JavaScript 中的 Undefined

JavaScript undefined 属性指定了是否已声明或赋值给变量一个值。

以下是实现 JavaScript undefined 属性的代码 −

示例

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>Document</title>
<style>
   body {
      font-family: "Segoe UI", Tahoma, Geneva, Verdana, sans-serif;
   }
   .sample {
      font-size: 18px;
      font-weight: 500;
      color: red;
   }
</style>
</head>
<body>
<h1>JavaScript undefined property</h1>
<div class="sample"></div>
<button class="Btn">CLICK HERE</button>
<h3>CLICK the above button to know if variable age has been defined or not</h3>
<script>
   let sampleEle = document.querySelector(".sample");
   let age;
   document.querySelector(".Btn").addEventListener("click", () => {
      if (age === undefined) {
         sampleEle.innerHTML =
         "The variable age has not been defined yet or no value is given to it";
      } else
         sampleEle.innerHTML =
         "The variable age is defined and its value = " + age;
   });
</script>
</body>
</html>