红绿灯问题

1. callback方法

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

function lightToggle(fn, timer, callback) {
      setTimeout(() => {
        if (fn === 'red') {
          red()
        }
        else if (fn === 'green') {
          green()
        }
        else if (fn === 'yellow') {
          yellow()
        }
        callback()
      }, timer)
    }
    function a() {
      lightToggle("red", 3000, b)
    }
    function b() {
      lightToggle("yellow", 2000, c)
    }
    function c() {
      lightToggle("green", 1000, a)
    }
    a()

2. promise方法

    // 使用promise实现
    function task(light, delay) {
      return new Promise((resolve, reject) => {
        setTimeout(() => {
          if (light === 'red') {
            red()
          }
          else if (light === 'green') {
            green()
          }
          else if (light === 'yellow') {
            yellow()
          }
          resolve()
        }, delay)
      })
    }
    async function task_all() {
      await task('red', 3000)
      await task('yellow', 2000)
      await task('green', 1000)
      task_all()
    }
    task_all()
    // const step = () => {
    //   task('red', 3000)
    //     .then(() => task('yellow', 2000))
    //     .then(() => task('green', 1000))
    //     .then(step)
    // }
    // step()