基本数据类型： undefined nul boolean string number object

ES6 引入了一种新的原始数据类型 Symbol，表示独一无二的值。

{
  // 声明
  let a1 = Symbol()
  let a2 = Symbol()
  console.log(a1 === a2) // false

  let a3 = Symbol.for('a3')
  let a4 = Symbol.for('a3')
  console.log(a3 === a4) // true
}

{
  const a5 = Symbol('a5')
  console.log(Symbol.keyFor(a5)) // undefined

  const a6 = Symbol.for('a5')
  console.log(Symbol.keyFor(a6)) // a5
}

{
  let a1 = Symbol.for('abc')
  let obj = {
    [a1]: '123',
    abc: 345,
    c: 456,
  }
  console.log('obj', obj) // obj { abc: 345, c: 456, [Symbol(abc)]: '123' }

  for (let [key, value] of Object.entries(obj)) {
    console.log('let of', key, value)
    // let of abc 345
    // let of c 456
  }

  Object.getOwnPropertySymbols(obj).forEach(function (item) {
    console.log(obj[item]) // 123
  })

  Reflect.ownKeys(obj).forEach(function (item) {
    console.log('ownkeys', item, obj[item])
    // ownkeys abc 345
    // ownkeys c 456
  })
}