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<!DOCTYPE html> <html> <body> <p>单击按钮以测试变量是否未定义。</p> <button onclick="myFunction()">试一试</button> <p id="demo"></p> <script> function myFunction() { var x; if (typeof x === "undefined") { txt = "x is undefined"; } else { txt = "x is defined"; } document.getElementById("demo").innerHTML = txt; } </script> </body> </html>