Nested ES6 classes?

问题:

It seems possible to nest a class in a constructor which can then be instantiated from anywhere within the class, is this official?

[EDIT] E.g.,

class C {

    constructor() {
        class D {
            constructor() { }
        }
    }

    method() {
        var a = new D();  // works fine
    }

}

//var a = new D();  // fails in outer scope

The traceur generated JS https://google.github.io/traceur-compiler/demo/repl.html

$traceurRuntime.ModuleStore.getAnonymousModule(function() {
  "use strict";
  var C = function C() {
    var D = function D() {};
    ($traceurRuntime.createClass)(D, {}, {});
  };
  ($traceurRuntime.createClass)(C, {method: function() {
      var a = new D();
    }}, {});
  return {};
});
//# sourceURL=traceured.js

回答1:

No, there are no nested classes in ES6, and there is no such thing as private members in the class syntax anyway if you mean that.

Of course you can put a second class as a static property on another class, like this:

class A {
    …
}
A.B = class {
    …
};

or you use an extra scope:

var C;
{
    class D {
        constructor() { }
    }
    C = class C {
        constructor() { }
        method() {
            var a = new D();  // works fine
        }
    }
}

^{(There seems to be a bug with traceur as it uses a hoisted var for the class declaration instead of block scope)}

---

With the proposed class field syntax, it will also be possible to write a single expression or declaration:

class A {
    …
    static B = class {
         …
    }
};

回答2:

You could use a getter:

class Huffman {
  constructor() { /* ... */ }
  static get Node() {
    return class Node {
      constructor() {  
        var API = this;
        API.symbol = 0; API.weight = 0;
        return API;    
      }
    };
  }
  get Node() {
    return Huffman.Node;
  }
  encode() { /* ... */ }
  decode() { /* ... */ }
  /* ... */
}

// usage
huffman = new Huffman;
new huffman.Node;
new Huffman.Node;

Which in latest Chrome Dev 44.0.2376.0 on Apple 10.10.2 gives in console

• new huffman.Node
• Node {symbol: 0, weight: 0}
• new Huffman.Node
• Node {symbol: 0, weight: 0}

In other news, getters are the secret sauce that let's you do a whole bunch of cool things in ES6.

Please Note The above construction breaks instanceof for Node (why? because a whole new class is defined with every get call). To not break instanceof define Node outside of the scope of a single getter, either in the constructor (disabling the Huffman.Node class property and causing instanceof to work within the namespace of a single Huffman instance, and break outside that), or define Node in a sibling or ancestor scope to Huffman (allowing instanceof to work in all scopes below that the one where Node is defined).

回答3:

something like that?

class A {
    constructor () {
        this.B = class {
            echo () {
                console.log('I am B class');
            }
        }
    }
    echo () {
        this.b = new this.B;
        this.b.echo();
    }
}

var a = new A;

a.echo();