利用js实现全选，全不选，以及数字选择功能
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>无标题文档</title>
</head>
<body>
<form>
请选择你的必修课:<br>
<input type="checkbox" name="hobby" id="hobby1"> PHP
<input type="checkbox" name="hobby" id="hobby2"> Mysql
<input type="checkbox" name="hobby" id="hobby3"> Div
<input type="checkbox" name="hobby" id="hobby4"> HTML
<input type="checkbox" name="hobby" id="hobby5"> C语言
<input type="checkbox" name="hobby" id="hobby6"> 面向对象 <br>
<input type="button" value = "全选" onclick = "checkall();">
<input type="button" value = "全不选" onclick = "clearall();">
<p>请输入您要选择必修课的序号，序号为1-6:</p>
<input id="wb" name="wb" type="text" >
<input name="ok" type="button" value="确定" onclick = "checkone();">
</form>
<script type="text/javascript">
function checkall(){
var hobby = document.getElementsByTagName("input");
for(var i=0;i<hobby.length;i++)
{
hobby[i].checked=true;
}
// 任务1
}
function clearall(){
var hobby = document.getElementsByName("hobby");
for(var i=0;i<hobby.length;i++)
{
hobby[i].checked=false;
}
// 任务2
}
function checkone(){
var j=document.getElementById("wb").value;
if(parseInt(j)>6||parseInt(j)<1)
{
alert("请输入1-6之间的数字");
}
// 任务3
var hobby=document.getElementsByName("hobby");
hobby[parseInt(j)-1].checked=true;
}
</script>
</body>
</html>