js sleep函数<meta name="viewport" content="width=device-width,

<!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>
</head>
<body>
  <script>
    //第一种方案(阻塞程序运行)
    function sleep(delay) {
      var start = (new Date()).getTime();
      while ((new Date()).getTime() - start < delay) {
        continue;
      }
    }

    console.log('start')
    sleep(3000)
    console.log('end')


    //第二种方案（不阻塞）
    let timeout = (ms) => {
      return new Promise(resolve => {
        setTimeout(() => resolve(), ms)
      })
    }

    let test = async () => {
      console.log('start2')
      await timeout(3000)
      console.log('end2')
    }
    
    test()

    //第三种方案（不阻塞）
    console.log('start3')
    new Promise(resolve => {
      setTimeout(() => resolve(), 3000)
    }).then(res => {
      console.log('end3')
    })

  </script>
</body>
</html>