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

   1 ---
   2 title: no-duplicate-case
   3 layout: doc
   4 rule_type: problem
   5 ---
   6
   7
   8
   9 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.
  10
  11 ## Rule Details
  12
  13 This rule disallows duplicate test expressions in `case` clauses of `switch` statements.
  14
  15 Examples of **incorrect** code for this rule:
  16
  17 :::incorrect
  18
  19 ```js
  20 /*eslint no-duplicate-case: "error"*/
  21
  22 var a = 1,
  23     one = 1;
  24
  25 switch (a) {
  26     case 1:
  27         break;
  28     case 2:
  29         break;
  30     case 1:         // duplicate test expression
  31         break;
  32     default:
  33         break;
  34 }
  35
  36 switch (a) {
  37     case one:
  38         break;
  39     case 2:
  40         break;
  41     case one:         // duplicate test expression
  42         break;
  43     default:
  44         break;
  45 }
  46
  47 switch (a) {
  48     case "1":
  49         break;
  50     case "2":
  51         break;
  52     case "1":         // duplicate test expression
  53         break;
  54     default:
  55         break;
  56 }
  57 ```
  58
  59 :::
  60
  61 Examples of **correct** code for this rule:
  62
  63 :::correct
  64
  65 ```js
  66 /*eslint no-duplicate-case: "error"*/
  67
  68 var a = 1,
  69     one = 1;
  70
  71 switch (a) {
  72     case 1:
  73         break;
  74     case 2:
  75         break;
  76     case 3:
  77         break;
  78     default:
  79         break;
  80 }
  81
  82 switch (a) {
  83     case one:
  84         break;
  85     case 2:
  86         break;
  87     case 3:
  88         break;
  89     default:
  90         break;
  91 }
  92
  93 switch (a) {
  94     case "1":
  95         break;
  96     case "2":
  97         break;
  98     case "3":
  99         break;
 100     default:
 101         break;
 102 }
 103 ```
 104
 105 :::
 106
 107 ## When Not To Use It
 108
 109 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.
 110
 111 ```js
 112 switch (a) {
 113     case i++:
 114         foo();
 115         break;
 116     case i++: // eslint-disable-line no-duplicate-case
 117         bar();
 118         break;
 119 }
 120 ```