<html><body><p>请单击按钮检查变量 y 是否已定义或未定义。</p><button onclick="myFunction()">试一试</button><p id="demo"></p><script>function myFunction() { if (typeof y === "undefined") { txt = "y is undefined"; } else { txt = "y is defined"; } document.getElementById("demo").innerHTML = txt;}</script></body></html>