no-case-declarations

不允许在 case 子句中进行词法声明

✅

在 配置文件 中使用来自 @eslint/js 的 recommended 配置可以启用此规则

💡

此规则报告的一些问题可通过编辑器 建议 手动修复

该规则不允许在 case/default 子句中使用词法声明（let、const、function 和 class）。原因是词法声明在整个 switch 块中可见，但只有在赋值时才会初始化，而赋值只有在到达定义它的 case 时才会发生。

¥This rule disallows lexical declarations (let, const, function and class) in case/default clauses. The reason is that the lexical declaration is visible in the entire switch block but it only gets initialized when it is assigned, which will only happen if the case where it is defined is reached.

为确保词法声明仅适用于当前 case 子句，请将子句封装在块中。

¥To ensure that the lexical declaration only applies to the current case clause wrap your clauses in blocks.

规则详情

¥Rule Details

此规则旨在防止访问未初始化的词法绑定以及跨 case 子句访问提升的函数。

¥This rule aims to prevent access to uninitialized lexical bindings as well as accessing hoisted functions across case clauses.

此规则的错误代码示例：

¥Examples of incorrect code for this rule:

在线运行 
/*eslint no-case-declarations: "error"*/

switch (foo) {
    case 1:
        let x = 1;
        break;
    case 2:
        const y = 2;
        break;
    case 3:
        function f() {}
        break;
    default:
        class C {}
}

此规则的正确代码示例：

¥Examples of correct code for this rule:

在线运行 
/*eslint no-case-declarations: "error"*/

// Declarations outside switch-statements are valid
const a = 0;

switch (foo) {
    // The following case clauses are wrapped into blocks using brackets
    case 1: {
        let x = 1;
        break;
    }
    case 2: {
        const y = 2;
        break;
    }
    case 3: {
        function f() {}
        break;
    }
    case 4:
        // Declarations using var without brackets are valid due to function-scope hoisting
        var z = 4;
        break;
    default: {
        class C {}
    }
}

何时不使用

¥When Not To Use It

如果你依赖于 fall through 行为并希望访问 case 块中引入的绑定。

¥If you depend on fall through behavior and want access to bindings introduced in the case block.

版本

此规则是在 ESLint v1.9.0 中引入。

资源