eslint/docs/src/rules/no-extra-boolean-cast.md

   1 ---
   2 title: no-extra-boolean-cast
   3 rule_type: suggestion
   4 ---
   5
   6
   7
   8
   9
  10 In contexts such as an `if` statement's test where the result of the expression will already be coerced to a Boolean, casting to a Boolean via double negation (`!!`) or a `Boolean` call is unnecessary. For example, these `if` statements are equivalent:
  11
  12 ```js
  13 if (!!foo) {
  14     // ...
  15 }
  16
  17 if (Boolean(foo)) {
  18     // ...
  19 }
  20
  21 if (foo) {
  22     // ...
  23 }
  24 ```
  25
  26 ## Rule Details
  27
  28 This rule disallows unnecessary boolean casts.
  29
  30 Examples of **incorrect** code for this rule:
  31
  32 ::: incorrect
  33
  34 ```js
  35 /*eslint no-extra-boolean-cast: "error"*/
  36
  37 var foo = !!!bar;
  38
  39 var foo = !!bar ? baz : bat;
  40
  41 var foo = Boolean(!!bar);
  42
  43 var foo = new Boolean(!!bar);
  44
  45 if (!!foo) {
  46     // ...
  47 }
  48
  49 if (Boolean(foo)) {
  50     // ...
  51 }
  52
  53 while (!!foo) {
  54     // ...
  55 }
  56
  57 do {
  58     // ...
  59 } while (Boolean(foo));
  60
  61 for (; !!foo; ) {
  62     // ...
  63 }
  64 ```
  65
  66 :::
  67
  68 Examples of **correct** code for this rule:
  69
  70 ::: correct
  71
  72 ```js
  73 /*eslint no-extra-boolean-cast: "error"*/
  74
  75 var foo = !!bar;
  76 var foo = Boolean(bar);
  77
  78 function foo() {
  79     return !!bar;
  80 }
  81
  82 var foo = bar ? !!baz : !!bat;
  83 ```
  84
  85 :::
  86
  87 ## Options
  88
  89 This rule has an object option:
  90
  91 * `"enforceForLogicalOperands"` when set to `true`, in addition to checking default contexts, checks whether the extra boolean cast is contained within a logical expression. Default is `false`, meaning that this rule by default does not warn about extra booleans cast inside logical expression.
  92
  93 ### enforceForLogicalOperands
  94
  95 Examples of **incorrect** code for this rule with `"enforceForLogicalOperands"` option set to `true`:
  96
  97 ::: incorrect
  98
  99 ```js
 100 /*eslint no-extra-boolean-cast: ["error", {"enforceForLogicalOperands": true}]*/
 101
 102 if (!!foo || bar) {
 103     //...
 104 }
 105
 106 while (!!foo && bar) {
 107     //...
 108 }
 109
 110 if ((!!foo || bar) && baz) {
 111     //...
 112 }
 113
 114 foo && Boolean(bar) ? baz : bat
 115
 116 var foo = new Boolean(!!bar || baz)
 117 ```
 118
 119 :::
 120
 121 Examples of **correct** code for this rule with `"enforceForLogicalOperands"` option set to `true`:
 122
 123 ::: correct
 124
 125 ```js
 126 /*eslint no-extra-boolean-cast: ["error", {"enforceForLogicalOperands": true}]*/
 127
 128 if (foo || bar) {
 129     //...
 130 }
 131
 132 while (foo && bar) {
 133     //...
 134 }
 135
 136 if ((foo || bar) && baz) {
 137     //...
 138 }
 139
 140 foo && bar ? baz : bat
 141
 142 var foo = new Boolean(bar || baz)
 143
 144 var foo = !!bar || baz;
 145 ```
 146
 147 :::