JavaScript undefined 속성은 변수가 선언되었거나 아직 값이 할당되었는지 여부를 지정합니다.
다음은 JavaScript 정의되지 않은 속성에 대한 코드입니다 -
예시
<!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> 출력
'여기를 클릭' 버튼을 클릭하면 -
