Does JavaScript have the interface type such as Java s interface

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I m learning how to make OOP with JavaScript  Does it have the interface concept  such as Java s interface     So I would be able to create a listener

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When you want to use a transcompiler  then you could give TypeScript a try  It supports draft ECMA features  in the proposal  interfaces are called  protocols   similar to what languages like coffeescript or babel do   In TypeScript your interface can look like   interface IMyInterface       id  number     TypeScript types are lowercase     name  string      callback   key  string  value  any  array  string      gt  void      type   test     notATest      so called  union type      What you can t do    Define RegExp patterns for type value Define validation like string length Number ranges etc

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there is no native interfaces in JavaScript  there are several ways to simulate an interface  i have written a package that does it  you can see the implantation here

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Hope  that anyone who s still looking for an answer finds it helpful   You can try out using a Proxy  It s standard since ECMAScript 2015   https   developer mozilla org en-US docs Web JavaScript Reference Global Objects Proxy  latLngLiteral   new Proxy          set  function obj  prop  val              only these two properties can be set         if   lng   lat   indexOf prop     -1                throw new ReferenceError  Key must be  lat  or  lng                           the dec format only accepts numbers         if typeof val      number                 throw new TypeError  Value must be numeric                         latitude is in range between 0 and 90         if prop     lat    amp  amp    0  lt  val  amp  amp  val  lt  90                 throw new RangeError  Position is out of range                         longitude is in range between 0 and 180         else if prop     lng   amp  amp    0  lt  val  amp  amp  val  lt  180                 throw new RangeError  Position is out of range                        obj prop    val           return true              Then you can easily say   myMap      myMap position   latLngLiteral

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There s no notion of  this class must have these functions   that is  no interfaces per se   because    JavaScript inheritance is based on objects  not classes   That s not a big deal until you realize  JavaScript is an extremely dynamically typed language -- you can create an object with the proper methods  which would make it conform to the interface  and then undefine all the stuff that made it conform  It d be so easy to subvert the type system -- even accidentally  -- that it wouldn t be worth it to try and make a type system in the first place    Instead  JavaScript uses what s called duck typing   If it walks like a duck  and quacks like a duck  as far as JS cares  it s a duck   If your object has quack    walk    and fly   methods  code can use it wherever it expects an object that can walk  quack  and fly  without requiring the implementation of some  Duckable  interface  The interface is exactly the set of functions that the code uses  and the return values from those functions   and with duck typing  you get that for free   Now  that s not to say your code won t fail halfway through  if you try to call some dog quack    you ll get a TypeError   Frankly  if you re telling dogs to quack  you have slightly bigger problems  duck typing works best when you keep all your ducks in a row  so to speak  and aren t letting dogs and ducks mingle together unless you re treating them as generic animals   In other words  even though the interface is fluid  it s still there  it s often an error to pass a dog to code that expects it to quack and fly in the first place   But if you re sure you re doing the right thing  you can work around the quacking-dog problem by testing for the existence of a particular method before trying to use it  Something like  if  typeof someObject quack      function            This thing can quack     So you can check for all the methods you can use before you use them  The syntax is kind of ugly  though  There s a slightly prettier way   Object prototype can   function methodName         return   typeof this methodName       function        if  someObject can  quack          someObject quack        This is standard JavaScript  so it should work in any JS interpreter worth using   It has the added benefit of reading like English   For modern browsers  that is  pretty much any browser other than IE 6-8   there s even a way to keep the property from showing up in for   in   Object defineProperty Object prototype   can         enumerable  false      value  function method            return  typeof this method       function              The problem is that IE7 objects don t have  defineProperty at all  and in IE8  it allegedly only works on host objects  that is  DOM elements and such    If compatibility is an issue  you can t use  defineProperty    I won t even mention IE6  because it s rather irrelevant anymore outside of China    Another issue is that some coding styles like to assume that everyone writes bad code  and prohibit modifying Object prototype in case someone wants to blindly use for   in  If you care about that  or are using  IMO broken  code that does  try a slightly different version   function can obj  methodName         return   typeof obj methodName       function       if  can someObject   quack          someObject quack

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abstract interface like this  const MyInterface       serialize       gt   throw  must implement serialize for MyInterface types      print       gt  console log this serialize        create an instance   function MyType       this serialize        gt   serialized     MyType prototype   MyInterface   and use it  let x   new MyType   x print

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I know this is an old one  but I ve recently found myself needing more and more to have a handy API for checking objects against interfaces  So I wrote this  https   github com tomhicks methodical  It s also available via NPM  npm install methodical  It basically does everything suggested above  with some options for being a bit more strict  and all without having to do loads of if  typeof x method      function   boilerplate   Hopefully someone finds it useful

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It bugged me too to find a solution to mimic interfaces with the lower impacts possible  One solution could be to make a tool        parameter  Array object  required   method name list or members types by their name  constructor    let Interface function required       this obj 0      if required instanceof Array           this obj             required forEach r  gt this obj r   function         else if typeof required     object            this obj required       else           throw  Interface invalid parameter required     required                check constructor instance  parameter  object  scope   instance to check   parameter  boolean   strict    if true - gt  throw an error if errors ar found   constructor    Interface prototype check function scope strict       let err    type res         for let k in this obj           type typeof scope k            if type   this obj k                err push                   key k                  type this obj k                   inputType type                  msg type    undefined   missing element   bad element type  quot   type   quot                                       res success  err length      if err length           res msg  Class bad structure             res errors err          if strict               let stk   new Error   stack split   n                stk shift                throw     res msg                  res errors map e  gt  -    e type      e key       e msg  join   n                        at   n t  stk join   n t                 join   n                          return res      Exemple of use      create interface tool let dataInterface new Interface   toData   fromData        abstract constructor let AbstractData function        dataInterface check this 1     check extended element       extended constructor let DataXY function        AbstractData apply this          this xy  0 0      DataXY prototype toData function        return  this xy 0  this xy 1           should throw an error because  fromData  is missing let dx new DataXY     With classes class AbstractData      constructor            dataInterface check this 1           class DataXY extends AbstractData      constructor            super            this xy  0 0             toData            return  this xy 0  this xy 1             It s still a bit performance consumming and require dependancy to the Interface class  but can be of use for debug or open api

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This is old but I implemented interfaces to use on ES6 without transpiller  https   github com jkutianski ES6-Interfaces

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This is an old question  nevertheless this topic never ceases to bug me   As many of the answers here and across the web focus on  enforcing  the interface  I d like to suggest an alternative view      I feel the lack of interfaces the most when I m using multiple classes that behave similarly  i e  implement an interface     For example  I have an Email Generator that expects to receive Email Sections Factories  that  know  how to generate the sections  content and HTML  Hence  they all need to have some sort of getContent id  and getHtml content  methods      The closest pattern to interfaces  albeit it s still a workaround  I could think of is using a class that ll get 2 arguments  which will define the 2 interface methods    The main challenge with this pattern is that the methods either have to be static  or to get as argument the instance itself  in order to access its properties  However there are cases in which I find this trade-off worth the hassle    x000D   x000D  class Filterable   x000D    constructor data    filter  toString      x000D      this data   data  x000D      this filter   filter  x000D      this toString   toString  x000D         You can also enforce here an Iterable interface  for example  x000D         which feels much more natural than having an external check x000D      x000D    x000D   x000D  const evenNumbersList   new Filterable  x000D     1  2  3  4  5  6     x000D      filter   lst    gt    x000D        const evenElements   lst data filter x   gt  x   2     0   x000D        lst data   evenElements  x000D         x000D      toString  lst   gt    lt    lst data toString     gt    x000D      x000D     x000D   x000D  console log  The whole list        evenNumbersList toString evenNumbersList    x000D  evenNumbersList filter evenNumbersList   x000D  console log  The filtered list     evenNumbersList toString evenNumbersList    x000D   x000D   x000D

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JavaScript Interfaces   Though JavaScript does not have the interface type  it is often times needed  For reasons relating to JavaScript s dynamic nature and use of Prototypical-Inheritance  it is difficult to ensure consistent interfaces across classes -- however  it is possible to do so  and frequently emulated   At this point  there are handfuls of particular ways to emulate Interfaces in JavaScript  variance on approaches usually satisfies some needs  while others are left unaddressed  Often times  the most robust approach is overly cumbersome and stymies the implementor  developer    Here is an approach to Interfaces   Abstract Classes that is not very cumbersome  is explicative  keeps implementations inside of Abstractions to a minimum  and leaves enough room for dynamic or custom methodologies   function resolvePrecept interfaceName        var interfaceName   interfaceName      return function curry value                    throw new Error interfaceName     requires an implementation for                       console warn   s requires an implementation for       interfaceName           return value            var iAbstractClass   function AbstractClass         var defaultTo   resolvePrecept  iAbstractClass         this datum1   this datum1    defaultTo new Number         this datum2   this datum2    defaultTo new String          this method1   this method1    defaultTo new Function  return new Boolean            this method2   this method2    defaultTo new Function  return new Object             var ConcreteImplementation   function ConcreteImplementation          this datum1   1      this datum2    str        this method1   function method1             return true             this method2   function method2             return                   Applies Interface  Implement iAbstractClass Interface      iAbstractClass apply this        call    apply after precept definitions      Participants  Precept Resolver  The resolvePrecept function is a utility  amp  helper function to use inside of your Abstract Class  Its job is to allow for customized implementation-handling of encapsulated Precepts  data  amp  behavior   It can throw errors or warn -- AND -- assign a default value to the Implementor class   iAbstractClass  The iAbstractClass defines the interface to be used  Its approach entails a tacit agreement with its Implementor class  This interface assigns each precept to the same exact precept namespace -- OR -- to whatever the Precept Resolver function returns  However  the tacit agreement resolves to a context -- a provision of Implementor   Implementor  The Implementor simply  agrees  with an Interface  iAbstractClass in this case  and applies it by the use of Constructor-Hijacking  iAbstractClass apply this   By defining the data  amp  behavior above  and then hijacking the Interface s constructor -- passing Implementor s context to the Interface constructor -- we can ensure that Implementor s overrides will be added  and that Interface will explicate warnings and default values   This is a very non-cumbersome approach which has served my team  amp  I very well for the course of time and different projects  However  it does have some caveats  amp  drawbacks   Drawbacks  Though this helps implement consistency throughout your software to a significant degree  it does not implement true interfaces -- but emulates them  Though definitions  defaults  and warnings or errors are explicated  the explication of use is enforced  amp  asserted by the developer  as with much of JavaScript development    This is seemingly the best approach to  Interfaces in JavaScript   however  I would love to see the following resolved    Assertions of return types Assertions of signatures Freeze objects from delete actions Assertions of anything else prevalent or needed in the specificity of the JavaScript community   That said  I hope this helps you as much as it has my team and I

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You need interfaces in Java since it is statically typed and the contract between classes should be known during compilation  In JavaScript it is different  JavaScript is dynamically typed  it means that when you get the object you can just check if it has a specific method and call it

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JavaScript  ECMAScript edition 3  has an implements reserved word saved up for future use  I think this is intended exactly for this purpose  however  in a rush to get the specification out the door they didn t have time to define what to do with it  so  at the present time  browsers don t do anything besides let it sit there and occasionally complain if you try to use it for something   It is possible and indeed easy enough to create your own Object implement Interface  method with logic that baulks whenever a particular set of properties functions are not implemented in a given object   I wrote an article on object-orientation where use my own notation as follows      Create a  Dog  class that inherits from  Animal     and implements the  Mammal  interface var Dog   Object extend Animal        constructor  function name            Dog superClass call this  name              bark  function             alert  woof             implement Mammal     There are many ways to skin this particular cat  but this is the logic I used for my own Interface implementation  I find I prefer this approach  and it is easy to read and use  as you can see above   It does mean adding an  implement  method to Function prototype which some people may have a problem with  but I find it works beautifully   Function prototype implement   function            Loop through each interface passed in and then check         that its members are implemented in the context object  this       for var i   0  i  lt  arguments length  i                   Check member s logic                 Remember to return the class being tested     return this

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Javascript does not have interfaces  But it can be duck-typed  an example can be found here   http   reinsbrain blogspot com 2008 10 interface-in-javascript html

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Pick up a copy of  JavaScript design patterns  by Dustin Diaz  There s a few chapters dedicated to implementing JavaScript interfaces through Duck Typing  It s a nice read as well  But no  there s no language native implementation of an interface  you have to Duck Type      example duck typing method var hasMethods   function obj     method list as strings          var i   1  methodName      while  methodName   arguments i               if typeof obj methodName      function                 return false                      return true        in your code if hasMethods obj   quak    flapWings   waggle              IT S A DUCK  do your duck thang

[javascript] Does JavaScript have the interface type (such as Java's 'interface')?

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