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<!DOCTYPE html> <html> <head> <style> .mystyle { width: 500px; height: 50px; } .anotherClass { background-color: lightblue; } .thirdClass { text-align: center; font-size: 25px; color: black; margin-bottom: 10px; } </style> </head> <body> <p>单击该按钮可显示 div 元素具有的类名称的数量。</p> <div id="myDIV" class="mystyle anotherClass thirdClass"> 我是一个 DIV 元素 with three classes </div> <button onclick="myFunction()">试一试</button> <p><strong>注意：</strong> Internet Explorer 9 及更早版本不支持 classList 属性。</p> <p id="demo"></p> <script> function myFunction() { var x = document.getElementById("myDIV").classList.length; document.getElementById("demo").innerHTML = x; } </script> </body> </html>