the hasOwnProperty method  This method can be used to determine whether an object has the specified property as a direct property of that object  unlike the in operator  this method does not check down the object s prototype chain  More about it    In my proposition we not have to change constructor every time after Object create like it is in current best answer by  Jord  o   Solution also enables to create many abstract classes in hierarchy  we need only to create abstract property in prototype

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version 1                                                                           var Animal   function params        this say   function                 console log params            var Cat   function         Animal call this   moes        var Dog   function         Animal call this   vewa         var cat   new Cat    var dog   new Dog     cat say    dog say                                                    version 2                                                                           var Cat   function params        this say   function                 console log params             var Dog   function params        this say   function                 console log params             var Animal   function type        var obj       var factory   function                 switch type                        case  cat                   obj   new Cat  bark                    break              case  dog                   obj   new Dog  meow                    break                       var init   function                 factory            return obj             return init         var cat   new Animal  cat    var dog   new Animal  dog     cat say    dog say

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One simple way to create an abstract class is this         constructor   abstract     var Animal   function         if  this constructor     Animal          throw new Error  Can t instantiate abstract class                  Animal initialization              abstract     Animal prototype say   function         throw new Error  Abstract method         The Animal  class  and the say method are abstract   Creating an instance would throw an error    new Animal       throws   This is how you  inherit  from it   var Cat   function         Animal apply this  arguments          Cat initialization       Cat prototype   Object create Animal prototype   Cat prototype constructor   Cat   Cat prototype say   function         console log  meow        Dog looks just like it   And this is how your scenario plays out   var cat   new Cat    var dog   new Dog     cat say    dog say      Fiddle here  look at the console output

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If you want to make sure that your base classes and their members are strictly abstract here is a base class that does this for you   class AbstractBase      constructor         checkConstructor c           if this constructor  c  return          throw new Error  Abstract class   this constructor name  cannot be instantiated              throwAbstract            throw new Error    this constructor name  must implement abstract member            class FooBase extends AbstractBase      constructor            super            this checkConstructor FooBase       doStuff   this throwAbstract         doOtherStuff   this throwAbstract        class FooBar extends FooBase      constructor            super         doOtherStuff     some code here        var fooBase   new FooBase       lt - Error  Abstract class FooBase cannot be instantiated var fooBar   new FooBar       lt - OK fooBar doStuff       lt - Error  FooBar must implement abstract member fooBar doOtherStuff       lt - OK   Strict mode makes it impossible to log the caller in the throwAbstract method but the error should occur in a debug environment that would show the stack trace

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function Animal type        if  type     cat             this   proto     Cat prototype        else if  type     dog             this   proto     Dog prototype        else if  type     fish             this   proto     Fish prototype          Animal prototype say   function         alert  This animal can t speak        function Cat            init cat   Cat prototype   new Animal    Cat prototype say   function         alert  Meow        function Dog            init dog   Dog prototype   new Animal    Dog prototype say   function         alert  Bark        function Fish            init fish   Fish prototype   new Animal     var newAnimal   new Animal  dog    newAnimal say      This isn t guaranteed to work as   proto   isn t a standard variable  but it works at least in Firefox and Safari   If you don t understand how it works  read about the prototype chain

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Reference  https   developer mozilla org pl docs Web JavaScript Reference Classes class Animal        constructor sound        this sound   sound             say         return  This animal does   this sound          const dog   new Animal  bark    console log dog say

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JavaScript Classes and Inheritance  ES6   According to ES6  you can use JavaScript classes and inheritance to accomplish what you need      JavaScript classes  introduced in ECMAScript 2015  are primarily syntactical sugar over JavaScript s existing prototype-based inheritance    Reference  https   developer mozilla org en-US docs Web JavaScript Reference Classes  First of all  we define our abstract class  This class can t be instantiated  but can be extended  We can also define functions that must be implemented in all classes that extends this one            Abstract Class Animal         class Animal     class Animal      constructor         if  this constructor    Animal          throw new Error  Abstract classes can t be instantiated                  say         throw new Error  Method  say    must be implemented            eat         console log  eating            After that  we can create our concrete Classes  These classes will inherit all functions and behaviour from abstract class          Dog         class Dog     extends  Animal      class Dog extends Animal     say         console log  bark                  Cat         class Cat     extends  Animal      class Cat extends Animal     say         console log  meow                  Horse         class Horse     extends  Animal      class Horse extends Animal      And the results        RESULTS  new Dog   eat       eating new Cat   eat       eating new Horse   eat       eating  new Dog   say       bark new Cat   say       meow new Horse   say       Error  Method say   must be implemented   new Animal       Error  Abstract classes can t be instantiated

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You might want to check out Dean Edwards  Base Class  http   dean edwards name weblog 2006 03 base   Alternatively  there is this example   article by Douglas Crockford on classical inheritance in JavaScript  http   www crockford com javascript inheritance html

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Animal   function      throw  abstract class     Animal prototype name    This animal   Animal prototype sound          Animal prototype say   function         console log  this name     says      this sound       Cat   function          this name    Cat       this sound    meow      Dog   function         this name    Dog       this sound     woof      Cat prototype   Object create Animal prototype   Dog prototype   Object create Animal prototype    new Cat   say         Cat says  meow new Dog   say         Dog says  woof  new Animal   say      Uncaught abstract class

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Your Abstract class Animal function Animal type        this say   type say     function catClass         this say   function              console log  I am a cat            function dogClass         this say   function              console log  I am a dog            var cat   new Animal new catClass     var dog   new Animal new dogClass      cat say      I am a cat  dog say      I am a dog

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We can use Factory design pattern in this case  Javascript use prototype to inherit the parent s members   Define the parent class constructor   var Animal   function       this type    animal     return this    Animal prototype tired   function       console log  sleeping  zzzZZZ          And then create children class      These are the child classes Animal cat   function       this type    cat     this says   function         console log  says  meow            Then define the children class constructor      Define the child class constructor -- Factory Design Pattern  Animal born   function type         Inherit all members and methods from parent class       and also keep its own members    Animal type  prototype   new Animal         Square bracket notation can deal with variable object    creature   new Animal type       return creature      Test it   var timmy   Animal born  cat    console log timmy type     cat timmy says       meow timmy tired       zzzZZZ    Here s the Codepen link for the full example coding

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Is it possible to simulate abstract base class in JavaScript    Certainly  There are about a thousand ways to implement class instance systems in JavaScript  Here is one      Classes magic  Define a new class with var C  Object subclass isabstract      add class members to C prototype     provide optional C prototype  init   method to initialise from constructor args     call base class methods using Base prototype call this           Function prototype subclass  function isabstract        if  isabstract            var c  new Function               if  arguments 0    Function prototype subclass FLAG  throw   Abstract class may not be constructed                        else           var c  new Function               if    this instanceof arguments callee   throw   Constructor called without  new                      if  arguments 0    Function prototype subclass FLAG  amp  amp  this  init  this  init apply this  arguments                          if  this   Object          c prototype  new this Function prototype subclass FLAG       return c    Function prototype subclass FLAG  new Object         var cat   new Animal  cat      That s not really an abstract base class of course  Do you mean something like   var Animal  Object subclass true      is abstract Animal prototype say  function         window alert this  noise          concrete classes var Cat  Animal subclass    Cat prototype  noise   meow   var Dog  Animal subclass    Dog prototype  noise   bark       usage var mycat  new Cat    mycat say       meow  var mygiraffe  new Animal       error

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I think All Those answers specially first two  by  some and jord  o  answer the question clearly with conventional prototype base JS concept  Now as you want the animal class constructor to behave  according to the passed parameter to the construction  I think this is very much similar to basic behavior of  Creational Patterns for example Factory Pattern    Here i made a little approach to make it work that way   var Animal   function type        this type type      if type   dog                 return new Dog              else if type   cat                 return new Cat                Animal prototype whoAreYou function         console log  I am a   this type      Animal prototype say   function        console log  Not implemented           var Cat  function          Animal call this       this type  cat       Cat prototype Object create Animal prototype   Cat prototype constructor   Cat   Cat prototype say function         console log  meow         var Dog  function          Animal call this       this type  dog       Dog prototype Object create Animal prototype   Dog prototype constructor   Dog   Dog prototype say function         console log  bark        var animal new Animal      var dog   new Animal  dog    var cat new Animal  cat     animal whoAreYou      I am a undefined animal say      Not implemented   dog whoAreYou      I am a dog dog say      bark  cat whoAreYou      I am a cat cat say      meow

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You can create abstract classes by using object prototypes  a simple example can be as follows     var SampleInterface        addItem   function item          You can change above method or not  it is up to you when you implement it  For a detailed observation  you may want to visit here

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Javascript can have inheritance  check out the URL below   http   www webreference com js column79   Andrew

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Do you mean something like this   function Animal         Initialization for all Animals      Function and properties shared by all instances of Animal Animal prototype init function name     this name name    Animal prototype say function        alert this name     who is a     this type     says     this whattosay     Animal prototype type  unknown    function Cat name        this init name          Make a cat somewhat unique     var s         for  var i Math ceil Math random   7   i gt  0  --i  s   e       this whattosay  Me    s   ow       Function and properties shared by all instances of Cat     Cat prototype new Animal    Cat prototype type  cat   Cat prototype whattosay  meow     function Dog           Call init with same arguments as Dog was called with     this init apply this arguments      Dog prototype new Animal    Dog prototype type  Dog   Dog prototype whattosay  bark     Override say  Dog prototype say   function             this openMouth              Call the original with the exact same arguments         Animal prototype say apply this arguments             or with other arguments           Animal prototype say call this  some   other   arguments            this closeMouth       Dog prototype openMouth function          Code   Dog prototype closeMouth function          Code    var dog   new Dog  Fido    var cat1   new Cat  Dash    var cat2   new Cat  Dot      dog say       Fido the Dog says bark cat1 say      Dash the Cat says M e  ow cat2 say      Dot the Cat says M e  ow   alert cat instanceof Cat     True alert cat instanceof Dog     False alert cat instanceof Animal     True

[javascript] How do I create an abstract base class in JavaScript?

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