html运行实例（jsck_undefined

代码示例：（标识：jsck_undefined_2）

<!DOCTYPE html>
<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>