Iterator 迭代器

// 手动创建一个迭代器
function createIterator(items) {
  let index = 0

  return {
    next() {
      if (index < items.length) {
        return { value: items[index++], done: false }
      }
      return { value: undefined, done: true }
    },
  }
}

const iterator = createIterator([1, 2, 3])
iterator.next() // { value: 1, done: false }
iterator.next() // { value: 2, done: false }
iterator.next() // { value: 3, done: false }
iterator.next() // { value: undefined, done: true }

const iterable = {
  data: [1, 2, 3],

  [Symbol.iterator]() {
    let index = 0
    const data = this.data

    return {
      next() {
        if (index < data.length) {
          return { value: data[index++], done: false }
        }
        return { done: true }
      },
    }
  },
}

// 现在可以用 for...of 遍历
for (const item of iterable) {
  console.log(item) // 1, 2, 3
}

// 也可以用扩展运算符
;[...iterable] // [1, 2, 3]

// 数组
for (const item of [1, 2, 3]) {
  console.log(item)
}

// 字符串
for (const char of 'hello') {
  console.log(char)
}

// Map
for (const [key, value] of new Map([
  ['a', 1],
  ['b', 2],
])) {
  console.log(key, value)
}

// Set
for (const item of new Set([1, 2, 3])) {
  console.log(item)
}

// arguments
function test() {
  for (const arg of arguments) {
    console.log(arg)
  }
}

// NodeList
for (const node of document.querySelectorAll('div')) {
  console.log(node)
}

const arr = [1, 2, 3]
const iterator = arr[Symbol.iterator]()

iterator.next() // { value: 1, done: false }
iterator.next() // { value: 2, done: false }

class Range {
  constructor(start, end, step = 1) {
    this.start = start
    this.end = end
    this.step = step
  }

  [Symbol.iterator]() {
    let current = this.start
    const { end, step } = this

    return {
      next() {
        if (current <= end) {
          const value = current
          current += step
          return { value, done: false }
        }
        return { done: true }
      },
    }
  }
}

const range = new Range(1, 10, 2)
;[...range] // [1, 3, 5, 7, 9]

for (const n of new Range(1, 5)) {
  console.log(n) // 1, 2, 3, 4, 5
}

class Node {
  constructor(value) {
    this.value = value
    this.next = null
  }
}

class LinkedList {
  constructor() {
    this.head = null
    this.tail = null
  }

  append(value) {
    const node = new Node(value)
    if (!this.head) {
      this.head = node
      this.tail = node
    } else {
      this.tail.next = node
      this.tail = node
    }
    return this
  }

  *[Symbol.iterator]() {
    let current = this.head
    while (current) {
      yield current.value
      current = current.next
    }
  }
}

const list = new LinkedList()
list.append(1).append(2).append(3)

;[...list] // [1, 2, 3]

class TreeNode {
  constructor(value, children = []) {
    this.value = value
    this.children = children
  }

  // 深度优先遍历
  *[Symbol.iterator]() {
    yield this.value
    for (const child of this.children) {
      yield* child
    }
  }
}

const tree = new TreeNode(1, [
  new TreeNode(2, [new TreeNode(4), new TreeNode(5)]),
  new TreeNode(3, [new TreeNode(6)]),
])

;[...tree] // [1, 2, 4, 5, 3, 6]

const arr = [1, 2, 3]

// for...of 遍历值
for (const item of arr) {
  console.log(item) // 1, 2, 3
}

// for...in 遍历键
for (const index in arr) {
  console.log(index) // '0', '1', '2'
}

// for...of 不能直接遍历普通对象
const obj = { a: 1, b: 2 }
// for (const item of obj) {} // TypeError

// 需要借助 Object.entries 等
for (const [key, value] of Object.entries(obj)) {
  console.log(key, value)
}

for (const item of [1, 2, 3, 4, 5]) {
  if (item === 3) break
  console.log(item) // 1, 2
}

for (const item of [1, 2, 3, 4, 5]) {
  if (item === 3) continue
  console.log(item) // 1, 2, 4, 5
}

function createIterator() {
  let index = 0
  const data = [1, 2, 3, 4, 5]

  return {
    [Symbol.iterator]() {
      return this
    },

    next() {
      if (index < data.length) {
        return { value: data[index++], done: false }
      }
      return { done: true }
    },

    return() {
      console.log('Iterator closed')
      return { done: true }
    },
  }
}

const iterator = createIterator()

for (const item of iterator) {
  console.log(item)
  if (item === 2) break
}
// 1
// 2
// Iterator closed

const arr = [...'hello'] // ['h', 'e', 'l', 'l', 'o']
const set = new Set([...'hello']) // Set {'h', 'e', 'l', 'o'}

const [a, b, c] = new Set([1, 2, 3])
console.log(a, b, c) // 1 2 3

const [first, ...rest] = 'hello'
console.log(first) // 'h'
console.log(rest) // ['e', 'l', 'l', 'o']

const map = new Map([
  ['a', 1],
  ['b', 2],
])
Array.from(map) // [['a', 1], ['b', 2]]
Array.from(map.keys()) // ['a', 'b']
Array.from(map.values()) // [1, 2]

const promises = new Set([
  Promise.resolve(1),
  Promise.resolve(2),
  Promise.resolve(3),
])

Promise.all(promises).then(console.log) // [1, 2, 3]

function* range(start, end, step = 1) {
  for (let i = start; i <= end; i += step) {
    yield i
  }
}

;[...range(1, 5)] // [1, 2, 3, 4, 5]
;[...range(0, 10, 2)] // [0, 2, 4, 6, 8, 10]

function* naturals() {
  let n = 1
  while (true) {
    yield n++
  }
}

function take(iterable, n) {
  const result = []
  for (const item of iterable) {
    if (result.length >= n) break
    result.push(item)
  }
  return result
}

take(naturals(), 5) // [1, 2, 3, 4, 5]

// map
function* map(iterable, fn) {
  for (const item of iterable) {
    yield fn(item)
  }
}

// filter
function* filter(iterable, predicate) {
  for (const item of iterable) {
    if (predicate(item)) {
      yield item
    }
  }
}

// take
function* take(iterable, n) {
  let count = 0
  for (const item of iterable) {
    if (count++ >= n) break
    yield item
  }
}

// 组合使用
const result = [
  ...take(
    filter(
      map([1, 2, 3, 4, 5], (x) => x * 2),
      (x) => x > 4
    ),
    2
  ),
]
// [6, 8]