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先上代码 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www./TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www./1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=gb2312" /> <title>级联特效</title> </head> <script type="text/javascript"> function change(){ var classes=document.getElementById("classes"); var t_class=classes.value; var people=document.getElementById("people"); //此处如果连写4个people.options.length=0;代码就正常了 //people.options.length=0; //people.options.length=0; //people.options.length=0; people.options.length=0; alert("options.length值为:"+people.options.length); switch(t_class){ case "三班": people.add(new Option("小王","小王"),null); people.add(new Option("小李","小李"),null); people.add(new Option("小红","小红"),null); people.add(new Option("小军","小军"),null); people.add(new Option("小王","小王"),null); people.add(new Option("小李","小李"),null); people.add(new Option("小红","小红"),null); people.add(new Option("小军","小军"),null); break; case "六班": people.add(new Option("老王","老王"),null); people.add(new Option("老李","老李"),null); people.add(new Option("老红","老红"),null); people.add(new Option("老军","老军"),null); people.add(new Option("老王","老王"),null); people.add(new Option("老李","老李"),null); people.add(new Option("老红","老红"),null); people.add(new Option("老军","老军"),null); break; } } </script> <body> <form> <select id="classes" onchange="change()"> <option>--选择班级--</option> <option value="三班">三班</option> <option value="六班">六班</option> </select> <select id="people"> <option>--选择人物--</option> </select> </form> </body> </html> 问题在于people.options.length=0;这句话 按道理people.options.length=0;是直接清除了下拉选项, 但是alert后发现,people.options.length=0;只是把下来选项除以2了 也就是说先选择3班,然偶从3班选到6班的时候,会发现people.options.length=0;这句话只是把3班一半的人去掉了,仍然有另一半在六班 而再写一个people.options.length=0;就会剩下1/4,写三个people.options.length=0;,就剩下1/8,写4个people.options.length=0;代码就正常了 想知道options.length=0;这句代码为什么会出现除以2的情况 |
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来自： 昵称27831725 > 《JavaScript》
