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

   1 ---
   2 title: no-duplicate-case
   3 rule_type: problem
   4 ---
   5
   6
   7
   8 If a `switch` statement has duplicate test expressions in `case` clauses, it is likely that a programmer copied a `case` clause but forgot to change the test expression.
   9
  10 ## Rule Details
  11
  12 This rule disallows duplicate test expressions in `case` clauses of `switch` statements.
  13
  14 Examples of **incorrect** code for this rule:
  15
  16 :::incorrect
  17
  18 ```js
  19 /*eslint no-duplicate-case: "error"*/
  20
  21 var a = 1,
  22     one = 1;
  23
  24 switch (a) {
  25     case 1:
  26         break;
  27     case 2:
  28         break;
  29     case 1:         // duplicate test expression
  30         break;
  31     default:
  32         break;
  33 }
  34
  35 switch (a) {
  36     case one:
  37         break;
  38     case 2:
  39         break;
  40     case one:         // duplicate test expression
  41         break;
  42     default:
  43         break;
  44 }
  45
  46 switch (a) {
  47     case "1":
  48         break;
  49     case "2":
  50         break;
  51     case "1":         // duplicate test expression
  52         break;
  53     default:
  54         break;
  55 }
  56 ```
  57
  58 :::
  59
  60 Examples of **correct** code for this rule:
  61
  62 :::correct
  63
  64 ```js
  65 /*eslint no-duplicate-case: "error"*/
  66
  67 var a = 1,
  68     one = 1;
  69
  70 switch (a) {
  71     case 1:
  72         break;
  73     case 2:
  74         break;
  75     case 3:
  76         break;
  77     default:
  78         break;
  79 }
  80
  81 switch (a) {
  82     case one:
  83         break;
  84     case 2:
  85         break;
  86     case 3:
  87         break;
  88     default:
  89         break;
  90 }
  91
  92 switch (a) {
  93     case "1":
  94         break;
  95     case "2":
  96         break;
  97     case "3":
  98         break;
  99     default:
 100         break;
 101 }
 102 ```
 103
 104 :::
 105
 106 ## When Not To Use It
 107
 108 In rare cases where identical test expressions in `case` clauses produce different values, which necessarily means that the expressions are causing and relying on side effects, you will have to disable this rule.
 109
 110 ```js
 111 switch (a) {
 112     case i++:
 113         foo();
 114         break;
 115     case i++: // eslint-disable-line no-duplicate-case
 116         bar();
 117         break;
 118 }
 119 ```