pseudo-private methods using a capabilities-based approach

pseudo-privates.es5.js

var ClassName = (function() {
  var PRIVATE = {};

  function ClassName(state) {
    this.state = state;
  }
  
  ClassName.prototype.publicMethod = function publicMethod() {
    this.privateMethod(PRIVATE, "...");
  };

  ClassName.prototype.privateMethod = function privateMethod(secret, otherArg) {
    if (secret !== PRIVATE) throw new Error("private method called without private capability");
    console.log(this.state);
    console.log(otherArg);
  };
  
  return ClassName;
})();

pseudo-privates.es6.js

const ClassName = (() => {
  const PRIVATE = {};

  return class ClassName {
    constructor(state) {
      this.state = state;
    }
  
    publicMethod() {
      this.privateMethod(PRIVATE, "...");
    }

    privateMethod(secret, otherArg) {
      if (secret !== PRIVATE) throw new Error("private method called without private capability");
      console.log(this.state);
      console.log(otherArg);
    }
  };
})();