javaScript计算li的个数，结果为啥不是4

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
pageEncoding="ISO-8859-1"%>
<!DOCTYPE html>



Insert title here




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<br><br> window.onload = function() {<br> var u = document.getElementsByID(&quot;ufo&quot;);<br> var linum = u.getElementsByTagName(&#39;li&#39;).length;<br> document.write(linum);<br> }<br>

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
pageEncoding=""ISO-8859-1"%>
<!DOCTYPE html>

计算

1

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1

window.onload=function(){ var q = document.getElementsByName("number"); var n= q.length; document.write(n); }
<html>
  <head>
    <meta charset="utf-8" />
  </head>
  <body>
    <ul>
      <li>一月</li>
      <li>二月</li>
      <li>三月</li>
      <li>四月</li>
    </ul>
    <button id="btn" onclick="getLength(this)">获取li个数</button>

    <script type="text/javascript">
      function getLength() {
        var len = document.getElementsByTagName('li').length;
        console.log('li的个数：' + len);
      }
    </script>
  </body>
</html>

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