How to implement private method in ES6 class with Traceur

Question

I use Traceur Compiler to have advantage with ES6 features now    I want to implement this stuff from ES5   function Animal         var self   this          sayHi       sayHi    function             self hi                this hi   function                   Currently traceur does not support private and public keywords  from harmony   And ES6 class syntax does not allow to use simple var  or let  statements in class body    The only way that I am find is to simulate privates before class declaration  Something like   var sayHi   function                do stuff     class Animal         It is better then nothing but as expected you can not pass correct this to private method without apply-ing or bind-ing it every time   So  is there any possibility to use private data in ES6 class compatible with traceur compiler

User · Answer

You can always use normal functions   function myPrivateFunction       console log  My property      this prop      class MyClass       constructor         this prop    myProp       myPrivateFunction bind this            new MyClass        My property  myProp

User · Answer

You can use Symbol   var say   Symbol    function Cat      this say       call private methos    Cat prototype say    function    alert  im a private       P S  alexpods is not correct  he get protect rather than private  since inheritance is a name conflict  Actually you can use var say   String Math random    instead Symbol  IN ES6   var say   Symbol    class Cat      constructor        this say       call private         say         alert  im private

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I hope this can be helpful      I  Declaring vars  functions inside IIFE Immediately-invoked function expression   those can be used only in the anonymous function   It can be good to use  let  const  keywords without using  var  when you need to change code for ES6     let Name    function       const  privateHello   function           class Name       constructor               publicMethod            privateHello               return Name          II  WeakMap object can be good for memoryleak trouble    Stored variables in the WeakMap will be removed when the instance will be removed  Check this article   Managing the private data of ES6 classes   let Name    function       const  privateName   new WeakMap            III  Let s put all together   let Name    function       const  privateName   new WeakMap      const  privateHello   function fullName        console log  Hello      fullName          class Name       constructor firstName  lastName           privateName set this   firstName  firstName  lastName  lastName              static printName name          let privateName    privateName get name         let  fullname   privateName firstName         privateName lastName         privateHello  fullname             printName           let privateName    privateName get this         let  fullname   privateName firstName         privateName lastName         privateHello  fullname                return Name         var aMan   new Name  JH    Son    aMan printName        Hello  JH Son  Name printName aMan       Hello  JH Son

User · Answer

Although currently there is no way to declare a method or property as private  ES6 modules are not in the global namespace  Therefore  anything that you declare in your module and do not export will not be available to any other part of your program  but will still be available to your module during run time  Thus  you have private properties and methods     Here is an example  in test js file    function tryMe1 a      console log a   2      var tryMe2   1234   class myModule     tryMe3 a        console log a   100          getTryMe1 a        tryMe1 a          getTryMe2         return tryMe2            Exports just myModule class  Not anything outside of it  export default myModule     In another file       import MyModule from    test    let bar   new MyModule     tryMe1 1      ReferenceError  tryMe1 is not defined tryMe2     ReferenceError  tryMe2 is not defined bar tryMe1 1      TypeError  bar tryMe1 is not a function bar tryMe2     undefined  bar tryMe3 1      101 bar getTryMe1 1      3 bar getTryMe2       1234

User · Answer

Have you considered using factory functions  They usually are a much better alternative to classes or constructor functions in Javascript   Here is an example of how it works   function car           var privateVariable   4      function privateFunction            return            color   red            drive  function  miles               stop  function                                Thanks to closures you have access to all private functions and variabels inside the returned object  but you can not access them from outside

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I came up with what I feel is a much better solution allowing    no need for  this     that self  weakmaps  symbols etc  Clear and straightforward  class  code private variables and methods are really private and have the correct  this  binding No use of  this  at all which means clear code that is much less error prone public interface is clear and separated from the implementation as a proxy to private methods allows easy composition   with this you can do    x000D   x000D  function Counter     x000D       public interface x000D    const proxy     x000D      advance      advance counter and get new value x000D      reset        reset value x000D      value        get value x000D      x000D    x000D       private variables and methods x000D    let count 0  x000D       x000D    function advance     x000D      return   count  x000D      x000D        x000D    function reset newCount    x000D      count  newCount    0   x000D      x000D        x000D    function value     x000D      return count  x000D      x000D       x000D    return proxy  x000D    x000D        x000D  let counter Counter New    x000D  console log counter instanceof Counter      true x000D  counter reset 100   x000D  console log  Counter next     counter advance        101 x000D  console log Object getOwnPropertyNames counter         advance    reset    value   x000D   lt script src  https   cdn rawgit com kofifus New 7987670c new js  gt  lt  script gt  x000D   x000D   x000D    see New for the code and more elaborate examples including constructor and composition

User · Answer

There are no private  public or protected keywords in current ECMAScript 6 specification   So Traceur does not support private and public  6to5  currently it s called  Babel   realizes this proposal for experimental purpose  see also this discussion   But it s just proposal  after all   So for now you can just simulate private properties through WeakMap  see here   Another alternative is Symbol - but it doesn t provide actual privacy as the property can be easily accessed through Object getOwnPropertySymbols   IMHO the best solution at this time - just use pseudo privacy  If you frequently use apply or call with your method  then this method is very object specific  So it s worth to declare it in your class just with underscore prefix   class Animal         sayHi                do stuff

User · Answer

As alexpods says  there is no dedicated way to do this in ES6  However  for those interested  there is also a proposal for the bind operator which enables this sort of syntax   function privateMethod       return  Hello   this name       export class Animal     constructor name        this name   name        publicMethod         this  privateMethod            Once again  this is just a proposal  Your mileage may vary

User · Answer

As Marcelo Lazaroni has already said      Although currently there is no way to declare a method or property as private  ES6 modules are not in the global namespace  Therefore  anything that you declare in your module and do not export will not be available to any other part of your program  but will still be available to your module during run time    But his example didn t show how the private method could access members of the instance of the class  Max shows us some good examples of how access instance members through binding or the alternative of using a lambda method in the constructor  but I would like to add one more simple way of doing it  passing the instance as a parameter to the private method  Doing it this way would lead Max s MyClass to look like this   function myPrivateFunction myClass      console log  My property      myClass prop      class MyClass       constructor         this prop    myProp         testMethod         myPrivateFunction this         module exports   MyClass    Which way you do it really comes down to personal preference

[javascript] How to implement private method in ES6 class with Traceur

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