How to simulate private scope with ES6 classes?

Question

How to simulate private scope with ES6 classes?

I'm trying to migrate my code from ES5 modules to ES6 classes. My current code (the module, not the class), looks as follows:

var appleModule = (function() {
    var yummy = true;
    var publicEat = function() { }
    var privateEat = function() { }

    return { "eat": publicEat }
})();

The above allows me to create private variables and methods that I don't expose to other Javascript objects/scopes in my environment. I like that. I figure in ES6 we have classes now so something like this becomes the refactored code:

exports class appleModule {
    constructor {
        this.yummy = true;
    }

    publicEat() {

    }

    privateEat() {
        // How to make private?
    }
}

However, there isn't a good way to make private variables and functions with this new class-based syntax. Is the new recommendation to use exports with the previous module pattern (closure returning object of public members)? If not, how can I simulate private scope like I did with the first example (hopefully without underscores)?

3 Answers

ES6 does not offer a specific feature for private variables. The ES6 class syntax is purely syntactic sugar on the regular .prototype so using pure ES6 syntax will just get you methods on the prototype that can't access private variables.

If you want private variables, you can define them in the constructor function like you did before and then you have to define the methods that want to access those privates in the constructor also (which will not be using the new ES6 syntax).

0 Created by jfriend00 on 2020-03-17 22:00:51 +0000 UTC

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I generally put functions I don't want exposed outside of the class definition and access it from within the class:

let privateMethod = function () {
  // Some magic and return some value
  // ...

  return true;
}

class Foo {
  constructor {
    this.bar = 'baz';
  }

  publicMethod() {
    // Calls method not visible outside to the caller
    return privateMethod();
  }
}

export Foo;

0 Created by stackunderflow on 2020-03-17 22:00:51 +0000 UTC

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Felix Kling · Accepted Answer · 2020-03-17 22:00:51 +0000 UTC

I figure in ES6 we have classes now so something like this becomes the refactored code.

Your original code seem to represent something like a module. Classes are not modules, but ES6 now has native support for modules. So your code should more likely become

var yummy = true;
var publicEat = function() { }
var privateEat = function() { }

export {publicEat as eat};

Not everything can, has or should be modeled as a class.