JavaScript What are extend and prototype used for

Question

I am relatively new to JavaScript and keep seeing  extend and  prototype in third party libraries I am using  I thought it had to do with the Prototype javascript library  but I am beginning to think that is not the case  What are these used for

User · Answer

extend   is added by many third-party libraries to make it easy to create objects from other objects   See http   api jquery com jQuery extend  or http   www prototypejs org api object extend for some examples    prototype refers to the  template   if you want to call it that  of an object  so by adding methods to an object s prototype  you see this a lot in libraries to add to String  Date  Math  or even Function  those methods are added to every new instance of that object

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Javascript s inheritance is prototype based  so you extend the prototypes of objects such as Date  Math  and even your own custom ones   Date prototype lol   function      alert  hi          new Date   lol      alert message   In the snippet above  I define a method for all Date objects   already existing ones and all new ones      extend is usually a high level function that copies the prototype of a new subclass that you want to extend from the base class   So you can do something like   extend  Fighter  Human     And the Fighter constructor object will inherit the prototype of Human  so if you define methods such as live and die on Human then Fighter will also inherit those   Updated Clarification    high level function  meaning  extend isn t built-in but often provided by a library such as jQuery or Prototype

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This seems to be the clearest and simplest example to me  this just appends property or replaces existing  function replaceProperties copyTo  copyFrom       for  var property in copyFrom       copyTo property     copyFrom property    return copyTo

User · Answer

extends    create a class which is a child of another class  behind the scenes  Child prototype   proto   sets its value to Parent prototype  so methods are inherited   prototype inherit features from one to another      proto    is a getter setter for Prototype

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Some extend functions in third party libraries are more complex than others  Knockout js for instance contains a minimally simple one that doesn t have some of the checks that jQuery s does   function extend target  source        if  source            for var prop in source                if source hasOwnProperty prop                     target prop    source prop                                     return target

User · Answer

Javascript inheritance seems to be like an open debate everywhere  It can be called  The curious case of Javascript language    The idea is that there is a base class and then you extend the base class to get an inheritance-like feature  not completely  but still    The whole idea is to get what prototype really means  I did not get it until I saw John Resig s code  close to what jQuery extend does  wrote a code chunk that does it and he claims that base2 and prototype libraries were the source of inspiration   Here is the code           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   amp  amp          typeof  super name      function   amp  amp  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  amp  amp  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               There are three parts which are doing the job  First  you loop through the properties and add them to the instance  After that  you create a constructor for later to be added to the object Now  the key lines are      Populate our constructed prototype object Class prototype   prototype      Enforce the constructor to be what we expect Class prototype constructor   Class    You first point the Class prototype to the desired prototype  Now  the whole object has changed meaning that you need to force the layout back to its own one   And the usage example   var Car   Class Extend     setColor  function clr       color   clr           var volvo   Car Extend      getColor  function            return color             Read more about it here at Javascript Inheritance by John Resig  s post

User · Answer

The extend method for example in jQuery or PrototypeJS  copies all properties from the source to the destination object   Now about the prototype property  it is a member of function objects  it is part of the language core   Any function can be used as a constructor  to create new object instances  All functions have this prototype property   When you use the new operator with on a function object  a new object will be created  and it will inherit from its constructor prototype   For example   function Foo        Foo prototype bar   true   var foo   new Foo     foo bar     true foo instanceof Foo     true Foo prototype isPrototypeOf foo      true

[javascript] JavaScript: What are .extend and .prototype used for?

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