​x
 
<!DOCTYPE html>
<html>
<body>
​
<p>请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>
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<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>
​