知识库

html
<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    #box {
      width: 300px;
      height: 100px;
      background: #ddd;
      text-align: center;
      line-height: 100px;
      color: black;
      font-size: 30px;
    }
  </style>
</head>
<body>
  <div id="box"></div>

  <script>
    // 从服务器服务器获取当前时间，应尽可能减小时间差。
    // 1.从响应头获取
    // 2.HEAD请求（只获取响应头信息）

    let currentTime,
        targetTime,
        timer

    const getTime = () => {
      return new Promise((resolve, reject) => {
        const xhr = new XMLHttpRequest()

        xhr.open('HEAD', 'http://localhost:3000/getTime')
        xhr.onreadystatechange = () => {
          const { readyState } = xhr

          if (readyState === 2) {
            // { 
            //   targetTime: '2021/10/07 20:00:00', 
            //   currentTime: '2021-10-07T11:04:58.696Z'
            // }
            const res = JSON.parse(xhr.getResponseHeader('X-Time'))

            currentTime = (new Date(res.currentTime)).getTime()
            targetTime = (new Date(res.targetTime)).getTime()

            if (currentTime && targetTime) {
              resolve()
            } else {
              reject()
            }
          }
        }
        xhr.send()
        xhr.onerror = (e) => {
          console.log(e)
        }
      })
    }

    const flow = () => {
      const res = targetTime - currentTime,
            hours = Math.floor(res / (1000 * 60 * 60)),
            minutes = Math.floor((res - hours * 60 * 60 * 1000) / (1000 * 60)),
            seconds = Math.floor(
              (res - hours * 60 * 60 * 1000 - minutes * 60 * 1000) / 1000
            )

      if (res <= 0) {
        box.innerText = `时间到~`
        clearInterval(timer)
        timer = null
      } else {
        box.innerText = `${ addZero(hours) }:${ addZero(minutes) }:${ addZero(seconds) }`
      }

      // 每次 +1s
      currentTime += 1000
    }

    function addZero (v) {
      v += ''
      
      return v.length === 2
        ? v
        : `0${ v }`
    }

    getTime().then(val => {
      flow()
      timer = setInterval(flow, 1000)
    })
  </script>
</body>
</html>