ES6 generator Symbol yield

Symbol  独一无二的值

const s1 = Symbol('a')
      const s2 = Symbol('a')
      console.log(s1 === s2) // false
      console.log(typeof s1) // 'symbol'
      let o = {
        a: 90,
      }
    let symbol = Symbol()
      o[symbol] = 100
      console.log(o)//{a: 90, Symbol(): 100}

普通函数一旦执行 函数体从上往下依次执行

 function f() {
        console.log(1)//1
        console.log(2)//2
        console.log(3)//3
      }
      f()

生成器函数(generator)-生成迭代器(Iterator)的函数,配合yield关键字来控制代码执行流程

  function* go(str) {
        console.log(1)//1
        let a = yield str
        console.log(2)//2
        let b = yield a
        console.log(3)//3
        return b
      }
 const it = go('hehe')
      console.log(it)
      const r1 = it.next(1)
      console.log(r1)//{value:'hehe',done: false}
      const r2 = it.next(2)
      console.log(r2)//{value: 2, done: false}
      const r3 = it.next(3)
      console.log(r3)//{value: 3, done: true}
      function* test(num) {
        let x = 3 * (yield num + 1)
        let y = yield x / 3
        return x + y + num
      }
  let n = test(3)
      console.log(n.next()) // {value: 4, done: false}
      console.log(n.next(5)) // {value: 5, done: false}
      console.log(n.next(8)) // {value: 26, done: true}

自己模拟生成器函数

 function* buy(books) {
        for (let i = 0; i < books.length; i++) {
          yield books[i]
        }
      }
      const it = buy(['js', 'vue'])
      console.log(it.next()) //{value: 'js', done: false}
      console.log(it.next()) // { value: 'vue', done:true}
      console.log(it.next())//{value: undefined, done: true}

for-of遍历迭代器

  function* demo() {
        yield 1
        yield 2
        yield 3
        return 4
      }
      const it = demo()
      for (let k of it) {
        console.log(k)
      }

yield是ES6的新关键字，使生成器函数执行暂停，yield关键字后面的表达式的值返回给生成器的调用者。它可以被认为是一个基于生成器的版本的return关键字。
yield关键字实际返回一个IteratorResult（迭代器）对象，它有两个属性，value和done，分别代表返回值和是否完成。
yield无法单独工作，需要配合generator(生成器)的其他函数，如next，懒汉式操作，展现强大的主动控制特性。

  function* test() {
        yield 'a'
        yield 'b'
      }

      function* test1() {
        yield 'x'
        // yield* test()
        // for (let k of test()) {
        //   console.log(k)
        // }
        yield 'y'
      }
      for (let v of test1()) {
        console.log(v)
      }

      function* Person() {
        yield (this.name = 'zs')
        yield (this.age = 18)
      }
      let person = {}
      let obj = Person.bind(person)()
      console.log(obj.next())//{value: 'zs', done: false}
      console.log(obj.next())//{value: 18, done: false}
      console.log(person)//{name: 'zs', age: 18}

generator应用 

 const oImg = document.querySelector('img')
      let flag = true // 第一张 
function* change() {
        while (true) {
          yield (oImg.src = './images/2.jpg')
          yield (oImg.src = './images/1.jpg')
        }
      }
      const it = change()
      document.querySelector('.btn').addEventListener('click', function () {
        it.next()
      })