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hobby[parseInt(j)-1].checked=true; 求問解釋

<!DOCTYPE HTML>
<html>
??? <head>
??????? <meta http-equiv="Content-Type" content="text/html; charset=gb2312">
??????? <title>無標題文檔</title>
??? </head>
?? ?
??? <body>
??????? <form>
????????? 請選擇你愛好:<br>
????????? <input type="checkbox" name="hobby" id="hobby1">? 音樂
????????? <input type="checkbox" name="hobby" id="hobby2">? 登山
????????? <input type="checkbox" name="hobby" id="hobby3">? 游泳
????????? <input type="checkbox" name="hobby" id="hobby4">? 閱讀
????????? <input type="checkbox" name="hobby" id="hobby5">? 打球
????????? <input type="checkbox" name="hobby" id="hobby6">? 跑步 <br>
????????? <input type="button" value = "全選" onclick = "checkall();">
????????? <input type="button" value = "全不選" onclick = "clearall();">
????????? <p>請輸入您要選擇愛好的序號，序號為1-6:</p>
????????? <input id="wb" name="wb" type="text" >
????????? <input name="ok" type="button" value="確定" onclick = "checkone();">
??????? </form>
??????? <script type="text/javascript">
??????? function checkall(){
??????????? var hobby = document.getElementsByTagName("input");
?????????? for(var i=0;i<bobby.length;i++){
?????????????? hobby[i].checked=true;
?????????? }
??????? }
????? ?
??????? function clearall(){
??????????? var hobby = document.getElementsByName("hobby");
?????????? ?
???????? for(var i=0;i<hobby.length;i++){
???????????? hobby[i].checked=false;
???????? }// 任務2?????? ?
??????? }
?????? ?
??????? function checkone(){
??????????? var j=document.getElementById("wb").value;
?????? ?
???????? for(var i=0;i<hobby.length;i++)// 任務3
????????? if(parseInt(j) > 6 || parseInt(j)? < 1? ){
??????????????????? alert("不在此范圍內(nèi)，請重新輸入!")
??????????????????? // document.write(j);
??????????????? }
??????????? }
??????????? var hobby = document.getElementsByName("hobby");??? ?
??????????? hobby[parseInt(j)-1].checked=true;
??????? }
?????? ?
??????? </script>
??? </body>
</html>