什么时候变量的值为 undefined
- 当一个变量声明了，但是没有赋值时，则其值默认为 undefined
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<head>
<meta charset="UTF-8">
<title>Document</title>
</style>
</style>
</head>
<body>
<script type="text/javascript">
var a;
console.log(a);
</script>
</body>
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- 一个变量声明了，并且赋值为 undefined
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
</style>
</style>
</head>
<body>
<script type="text/javascript">
var a = undefined;
console.log(a);
</script>
</body>
</html>
- 获取一个对象中某个不存在的属性时，该属性的值为 undefined
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
</style>
</style>
</head>
<body>
<script type="text/javascript">
var a = {};
console.log(a.age);
</script>
</body>
</html>
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