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How does jquery determine whether a checkbox is selected? (Source code)

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Release: 2018-10-29 14:24:40
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How does jquery determine checkbox? I believe that many friends who have just come into contact with jquery will have such questions. This chapter will introduce to you how jquery determines whether a checkbox is selected. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

Everyone knows that in html, if a checkbox is checked, it is checked="checked". But if we use jquery alert($("#id").attr("checked")), it will prompt you that it is true instead of checked. Therefore, many friends judge that if($("#id").attr("checked")=="true") is wrong. In fact, it should be if($("#id").attr("checked") ==true).

The example includes the following functions:

<input type="button" id="btn1" value="全选">
<input type="button" id="btn2" value="取消全选">
<input type="button" id="btn3" value="选中所有奇数">
<input type="button" id="btn4" value="反选">
<input type="button" id="btn5" value="获得选中的所有值">
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Code:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
    <TITLE> New Document </TITLE>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
        <SCRIPT LANGUAGE="JavaScript" src="http://www.cnjquery.com/demo/jquery.js"></script>
        <SCRIPT LANGUAGE="JavaScript">
           $("document").ready(function(){
              $("#btn1").click(function(){
                  $("[name=&#39;checkbox&#39;]").attr("checked",&#39;true&#39;);//全选
   
              });
              $("#btn2").click(function(){
                  $("[name=&#39;checkbox&#39;]").removeAttr("checked");//取消全选
   
              });
              $("#btn3").click(function(){
                  $("[name=&#39;checkbox&#39;]:even").attr("checked",&#39;true&#39;);//选中所有奇数
   
              })
              $("#btn4").click(function(){
                  $("[name=&#39;checkbox&#39;]").each(function(){
                  if($(this).attr("checked"))
                  {
                      $(this).removeAttr("checked");
     
                  }
                  else
                  {
                      $(this).attr("checked",&#39;true&#39;);
     
                  }
              })
          })
               $("#btn5").click(function(){
              var str="";
              $("[name=&#39;checkbox&#39;][checked]").each(function(){
               str+=$(this).val()+""r"n";
             //alert($(this).val());
              })
             alert(str);
              })
   })
  </SCRIPT>
 </HEAD>
 <BODY>
 <form name="form1" method="post" action="">
   <input type="button" id="btn1" value="全选">
   <input type="button" id="btn2" value="取消全选">
   <input type="button" id="btn3" value="选中所有奇数">
   <input type="button" id="btn4" value="反选">
   <input type="button" id="btn5" value="获得选中的所有值">
   <br>
   <input type="checkbox" name="checkbox" value="checkbox1">
   checkbox1
   <input type="checkbox" name="checkbox" value="checkbox2">
   checkbox2
   <input type="checkbox" name="checkbox" value="checkbox3">
   checkbox3
   <input type="checkbox" name="checkbox" value="checkbox4">
   checkbox4
   <input type="checkbox" name="checkbox" value="checkbox5">
   checkbox5
   <input type="checkbox" name="checkbox" value="checkbox6">
   checkbox6
   <input type="checkbox" name="checkbox" value="checkbox7">
   checkbox7
   <input type="checkbox" name="checkbox" value="checkbox8">
 checkbox8
 </form>
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