eslint/docs/rules/no-dupe-else-if.md

   1 # Disallow duplicate conditions in `if-else-if` chains (no-dupe-else-if)
   2
   3 `if-else-if` chains are commonly used when there is a need to execute only one branch (or at most one branch) out of several possible branches, based on certain conditions.
   4
   5 ```js
   6 if (a) {
   7     foo();
   8 } else if (b) {
   9     bar();
  10 } else if (c) {
  11     baz();
  12 }
  13 ```
  14
  15 Two identical test conditions in the same chain are almost always a mistake in the code. Unless there are side effects in the expressions, a duplicate will evaluate to the same `true` or `false` value as the identical expression earlier in the chain, meaning that its branch can never execute.
  16
  17 ```js
  18 if (a) {
  19     foo();
  20 } else if (b) {
  21     bar();
  22 } else if (b) {
  23     baz();
  24 }
  25 ```
  26
  27 In the above example, `baz()` can never execute. Obviously, `baz()` could be executed only when `b` evaluates to `true`, but in that case `bar()` would be executed instead, since it's earlier in the chain.
  28
  29 ## Rule Details
  30
  31 This rule disallows duplicate conditions in the same `if-else-if` chain.
  32
  33 Examples of **incorrect** code for this rule:
  34
  35 ```js
  36 /*eslint no-dupe-else-if: "error"*/
  37
  38 if (isSomething(x)) {
  39     foo();
  40 } else if (isSomething(x)) {
  41     bar();
  42 }
  43
  44 if (a) {
  45     foo();
  46 } else if (b) {
  47     bar();
  48 } else if (c && d) {
  49     baz();
  50 } else if (c && d) {
  51     quux();
  52 } else {
  53     quuux();
  54 }
  55
  56 if (n === 1) {
  57     foo();
  58 } else if (n === 2) {
  59     bar();
  60 } else if (n === 3) {
  61     baz();
  62 } else if (n === 2) {
  63     quux();
  64 } else if (n === 5) {
  65     quuux();
  66 }
  67 ```
  68
  69 Examples of **correct** code for this rule:
  70
  71 ```js
  72 /*eslint no-dupe-else-if: "error"*/
  73
  74 if (isSomething(x)) {
  75     foo();
  76 } else if (isSomethingElse(x)) {
  77     bar();
  78 }
  79
  80 if (a) {
  81     foo();
  82 } else if (b) {
  83     bar();
  84 } else if (c && d) {
  85     baz();
  86 } else if (c && e) {
  87     quux();
  88 } else {
  89     quuux();
  90 }
  91
  92 if (n === 1) {
  93     foo();
  94 } else if (n === 2) {
  95     bar();
  96 } else if (n === 3) {
  97     baz();
  98 } else if (n === 4) {
  99     quux();
 100 } else if (n === 5) {
 101     quuux();
 102 }
 103 ```
 104
 105 This rule can also detect some cases where the conditions are not identical, but the branch can never execute due to the logic of `||` and `&&` operators.
 106
 107 Examples of additional **incorrect** code for this rule:
 108
 109 ```js
 110 /*eslint no-dupe-else-if: "error"*/
 111
 112 if (a || b) {
 113     foo();
 114 } else if (a) {
 115     bar();
 116 }
 117
 118 if (a) {
 119     foo();
 120 } else if (b) {
 121     bar();
 122 } else if (a || b) {
 123     baz();
 124 }
 125
 126 if (a) {
 127     foo();
 128 } else if (a && b) {
 129     bar();
 130 }
 131
 132 if (a && b) {
 133     foo();
 134 } else if (a && b && c) {
 135     bar();
 136 }
 137
 138 if (a || b) {
 139     foo();
 140 } else if (b && c) {
 141     bar();
 142 }
 143
 144 if (a) {
 145     foo();
 146 } else if (b && c) {
 147     bar();
 148 } else if (d && (c && e && b || a)) {
 149     baz();
 150 }
 151 ```
 152
 153 Please note that this rule does not compare conditions from the chain with conditions inside statements, and will not warn in the cases such as follows:
 154
 155 ```js
 156 if (a) {
 157     if (a) {
 158         foo();
 159     }
 160 }
 161
 162 if (a) {
 163     foo();
 164 } else {
 165     if (a) {
 166         bar();
 167     }
 168 }
 169 ```
 170
 171 ## When Not To Use It
 172
 173 In rare cases where you really need identical test conditions in the same chain, which necessarily means that the expressions in the chain are causing and relying on side effects, you will have to turn this rule off.
 174
 175 ## Related Rules
 176
 177 * [no-duplicate-case](no-duplicate-case.md)
 178 * [no-lonely-if](no-lonely-if.md)