eslint/docs/rules/no-implicit-coercion.md

   1 # Disallow the type conversion with shorter notations. (no-implicit-coercion)
   2
   3 In JavaScript, there are a lot of different ways to convert value types.
   4 Some of them might be hard to read and understand.
   5
   6 Such as:
   7
   8 ```js
   9 var b = !!foo;
  10 var b = ~foo.indexOf(".");
  11 var n = +foo;
  12 var n = 1 * foo;
  13 var s = "" + foo;
  14 foo += ``;
  15 ```
  16
  17 Those can be replaced with the following code:
  18
  19 ```js
  20 var b = Boolean(foo);
  21 var b = foo.indexOf(".") !== -1;
  22 var n = Number(foo);
  23 var n = Number(foo);
  24 var s = String(foo);
  25 foo = String(foo);
  26 ```
  27
  28 ## Rule Details
  29
  30 This rule is aimed to flag shorter notations for the type conversion, then suggest a more self-explanatory notation.
  31
  32 ## Options
  33
  34 This rule has three main options and one override option to allow some coercions as required.
  35
  36 -   `"boolean"` (`true` by default) - When this is `true`, this rule warns shorter type conversions for `boolean` type.
  37 -   `"number"` (`true` by default) - When this is `true`, this rule warns shorter type conversions for `number` type.
  38 -   `"string"` (`true` by default) - When this is `true`, this rule warns shorter type conversions for `string` type.
  39 -   `"disallowTemplateShorthand"` (`false` by default) - When this is `true`, this rule warns `string` type conversions using `${expression}` form.
  40 -   `"allow"` (`empty` by default) - Each entry in this array can be one of `~`, `!!`, `+` or `*` that are to be allowed.
  41
  42 Note that operator `+` in `allow` list would allow `+foo` (number coercion) as well as `"" + foo` (string coercion).
  43
  44 ### boolean
  45
  46 Examples of **incorrect** code for the default `{ "boolean": true }` option:
  47
  48 ```js
  49 /*eslint no-implicit-coercion: "error"*/
  50
  51 var b = !!foo;
  52 var b = ~foo.indexOf(".");
  53 // bitwise not is incorrect only with `indexOf`/`lastIndexOf` method calling.
  54 ```
  55
  56 Examples of **correct** code for the default `{ "boolean": true }` option:
  57
  58 ```js
  59 /*eslint no-implicit-coercion: "error"*/
  60
  61 var b = Boolean(foo);
  62 var b = foo.indexOf(".") !== -1;
  63
  64 var n = ~foo; // This is a just bitwise not.
  65 ```
  66
  67 ### number
  68
  69 Examples of **incorrect** code for the default `{ "number": true }` option:
  70
  71 ```js
  72 /*eslint no-implicit-coercion: "error"*/
  73
  74 var n = +foo;
  75 var n = 1 * foo;
  76 ```
  77
  78 Examples of **correct** code for the default `{ "number": true }` option:
  79
  80 ```js
  81 /*eslint no-implicit-coercion: "error"*/
  82
  83 var n = Number(foo);
  84 var n = parseFloat(foo);
  85 var n = parseInt(foo, 10);
  86 ```
  87
  88 ### string
  89
  90 Examples of **incorrect** code for the default `{ "string": true }` option:
  91
  92 ```js
  93 /*eslint no-implicit-coercion: "error"*/
  94
  95 var s = "" + foo;
  96 var s = `` + foo;
  97 foo += "";
  98 foo += ``;
  99 ```
 100
 101 Examples of **correct** code for the default `{ "string": true }` option:
 102
 103 ```js
 104 /*eslint no-implicit-coercion: "error"*/
 105
 106 var s = String(foo);
 107 foo = String(foo);
 108 ```
 109
 110 ### disallowTemplateShorthand
 111
 112 This option is **not** affected by the `string` option.
 113
 114 Examples of **incorrect** code for the `{ "disallowTemplateShorthand": true }` option:
 115
 116 ```js
 117 /*eslint no-implicit-coercion: ["error", { "disallowTemplateShorthand": true }]*/
 118
 119 var s = `${foo}`;
 120 ```
 121
 122 Examples of **correct** code for the `{ "disallowTemplateShorthand": true }` option:
 123
 124 ```js
 125 /*eslint no-implicit-coercion: ["error", { "disallowTemplateShorthand": true }]*/
 126
 127 var s = String(foo);
 128
 129 var s = `a${foo}`;
 130
 131 var s = `${foo}b`;
 132
 133 var s = `${foo}${bar}`;
 134
 135 var s = tag`${foo}`;
 136 ```
 137
 138 Examples of **correct** code for the default `{ "disallowTemplateShorthand": false }` option:
 139
 140 ```js
 141 /*eslint no-implicit-coercion: ["error", { "disallowTemplateShorthand": false }]*/
 142
 143 var s = `${foo}`;
 144 ```
 145
 146 ### allow
 147
 148 Using `allow` list, we can override and allow specific operators.
 149
 150 Examples of **correct** code for the sample `{ "allow": ["!!", "~"] }` option:
 151
 152 ```js
 153 /*eslint no-implicit-coercion: [2, { "allow": ["!!", "~"] } ]*/
 154
 155 var b = !!foo;
 156 var b = ~foo.indexOf(".");
 157 ```
 158
 159 ## When Not To Use It
 160
 161 If you don't want to be notified about shorter notations for the type conversion, you can safely disable this rule.