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法律学JavaScript 未定义的属性
JavaScript undefined 属性指定了变量是否已被声明或赋值。
以下是 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>输出
单击“单击此处”按钮后 -
更新于： 12-May-2020
118 次浏览
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