eslint/docs/src/rules/no-case-declarations.md

   1 ---
   2 title: no-case-declarations
   3 rule_type: suggestion
   4 related_rules:
   5 - no-fallthrough
   6 ---
   7
   8
   9
  10 This rule disallows lexical declarations (`let`, `const`, `function` and `class`)
  11 in `case`/`default` clauses. The reason is that the lexical declaration is visible
  12 in the entire switch block but it only gets initialized when it is assigned, which
  13 will only happen if the case where it is defined is reached.
  14
  15 To ensure that the lexical declaration only applies to the current case clause
  16 wrap your clauses in blocks.
  17
  18 ## Rule Details
  19
  20 This rule aims to prevent access to uninitialized lexical bindings as well as accessing hoisted functions across case clauses.
  21
  22 Examples of **incorrect** code for this rule:
  23
  24 ::: incorrect
  25
  26 ```js
  27 /*eslint no-case-declarations: "error"*/
  28 /*eslint-env es6*/
  29
  30 switch (foo) {
  31     case 1:
  32         let x = 1;
  33         break;
  34     case 2:
  35         const y = 2;
  36         break;
  37     case 3:
  38         function f() {}
  39         break;
  40     default:
  41         class C {}
  42 }
  43 ```
  44
  45 :::
  46
  47 Examples of **correct** code for this rule:
  48
  49 ::: correct
  50
  51 ```js
  52 /*eslint no-case-declarations: "error"*/
  53 /*eslint-env es6*/
  54
  55 // Declarations outside switch-statements are valid
  56 const a = 0;
  57
  58 switch (foo) {
  59     // The following case clauses are wrapped into blocks using brackets
  60     case 1: {
  61         let x = 1;
  62         break;
  63     }
  64     case 2: {
  65         const y = 2;
  66         break;
  67     }
  68     case 3: {
  69         function f() {}
  70         break;
  71     }
  72     case 4:
  73         // Declarations using var without brackets are valid due to function-scope hoisting
  74         var z = 4;
  75         break;
  76     default: {
  77         class C {}
  78     }
  79 }
  80 ```
  81
  82 :::
  83
  84 ## When Not To Use It
  85
  86 If you depend on fall through behavior and want access to bindings introduced in the case block.