如何在JavaScript中以HH：MM：SS格式显示当前时间？

您可以使用本机函数：Date.toLocaleTimeString()

var d = new Date();
var n = d.toLocaleTimeString();

这将显示例如：

"11:33:01"

MDN： Date toLocaleTimeString

var d = new Date();
var n = d.toLocaleTimeString();
alert("The time is: \n"+n);

function checkTime(i) {
  if (i < 10) {
    i = "0" + i;
  }
  return i;
}

function startTime() {
  var today = new Date();
  var h = today.getHours();
  var m = today.getMinutes();
  var s = today.getSeconds();
  // add a zero in front of numbers<10
  m = checkTime(m);
  s = checkTime(s);
  document.getElementById('time').innerHTML = h + ":" + m + ":" + s;
  t = setTimeout(function() {
    startTime()
  }, 500);
}
startTime();

<div id="time"></div>

仅使用 javaScript 的演示

更新

Updated Demo

(function () {
    function checkTime(i) {
        return (i < 10) ? "0" + i : i;
    }

    function startTime() {
        var today = new Date(),
            h = checkTime(today.getHours()),
            m = checkTime(today.getMinutes()),
            s = checkTime(today.getSeconds());
        document.getElementById('time').innerHTML = h + ":" + m + ":" + s;
        t = setTimeout(function () {
            startTime()
        }, 500);
    }
    startTime();
})();