Symbol Polyfill

symbol-polyfill.js

const GlobalSymbolRegistry = {}

const INTERNAL = {
  hasInstance: true,
  isConcatSpreadable: true,
  iterator: true,
  match: true,
  replace: true,
  search: true,
  species: true,
  split: true,
  toPrimitive: true,
  toStringTag: true,
  unscopables: true,
}

const generateName = (function () {
  const created = {}

  return function (description, internal) {
    let postfix
    if (INTERNAL[description]) {
      postfix = ''
      INTERNAL[description] = false
    } else {
      postfix = created[description] =  created[description] === undefined ?  1 : created[description] + 1
    }
    return `@@${ description || '' }${ postfix }`
  }
}())

function d (mode, value) {
  const map = {
    c: 'configurable',
    e: 'enumerable',
    w: 'writable',
  }

  return mode
    .split('')
    .reduce((desc, m) => {
      desc[map[m]] = true
      return desc
    }, {
      value,
      configurable: false,
      enumerable: false,
      writable: false,
    })
}

function isSymbol (value) {
  if (!value) return false
  if (typeof value !== 'object') return false
  if (!value.constructor) return false
  if (value.constructor.name !== 'Symbol') return false
  if (!value.__SymbolData__ || value.__SymbolData__ !== value) return false
  if (value[value.constructor.toStringTag] !== 'Symbol') return false
  return true
}

function validateSymbol (value) {
  if (!isSymbol(value)) return new TypeError(value + 'is not a symbol')
  return value
}

const {
  defineProperty,
  defineProperties
} = Object

const HiddenSymbol = function Symbol (description) {
  if (this instanceof HiddenSymbol) throw new TypeError('Symbol is not a constructor')
  return SymbolPolyfill(description)
}

const SymbolPolyfill= function Symbol (description) {
  if (this instanceof SymbolPolyfill) throw new TypeError('Symbol is not a constructor')

  let descString
  description === undefined ? descString = undefined : descString = String(description)

  let symbol = Object.create(HiddenSymbol.prototype)

  defineProperties(symbol, {
    __Description__: d('c', descString),
    __Name__: d('c', generateName(descString)),
    __SymbolData__: d('c', symbol),
  })

  return symbol
}

defineProperties(SymbolPolyfill, {
  for: d('cw', function (key) {
    key = String(key)
    return GlobalSymbolRegistry[key] ? GlobalSymbolRegistry[key] : GlobalSymbolRegistry[key] = SymbolPolyfill(key)
  }),
  keyFor: d('cw', function (symbol) {
    for (let key in globalSymbols) {
      if (globalSymbols[key] === symbol) return key
    }
  }),
  prototype: d('', SymbolPolyfill.prototype),
})

Object
  .keys(INTERNAL)
  .forEach(key => {
    defineProperty(SymbolPolyfill, key, d('', SymbolPolyfill(key)))
  })

defineProperties(HiddenSymbol.prototype, {
  constructor: d('cw', SymbolPolyfill),
  toString: d('cw', function () { return this.__Name__; }),
})

defineProperties(SymbolPolyfill.prototype, {
  toString: d('cw', function () { return `Symbol(${ validateSymbol(this).__Description__ || '' })` }),
  valueOf: d('cw', function () { return validateSymbol(this).__SymbolData__ }),
})

defineProperty(SymbolPolyfill.prototype, SymbolPolyfill.toPrimitive, d('c', function () { return validateSymbol(this).__SymbolData__} ))
defineProperty(SymbolPolyfill.prototype, SymbolPolyfill.toStringTag, d('c', 'Symbol'))

defineProperty(HiddenSymbol.prototype, SymbolPolyfill.toPrimitive, d('c', SymbolPolyfill.prototype[SymbolPolyfill.toPrimitive]));
defineProperty(HiddenSymbol.prototype, SymbolPolyfill.toStringTag, d('c', SymbolPolyfill.prototype[SymbolPolyfill.toStringTag]));