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<!DOCTYPE html> <html> <body> <p>单击该按钮返回字符串“Hello World!”中的字符数。</p> <button onclick="myFunction()">试一试</button> <p id="demo"></p> <script> function myFunction() { var str = "Hello World!"; var n = str.length; document.getElementById("demo").innerHTML = n; } </script> </body> </html>