es5_Set.js

// approximate implementation of Set using Symbol (ES6)
(() => {
  function aSet() {
    this.hash = {}
  }
  aSet.prototype = {
    add(a) { this.hash[a] = true },
    has(a) { return a in this.hash },
    values() { return Object.keys(this.hash) },
    delete(a) { delete this.hash[a] },
    clear() { this.hash = {} }
  }

  var mySet = new aSet()
  var myObj = Object(Symbol('myObj'))
  myObj.a = 1
  myObj.b = 2

  mySet.add(myObj)
  console.log(mySet.has(myObj)) // true
})()



// approximate implementation of Set without Symbol
// ES5 compatible
//  - create unique hash keys
//  - local override #toString
(() => {
  function aSet() {
    this.hash = {}
  }
  aSet.prototype = {
    add: function(a) { this.hash[a] = true },
    has: function(a) { return a in this.hash },
    values: function() { return Object.keys(this.hash) },
    delete: function(a) { delete this.hash[a] },
    clear: function() { this.hash = {} }
  }

  var count = 0
  function id(i) {
    count++
    return function() {
      return String(i)
    }
  }

  var mySet = new aSet()
  var myObj = { a: 1, b: 2, toString: id(count) }

  mySet.add(myObj)
  console.log(mySet.has(myObj)) // true
})()

// same as above with a less fragile SetObject abstraction
(() => {
  function aSet() {
    this.hash = {}
  }
  aSet.prototype = {
    add: function(a) { this.hash[a] = true },
    has: function(a) { return a in this.hash },
    values: function() { return Object.keys(this.hash) },
    delete: function(a) { delete this.hash[a] },
    clear: function() { this.hash = {} }
  }

  var SetObject = (function() {
    var count = 0
    return function(obj) {
      Object.defineProperty(obj, 'toString', {
        value: (function(i) {
          return function() {
            return '__SetObject__' + i
          }
        })(count),
        enumerable: false
      })
      count++
      return obj
    }
  })()

  var mySet = new aSet()
  var mySetObj = SetObject({ a: 1, b: 2 })

  mySet.add(mySetObj)
  console.log(mySet.has(mySetObj)) // true
})()

Compare SetObject to Data Warehouse: https://gist.github.com/thurt/c654dd88e3981f410d50