Implementing private instance variables in Javascript

Question

I don\'t know how I\'ve missed this for so long. I\'ve been presuming private instance variables to work like this, but they don\'t. They\'re private (as in non-global), cer

Answer 1

Privates are expensive, avoid them if possible

Private doesn't exist. You can do one of two things to emulate this.

• closures
• Weakmaps

Closures

function makePrinter(word) {
  return {
    print: function () {
      console.log(word)
    }
  }
}

WeakMap

Browser support for weakmaps is awful. You will probably need an emulation, I recommend pd.Name

var Printer = (function () {
  var privates = function (obj) {
    var v = map.get(obj)
    if (v === undefined) {
      v = {}
      map.set(obj, v)
    } 
    return v
  }, map = new WeakMap()

  return {
    constructor: function (word) {
      privates(this).word = word
    },
    print: function () {
      console.log(privates(this).word)
    }
  }
}());

Sensible objects

var Printer = {
  constructor: function (word) {
    this._word = word
  },
  print: function () {
    console.log(this._word)
  }
}