如何验证 JavaScript 变量是否已加载？

要验证一个 JavaScript 变量是否已加载，我们可以通过对其未定义或为空值的情况进行比较来检查。我们也可以使用 typeof()，因为它总会返回字符串来了解变量是否已加载。

以下是验证 JavaScript 变量是否已加载的代码 −

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;
   }
   .result {
      font-size: 20px;
      font-weight: 500;
   }
</style>
</head>
<body>
<h1>Check if a variable is loaded or not</h1>
<div style="color: green;" class="result"></div>
<button class="Btn">CLICK HERE</button>
<h3>
Click on the above button to see if the object obj has been loaded or not
</h3>
<script>
   let resEle = document.querySelector(".result");
   let BtnEle = document.querySelector(".Btn");
   let obj;
   BtnEle.addEventListener("click", () => {
      if (obj === null || obj === undefined) {
         resEle.innerHTML = "The object obj hasn't loaded yet";
         obj = { firstName: "Rohan", lastName: "Sharma" };
      }
      else {
         resEle.innerHTML = "The object obj has been loaded";
      }
   });
</script>
</body>
</html>