1. promise

1.1 实现红绿灯

function red() {
  console.log('red')
}
function green() {
  console.log('green')
}
function yellow() {
  console.log('yellow')
}

const light = (timer, cb) => {
  return new Promise(resolve => {
    setTimeout(() => {
      cb()
      resolve()
    }, timer)
  })
}

const step = function () {
  Promise.resolve()
    .then(() => {
      return light(3000, red)
    })
    .then(() => {
      return light(2000, yellow)
    })
    .then(() => {
      return light(1000, green)
    })
    .then(() => {
      return step()
    })
}

step()

1.2 实现每秒自动加一

其实，这道题就是上面那道题的变形。

let count = 0

const addOne = () => {
  count = count + 1
  console.log(count)
}
const addOneByOneSecond = (timer, cb) => {
  return new Promise(resolve => {
    setTimeout(() => {
      cb()
      resolve()
    }, timer)
  })
}

const finalFunc = () => {
  Promise.resolve()
    .then(() => {
      return addOneByOneSecond(1000, addOne)
    })
    .then(() => {
      return finalFunc()
    })
}

finalFunc()

1.3 Promise.all

const myPromiseAll = (iterable) => {
  return new Promise((resolve, reject) => {
    const promises = Array.from(iterable)
    // 保存执行成功的个数
    let count = 0
    // 保存执行成功的结果
    const result = []

    promises.map((promise, index) => {
      promise
        .then((res) => {
          result[index] = res
          count += 1

          if (promises.length === count) {
            resolve(result)
          }
        })
        .catch((err) => {
          reject(err)
        })
    })
  })
}