JS Data Structures: Sets, see more at: https://medium.com/p/5c0c7e52203

DATA_STRUCTURES_setWithoutES6.js

function Set() {
  let items = {}
  let size = 0

  class PublicSet {
    has(item) {
      return item in items
    }

    add(item) {
      if (this.has(item)) {
        return false
      }

      items[item] = item
      size++
      return true
    }

    delete(item) {
      if (!this.has(item)) {
        return false
      }

      delete items[item]

      size--

      return true
    }

    clear() {
      items = {}
      size = 0

      return true
    }

    size() {
      return size
    }

    values() {
      let itemsValue = []

      for (let value in items) {
        itemsValue.push(items[value])
      }

      return itemsValue
    }
  }

  return new PublicSet()
}