我希望浏览器显示自上次刷新以来的秒数.我不明白为什么Code 1不起作用; 守则2; 如果代码1不起作用,为什么代码3有效？该setInterval呼叫是在码1和码3的方式类似定义的函数是不同的.但我不清楚为什么会有所作为.
非常感谢您的帮助.我刚开始学习JavaScript.
代码1
<html>
<head>
<title>Time Since Refresh</title>
</head>
<body>
<br/>
<br/>
<span id="timeSinceStart"></span>
<script language="JavaScript">
var timeRefreshed = new Date();
function displayTimeSinceStart(){
var now = new Date();
//compute elapsed time
var elapsed = Math.round((now - timeRefreshed)/1000);
document.getElementById("timeSinceStart").innerHTML = "Time Elapsed: " + elapsed + " seconds.";
}
// Update seconds counter
setInterval(displayTimeSinceStart(), 1000);
</script>
</body>
</html>
代码2
与CODE 1相同,但setInterval()函数写为
setInterval("displayTimeSinceStart();", 1000);
代码3
<html>
<head>
<title>Time Since Refresh</title>
</head>
<body>
<br/>
<br/>
<span id="timeSinceStart"></span>
<script language="JavaScript">
var timeRefreshed = new Date();
var timeSinceStart = {
displayTimeSinceStart: function(){
var now = new Date();
//compute elapsed time
var elapsed = Math.round((now - timeRefreshed)/1000);
document.getElementById("timeSinceStart").innerHTML = "Time Elapsed: " + elapsed + " seconds.";
}
}
// Update seconds counter
setInterval(timeSinceStart.displayTimeSinceStart, 1000);
</script>
</body>
</html>
代码1 调用 displayTimeSinceStart(因为"()")而不是传递对它的引用:setInterval获取该函数的返回值(undefined).删除括号以修复.
代码2传递一个字符串以setInterval进行评估:由于您希望在间隔超时时调用该方法,因此需要填充.
代码3传递一个参考,相当于代码1没有"()",所以它的工作原理.
setInterval 期望函数引用(首选)或要评估的字符串.
更多细节(包括为什么有时将方法调用作为参数setTimeout才有意义).