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JavaScript --函数作用域变量的使用规则（局部和访问）

news 2025/2/25 18:29:14

访问规则，就近原则

<!DOCTYPE html>
<html lang="en">
<head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title>
</head>
<body><script>// 全局变量100var num = 100function test1(){var num = 200function child(){var num = 300console.log(num)}child()}test1()</script>
</body>
</html>

就近原则 num 等于100

<script>// 全局变量100var num = 100function test1(){var num = 200function child(){// var num = 300console.log(num)}child()}test1()</script>

num等于300的注释掉，往上找，所以num等于200

num=300和num = 200注释掉，在往上找，所以num等于100

 <script>// 全局变量100var num = 100function test1(){// var num = 200function child(){// var num = 300console.log(num)}child()}test1()</script>

赋值规则，就近原则

<!DOCTYPE html>
<html lang="en">
<head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title>
</head>
<body><script>// 全局变量100// num值不改变var num = 100function test1(){// num值不改变var num = 200function child(){var num = 300// 只改变最近的num的值num = 400console.log(num)}child()}test1()</script>
</body>
</html>

<body><script>// 全局变量100// num值不改变var num = 100function test1(){// num值变成400var num = 200function child(){// var num = 300// 只改变最近的num的值num = 400console.log(num)}console.log("修改前",num)child()console.log("修改后",num)}test1()</script>

特殊情况

赋值的时候往上找找到最外层都没找到，就把这个num=400变成全局变量

<script>// 全局变量100// num值不改变// var num = 100function test1(){// num值变成400// var num = 200function child(){// var num = 300// 只改变最近的num的值num = 400console.log(num)}// console.log("修改前",num)child()console.log("修改后",num)}test1()</script>

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