eslint/docs/rules/strict.md

   1 # require or disallow strict mode directives (strict)
   2
   3 A strict mode directive is a `"use strict"` literal at the beginning of a script or function body. It enables strict mode semantics.
   4
   5 When a directive occurs in global scope, strict mode applies to the entire script:
   6
   7 ```js
   8 "use strict";
   9
  10 // strict mode
  11
  12 function foo() {
  13     // strict mode
  14 }
  15 ```
  16
  17 When a directive occurs at the beginning of a function body, strict mode applies only to that function, including all contained functions:
  18
  19 ```js
  20 function foo() {
  21     "use strict";
  22     // strict mode
  23 }
  24
  25 function foo2() {
  26     // not strict mode
  27 };
  28
  29 (function() {
  30     "use strict";
  31     function bar() {
  32         // strict mode
  33     }
  34 }());
  35 ```
  36
  37 In the **CommonJS** module system, a hidden function wraps each module and limits the scope of a "global" strict mode directive.
  38
  39 In **ECMAScript** modules, which always have strict mode semantics, the directives are unnecessary.
  40
  41 ## Rule Details
  42
  43 This rule requires or disallows strict mode directives.
  44
  45 This rule disallows strict mode directives, no matter which option is specified, if ESLint configuration specifies either of the following as [parser options](/docs/user-guide/configuring/language-options.md#specifying-parser-options):
  46
  47 * `"sourceType": "module"` that is, files are **ECMAScript** modules
  48 * `"impliedStrict": true` property in the `ecmaFeatures` object
  49
  50 This rule disallows strict mode directives, no matter which option is specified, in functions with non-simple parameter lists (for example, parameter lists with default parameter values) because that is a syntax error in **ECMAScript 2016** and later. See the examples of the [function](#function) option.
  51
  52 The `--fix` option on the command line does not insert new `"use strict"` statements, but only removes unneeded statements.
  53
  54 ## Options
  55
  56 This rule has a string option:
  57
  58 * `"safe"` (default) corresponds either of the following options:
  59     * `"global"` if ESLint considers a file to be a **CommonJS** module
  60     * `"function"` otherwise
  61 * `"global"` requires one strict mode directive in the global scope (and disallows any other strict mode directives)
  62 * `"function"` requires one strict mode directive in each top-level function declaration or expression (and disallows any other strict mode directives)
  63 * `"never"` disallows strict mode directives
  64
  65 ### safe
  66
  67 The `"safe"` option corresponds to the `"global"` option if ESLint considers a file to be a **Node.js** or **CommonJS** module because the configuration specifies either of the following:
  68
  69 * `node` or `commonjs` [environments](/docs/user-guide/configuring/language-options.md#specifying-environments)
  70 * `"globalReturn": true` property in the `ecmaFeatures` object of [parser options](/docs/user-guide/configuring/language-options.md#specifying-parser-options)
  71
  72 Otherwise the `"safe"` option corresponds to the `"function"` option. Note that if `"globalReturn": false` is explicitly specified in the configuration, the `"safe"` option will correspond to the `"function"` option regardless of the specified environment.
  73
  74 ### global
  75
  76 Examples of **incorrect** code for this rule with the `"global"` option:
  77
  78 ```js
  79 /*eslint strict: ["error", "global"]*/
  80
  81 function foo() {
  82 }
  83 ```
  84
  85 ```js
  86 /*eslint strict: ["error", "global"]*/
  87
  88 function foo() {
  89     "use strict";
  90 }
  91 ```
  92
  93 ```js
  94 /*eslint strict: ["error", "global"]*/
  95
  96 "use strict";
  97
  98 function foo() {
  99     "use strict";
 100 }
 101 ```
 102
 103 Examples of **correct** code for this rule with the `"global"` option:
 104
 105 ```js
 106 /*eslint strict: ["error", "global"]*/
 107
 108 "use strict";
 109
 110 function foo() {
 111 }
 112 ```
 113
 114 ### function
 115
 116 This option ensures that all function bodies are strict mode code, while global code is not. Particularly if a build step concatenates multiple scripts, a strict mode directive in global code of one script could unintentionally enable strict mode in another script that was not intended to be strict code.
 117
 118 Examples of **incorrect** code for this rule with the `"function"` option:
 119
 120 ```js
 121 /*eslint strict: ["error", "function"]*/
 122
 123 "use strict";
 124
 125 function foo() {
 126 }
 127 ```
 128
 129 ```js
 130 /*eslint strict: ["error", "function"]*/
 131
 132 function foo() {
 133 }
 134
 135 (function() {
 136     function bar() {
 137         "use strict";
 138     }
 139 }());
 140 ```
 141
 142 ```js
 143 /*eslint strict: ["error", "function"]*/
 144 /*eslint-env es6*/
 145
 146 // Illegal "use strict" directive in function with non-simple parameter list.
 147 // This is a syntax error since ES2016.
 148 function foo(a = 1) {
 149     "use strict";
 150 }
 151
 152 // We cannot write "use strict" directive in this function.
 153 // So we have to wrap this function with a function with "use strict" directive.
 154 function foo(a = 1) {
 155 }
 156 ```
 157
 158 Examples of **correct** code for this rule with the `"function"` option:
 159
 160 ```js
 161 /*eslint strict: ["error", "function"]*/
 162
 163 function foo() {
 164     "use strict";
 165 }
 166
 167 (function() {
 168     "use strict";
 169
 170     function bar() {
 171     }
 172
 173     function baz(a = 1) {
 174     }
 175 }());
 176
 177 var foo = (function() {
 178     "use strict";
 179
 180     return function foo(a = 1) {
 181     };
 182 }());
 183 ```
 184
 185 ### never
 186
 187 Examples of **incorrect** code for this rule with the `"never"` option:
 188
 189 ```js
 190 /*eslint strict: ["error", "never"]*/
 191
 192 "use strict";
 193
 194 function foo() {
 195 }
 196 ```
 197
 198 ```js
 199 /*eslint strict: ["error", "never"]*/
 200
 201 function foo() {
 202     "use strict";
 203 }
 204 ```
 205
 206 Examples of **correct** code for this rule with the `"never"` option:
 207
 208 ```js
 209 /*eslint strict: ["error", "never"]*/
 210
 211 function foo() {
 212 }
 213 ```
 214
 215 ### earlier default (removed)
 216
 217 (removed) The default option (that is, no string option specified) for this rule was **removed** in ESLint v1.0. The `"function"` option is most similar to the removed option.
 218
 219 This option ensures that all functions are executed in strict mode. A strict mode directive must be present in global code or in every top-level function declaration or expression. It does not concern itself with unnecessary strict mode directives in nested functions that are already strict, nor with multiple strict mode directives at the same level.
 220
 221 Examples of **incorrect** code for this rule with the earlier default option which has been removed:
 222
 223 ```js
 224 // "strict": "error"
 225
 226 function foo() {
 227 }
 228 ```
 229
 230 ```js
 231 // "strict": "error"
 232
 233 (function() {
 234     function bar() {
 235         "use strict";
 236     }
 237 }());
 238 ```
 239
 240 Examples of **correct** code for this rule with the earlier default option which has been removed:
 241
 242 ```js
 243 // "strict": "error"
 244
 245 "use strict";
 246
 247 function foo() {
 248 }
 249 ```
 250
 251 ```js
 252 // "strict": "error"
 253
 254 function foo() {
 255     "use strict";
 256 }
 257 ```
 258
 259 ```js
 260 // "strict": "error"
 261
 262 (function() {
 263     "use strict";
 264     function bar() {
 265         "use strict";
 266     }
 267 }());
 268 ```
 269
 270 ## When Not To Use It
 271
 272 In a codebase that has both strict and non-strict code, either turn this rule off, or [selectively disable it](/docs/user-guide/configuring/rules.md#disabling-rules) where necessary. For example, functions referencing `arguments.callee` are invalid in strict mode. A [full list of strict mode differences](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode/Transitioning_to_strict_mode#Differences_from_non-strict_to_strict) is available on MDN.