Extending an Object in Javascript

Question

I am currently transforming from Java to Javascript  and it s a bit hard for me to figure out how to extend objects the way I want it to do   I ve seen several people on the internet use a method called extend on object  The code will look like this   var Person        name    Blank      age    22    var Robot   Person extend      name    Robo      age    4     var robot   new Robot    alert robot name     Should return  Robo    Does anyone know how to make this work  I ve heard that you need to write   Object prototype extend   function         But I don t know how to make this system work  If it is not possible  please show me another alternative that extends an object

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PLEASE ADD REASON FOR DOWNVOTE   No need to use any external library to extend In JavaScript  everything is an object   except for the three primitive datatypes  and even they are automatically wrapped with objects when needed    Furthermore  all objects are mutable       Class Person in JavaScript   function Person name  age        this name   name      this age   age    Person prototype         getName  function             return this name             getAge  function             return this age              Instantiate the class     var alice   new Person  Alice   93   var bill   new Person  Bill   30        Modify a specific instance object    alice displayGreeting   function          alert this getGreeting            Modify the class   Person prototype getGreeting   function          return  Hi     this getName                  Or simply say   extend JSON and OBJECT both are same   var k         name    jack       age   30    k gender    male     object or json k got extended with new property gender     thanks to ross harmes   dustin diaz

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People who are still struggling for the simple and best approach  you can use Spread Syntax for extending object    x000D   x000D  var person1     x000D        name   Blank   x000D        age  22 x000D         x000D   x000D  var person2     x000D        name   Robo   x000D        age  4  x000D        height   6 feet  x000D         x000D     spread syntax x000D  let newObj        person1     person2    x000D  console log newObj height   x000D   x000D   x000D    Note  Remember that  the property is farthest to the right will have the priority  In this example  person2 is at right side  so newObj will have name Robo in it

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Summary   Javascript uses a mechanism which is called prototypal inheritance  Prototypal inheritance is used when looking up a property on an object  When we are extending properties in javascript we are inheriting these properties from an actual object  It works in the following manner    When an object property is requested   e g myObj foo or myObj  foo    the JS engine will first look for that property on the object itself When this property isn t found on the object itself it will climb the prototype chain look at the prototype object  If this property is also not found here it will keep climbing the prototype chain until the property is found  If the property is not found it will throw a reference error    When we want to extend from a object in javascript we can simply link this object in the prototype chain  There are numerous ways to achieve this  I will describe 2 commonly used methods   Examples   1  Object create    Object create   is a function that takes an object as an argument and creates a new object  The object which was passed as an argument will be the prototype of the newly create object  For example    x000D   x000D     prototype of the dog x000D  const dogPrototype     x000D    woof  function      console log  woof      x000D    x000D   x000D     create 2 dog objects  pass prototype as an argument x000D  const fluffy   Object create dogPrototype   x000D  const notFluffy   Object create dogPrototype   x000D   x000D     both newly created object inherit the woof  x000D     function from the dogPrototype x000D  fluffy woof    x000D  notFluffy woof    x000D   x000D   x000D    2  Explicitly setting the prototype property  When creating objects using constructor functions  we can set add properties to its prototype object property  Objects which are created form a constructor function when using the new keyword  have their prototype set to the prototype of the constructor function  For example    x000D   x000D     Constructor function object x000D  function Dog  name    x000D     name   this name  x000D    x000D   x000D     Functions are just objects x000D     All functions have a prototype property x000D     When a function is used as a constructor  with the new keyword  x000D     The newly created object will have the consturctor function s x000D     prototype as its prototype property x000D  Dog prototype woof   function      x000D    console log  woof    x000D    x000D   x000D     create a new dog instance x000D  const fluffy   new Dog  fluffyGoodBoyyyyy    x000D     fluffy inherits the woof method x000D  fluffy woof    x000D   x000D     can check the prototype in the following manner x000D  console log Object getPrototypeOf fluffy    x000D   x000D   x000D

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This will make extend your properties create a new Object with the object parameter prototypes without altering the passed object   function extend object        if  object     null          throw TypeError      if  typeof object      object   amp  amp  typeof object      function           throw TypeError      if  Object create          return Object create object       function f                f prototype   p      return new f        But if you want to extend your Object without modifying it parameters  you can add extendProperty to your object    var Person    some code extend  extendProperty      Enforce type checking an Error report as you wish     function extendProperty object            if   object     null  amp  amp   typeof object      object     typeof object      function                  for  var prop in object                    if  object hasOwnProperty prop                       this prop    object prop                          else              throw TypeError    Not an object

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Mozilla  announces  object extending from ECMAScript 6 0   https   developer mozilla org en-US docs Web JavaScript Reference Classes extends  NOTE  This is an experimental technology  part of the ECMAScript 6  Harmony  proposal   class Square extends Polygon     constructor length           Here  it calls the parent class  constructor with lengths        provided for the Polygon s width and height     super length  length          Note  In derived classes  super   must be called before you        can use  this   Leaving this out will cause a reference error      this name    Square          get area         return this height   this width         set area value        this area   value             This technology is available in Gecko  Google Chrome   Firefox  - 03 2015 nightly builds

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Function prototype extends function ParentClass        this prototype   new ParentClass        this prototype constructor   this      Then   function Person         this name    anonym      this skills     abc      Person prototype profile   function         return this skills length    1     function Student        well extends fom Person Class Student extends Person   var s1   new Student    s1 skills push     s1 profile      2   Update 01 2017   Please  Ignore my answer of 2015 since Javascript is now supports extends keyword since ES6  Ecmasctipt6    - ES6    class Person      constructor          this name    anonym       this skills     abc             profile         return this skills length    1          Person MAX SKILLS   10  class Student extends Person         well extends from Person Class    ----------------- var s1   new Student    s1 skills push     s1 profile      2   - ES7    class Person       static MAX SKILLS   10      name    anonym      skills     abc         profile           return this skills length    1          class Student extends Person         well extends from Person Class    ----------------- var s1   new Student    s1 skills push     s1 profile      2

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If you haven t yet figured out a way  use the associative property of JavaScript objects to add an extend function to the Object prototype as shown below   Object prototype extend   function obj       for  var i in obj          if  obj hasOwnProperty i              this i    obj i                     You can then use this function as shown below   var o     member   some member     var x     extension   some extension      o extend x

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Different approach  Object create Per  osahyoun answer  I find the following as a better and efficient way to  inherit  from Person s prototype object  function Person name       this name   name      this type    human      Person prototype info   function        console log  quot Name  quot   this name   quot Type  quot   this type      function Robot name       Person call this  name      this type    robot         Set Robot s prototype to Person s prototype by    creating a new object that inherits from Person prototype     and assigning it to Robot prototype Robot prototype   Object create Person prototype       Set constructor back to Robot Robot prototype constructor   Robot   Create new instances  var person   new Person  quot Bob quot    var robot   new Robot  quot Boutros quot     person info       Name  Bob Type  human robot info        Name  Boutros Type  robot  Now  by using Object create  Person prototype constructor     Robot  Check also the MDN documentation

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You might want to consider using helper library like underscore js  which has it s own implementation of extend     And it s also a good way to learn by looking at it s source code  The annotated source code page is quite useful

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You can simply do it by using   Object prototype extend   function object         loop through object    for  var i in object           check if the extended object has that property     if  object hasOwnProperty i              mow check if the child is also and object so we go through it recursively       if  typeof this i      object   amp  amp  this hasOwnProperty i   amp  amp  this i     null            this i  extend object i            else           this i    object i                       return this          update  I checked for this i     null since null is an object   Then use it like   var options           foo   bar         baz   dar             var defaults           foo  false        baz   car         nat  0        defaults extend options     This well result in      defaults will now be     foo   bar     baz   dar     nat  0

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In ES6  you may use spread operator like  var mergedObj        Obj1     Obj2         Note that Object assign   triggers setters whereas spread syntax doesn t    For more info see link  MDN -Spread Syntax    Old Answer    In ES6  there is Object assign for copying property values  Use    as first param if you don t want to modify the target object  the first param passed    var mergedObj   Object assign     Obj1  Obj2     For more details see link  MDN - Object assign    In case if you need is a Polyfill for ES5  the link offers it too

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You want to  inherit  from Person s prototype object   var Person   function  name        this name   name      this type    human       Person prototype info   function          console log  Name    this name   Type    this type       var Robot   function  name        Person apply this  arguments       this type    robot       Robot prototype   Person prototype      Set prototype to Person s Robot prototype constructor   Robot     Set constructor back to Robot  person   new Person  Bob    robot   new Robot  Boutros     person info       Name  Bob Type  human  robot info       Name  Boutros Type  robot

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In the majority of project there are some implementation of object extending  underscore  jquery  lodash  extend   There is also pure javascript implementation  that is a part of ECMAscript 6  Object assign  https   developer mozilla org en-US docs Web JavaScript Reference Global Objects Object assign

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World without the  quot new quot  keyword  And simpler  quot prose-like quot  syntax with Object create      This example is updated for ES6 classes and TypeScript   First off  and factually  Javascript is a prototypal language  not class-based  Its true nature is expressed in the prototypial form below  which you may come to see that is very simple  prose-like  yet powerful  TLDR  const Person          name   Anonymous      person has a name     greet  function     console log  Hi  I am   this name              const jack   Object create Person       jack is a person jack name    Jack                       and has a name  Jack  jack greet                              outputs  quot Hi  I am Jack  quot   This absolves the sometimes convoluted constructor pattern  A new object inherits from the old one  but is able to have its own properties  If we attempt to obtain a member from the new object   greet    which the new object jack lacks  the old object Person will supply the member  In Douglas Crockford s words   quot Objects inherit from objects  What could be more object-oriented than that  quot  You don t need constructors  no new instantiation  read why you shouldn t use new   no super  no self-made   construct  You simply create Objects and then extend or morph them  This pattern also offers immutability  partial or full   and getters setters  TypeScript The TypeScript equivalent looks the same  interface Person        name   string        greet  Function    const Person          name    Anonymous         greet   function    void    console log  Hi  I am   this name          const jack  Person   Object create Person  jack name    Jack  jack greet    Prose-like syntax  Person protoype const Person           attributes    firstName    Anonymous       lastName   Anonymous      birthYear    0     type    human         methods    name     return this firstName         this lastName       greet            console log  Hi  my name is     this name       and I am a     this type                  age              age is a function of birth time          const person   Object create Person      that s it   Clean and clear  It s simplicity does not compromise features  Read on  Creating an descendant copy of Person  Note  The correct terms are prototypes  and their descendants copies  There are no classes  and no need for instances   const Skywalker   Object create Person  Skywalker lastName    Skywalker   const anakin   Object create Skywalker  anakin firstName    Anakin  anakin birthYear    442 BBY  anakin gender    male     you can attach new properties  anakin greet       Hi  my name is Anakin Skywalker and I am a human    Person isPrototypeOf Skywalker     outputs true Person isPrototypeOf anakin     outputs true Skywalker isPrototypeOf anakin     outputs true  If you feel less safe throwing away the constructors in-lieu of direct assignments  fair point  One common way is to attach a  create method  Skywalker create   function firstName  gender  birthYear         let skywalker   Object create Skywalker       Object assign skywalker            firstName          birthYear          gender          lastName   Skywalker           type   human              return skywalker    const anakin   Skywalker create  Anakin    male    442 BBY    Branching the Person prototype to Robot When you branch the Robot descendant from Person prototype  you do not affect Skywalker and anakin     create a  Robot  prototype by extending the  Person  prototype  const Robot   Object create Person  Robot type    robot   Attach methods unique to Robot Robot machineGreet   function            some function to convert strings to binary           Mutating the  Robot  object doesn t affect  Person  prototype and its descendants anakin machineGreet      error  Person isPrototypeOf Robot     outputs true Robot isPrototypeOf Skywalker     outputs false  In TypeScript you would also need to extend the Person interface  interface Robot extends Person       machineGreet  Function   const Robot  Robot   Object create Person  Robot machineGreet   function    void   console log 101010     And You Can Have Mixins -- Because   is Darth Vader a human or robot  const darthVader   Object create anakin     for brevity  property assignments are skipped because you get the point by now  Object assign darthVader  Robot   Darth Vader gets the methods of Robot  darthVader greet      inherited from  Person   outputs  quot Hi  my name is Darth Vader    quot  darthVader machineGreet      inherited from  Robot   outputs 001010011010     Along with other odd things  console log darthVader type     outputs robot  Robot isPrototypeOf darthVader     returns false  Person isPrototypeOf darthVader     returns true   Which elegantly reflects the  quot real-life quot  subjectivity    quot He s more machine now than man  twisted and evil  quot  - Obi-Wan Kenobi    quot I know there is good in you  quot  - Luke Skywalker  Compare to the pre-ES6  quot classical quot  equivalent  function Person  firstName  lastName  birthYear  type        this firstName   firstName      this lastName   lastName     this birthYear   birthYear     this type   type       attaching methods Person prototype name   function     return firstName         lastName   Person prototype greet   function           Person prototype age   function            function Skywalker firstName  birthYear        Person apply this   firstName   Skywalker   birthYear   human          confusing re-pointing    Skywalker prototype   Person prototype Skywalker prototype constructor   Skywalker  const anakin   new Skywalker  Anakin    442 BBY        isPrototypeOf won t work Person isPrototypeOf anakin     returns false Skywalker isPrototypeOf anakin     returns false  If you want to increase code readability  you have to go for ES6 classes  which has increased in adoptation and browser compatibility  or a non-concern with babel   ES6 Classes class Person       constructor firstName  lastName  birthYear  type            this firstName   firstName          this lastName   lastName         this birthYear   birthYear         this type   type           name     return this firstName         this lastName       greet     console log  Hi  my name is     this name       and I am a     this type              class Skywalker extends Person       constructor firstName  birthYear            super firstName   Skywalker   birthYear   human            const anakin   new Skywalker  Anakin    442 BBY       prototype chain inheritance checking is partially fixed  Person isPrototypeOf anakin     returns false  Skywalker isPrototypeOf anakin     returns true  Admittedly  some of these problems are eradicated by the ES6 classes  When I wrote my answer five years ago  ES6 classes were budding  and has now matured for use  But underneath the hood of ES6 classes is the obscured true prototypial nature of Javascript  So I was naturally disappointed of its implementation  Nonetheless  that s not to say ES6 classes are bad  It provides a lot of new features and standardised a manner that is reasonably readable  Though it really should have not used the operator class and new to confuse the whole issue  Further reading Writability  Configurability and Free Getters and Setters  For free getters and setters  or extra configuration  you can use Object create   s second argument a k a propertiesObject  It is also available in  Object defineProperty  and  Object defineProperties  To illustrate its usefulness  suppose we want all Robot to be strictly made of metal  via writable  false   and standardise powerConsumption values  via getters and setters   const Robot   Object create Person           define your property attributes     madeOf             value   quot metal quot           writable  false          configurable  false          enumerable  true               getters and setters     powerConsumption            get     return this  powerConsumption            set value                 if  value indexOf  MWh    return this  powerConsumption   value replace  M     000k                this  powerConsumption   value             throw new Error  Power consumption format not recognised                          const newRobot   Object create Robot  newRobot powerConsumption    5MWh  console log newRobot powerConsumption     outputs 5 000kWh  And all prototypes of Robot cannot be madeOf something else  const polymerRobot   Object create Robot  polymerRobot madeOf    polymer  console log polymerRobot madeOf     outputs  metal

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And another year later  I can tell you there is another nice answer   If you don t like the way prototyping works in order to extend on objects classes  take alook at this  https   github com haroldiedema joii  Quick example code of possibilities  and many more    var Person   Class        username   John       role   Employee          construct  function name  role            this username   name          this role   role              getNameAndRole  function             return this username     -     this role              var Manager   Class   extends  Person           construct  function name            this super    construct   name   Manager              var m   new Manager  John    console log m getNameAndRole        Prints   John - Manager

[javascript] Extending an Object in Javascript

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