eslint/docs/src/use/migrating-to-6.0.0.md

   1 ---
   2 title: Migrating to v6.0.0
   3
   4 ---
   5
   6 ESLint v6.0.0 is a major release of ESLint. We have made a few breaking changes in this release. This guide is intended to walk you through the breaking changes.
   7
   8 The lists below are ordered roughly by the number of users each change is expected to affect, where the first items are expected to affect the most users.
   9
  10 ## Breaking changes for users
  11
  12 1. [Node.js 6 is no longer supported](#drop-node-6)
  13 1. [`eslint:recommended` has been updated](#eslint-recommended-changes)
  14 1. [Plugins and shareable configs are no longer affected by ESLint's location](#package-loading-simplification)
  15 1. [The default parser now validates options more strictly](#espree-validation)
  16 1. [Rule configuration are validated more strictly](#rule-config-validating)
  17 1. [The `no-redeclare` rule is now more strict by default](#no-redeclare-updates)
  18 1. [The `comma-dangle` rule is now more strict by default](#comma-dangle-updates)
  19 1. [The `no-confusing-arrow` rule is now more lenient by default](#no-confusing-arrow-updates)
  20 1. [Overrides in a config file can now match dotfiles](#overrides-dotfiles)
  21 1. [Overrides in an extended config file can now be overridden by a parent config file](#overrides-precedence)
  22 1. [Configuration values for globals are now validated](#globals-validation)
  23 1. [The deprecated `experimentalObjectRestSpread` option has been removed](#experimental-object-rest-spread)
  24 1. [User-provided regular expressions in rule options are parsed with the unicode flag](#unicode-regexes)
  25
  26 ## Breaking changes for plugin/custom rule developers
  27
  28 1. [Plugin authors may need to update installation instructions](#plugin-documentation)
  29 1. [`RuleTester` now validates against invalid `default` keywords in rule schemas](#rule-tester-defaults)
  30 1. [`RuleTester` now requires an absolute path on `parser` option](#rule-tester-parser)
  31 1. [The `eslintExplicitGlobalComment` scope analysis property has been removed](#eslintExplicitGlobalComment)
  32
  33 ## Breaking changes for integration developers
  34
  35 1. [Plugins and shareable configs are no longer affected by ESLint's location](#package-loading-simplification)
  36 1. [`Linter` no longer tries to load missing parsers from the filesystem](#linter-parsers)
  37
  38 ---
  39
  40 ## <a name="drop-node-6"></a> Node.js 6 is no longer supported
  41
  42 As of April 2019, Node.js 6 will be at EOL and will no longer be receiving security updates. As a result, we have decided to drop support for it in ESLint v6. We now support the following versions of Node.js:
  43
  44 * Node.js 8 (8.10.0 and above)
  45 * Node.js 10 (10.13.0 and above)
  46 * Anything above Node.js 11.10.1
  47
  48 **To address:** Make sure you upgrade to at least Node.js 8 when using ESLint v6. If you are unable to upgrade, we recommend continuing to use ESLint v5.x until you are able to upgrade Node.js.
  49
  50 **Related issue(s):** [eslint/eslint#11546](https://github.com/eslint/eslint/issues/11456)
  51
  52 ## <a name="eslint-recommended-changes"></a> `eslint:recommended` has been updated
  53
  54 The following rules have been added to the [`eslint:recommended`](../use/configure#using-eslintrecommended) config:
  55
  56 * [`no-async-promise-executor`](../rules/no-async-promise-executor) disallows using an `async` function as the argument to the `Promise` constructor, which is usually a bug.
  57 * [`no-misleading-character-class`](../rules/no-misleading-character-class) reports character classes in regular expressions that might not behave as expected.
  58 * [`no-prototype-builtins`](../rules/no-prototype-builtins) reports method calls like `foo.hasOwnProperty("bar")` (which are a frequent source of bugs), and suggests that they be replaced with `Object.prototype.hasOwnProperty.call(foo, "bar")` instead.
  59 * [`no-shadow-restricted-names`](../rules/no-shadow-restricted-names) disallows shadowing variables like `undefined` (e.g. with code like `let undefined = 5;`), since is likely to confuse readers.
  60 * [`no-useless-catch`](../rules/no-useless-catch) reports `catch` clauses that are redundant and can be removed from the code without changing its behavior.
  61 * [`no-with`](../rules/no-with) disallows use of the [`with` statement](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/with), which can make code difficult to understand and cause compatibility problems.
  62 * [`require-atomic-updates`](../rules/require-atomic-updates) reports race condition bugs that can occur when reassigning variables in async functions.
  63
  64 Additionally, the following rule has been *removed* from `eslint:recommended`:
  65
  66 * [`no-console`](../rules/no-console) disallows calling functions like `console.log`. While this rule is useful in many cases (e.g. to avoid inadvertently leaving debugging statements in production code), it is not as broadly applicable as the other rules in `eslint:recommended`, and it was a source of false positives in cases where `console.log` is acceptable (e.g. in CLI applications).
  67
  68 Finally, in ESLint v5 `eslint:recommended` would explicitly disable all core rules that were not considered "recommended". This could cause confusing behavior if `eslint:recommended` was loaded after another config, since `eslint:recommended` would have the effect of turning off some rules. In ESLint v6, `eslint:recommended` has no effect on non-recommended rules.
  69
  70 **To address:** To mimic the `eslint:recommended` behavior from 5.x, you can explicitly disable/enable rules in a config file as follows:
  71
  72 ```json
  73 {
  74   "extends": "eslint:recommended",
  75
  76   "rules": {
  77     "no-async-promise-executor": "off",
  78     "no-misleading-character-class": "off",
  79     "no-prototype-builtins": "off",
  80     "no-shadow-restricted-names": "off",
  81     "no-useless-catch": "off",
  82     "no-with": "off",
  83     "require-atomic-updates": "off",
  84
  85     "no-console": "error"
  86   }
  87 }
  88 ```
  89
  90 In rare cases (if you were relying on the previous behavior where `eslint:recommended` disables core rules), you might need to disable additional rules to restore the previous behavior.
  91
  92 **Related issue(s):** [eslint/eslint#10768](https://github.com/eslint/eslint/issues/10768), [eslint/eslint#10873](https://github.com/eslint/eslint/issues/10873)
  93
  94 ## <a name="package-loading-simplification"></a> Plugins and shareable configs are no longer affected by ESLint's location
  95
  96 Previously, ESLint loaded plugins relative to the location of the ESLint package itself. As a result, we suggested that users with global ESLint installations should also install plugins globally, and users with local ESLint installations should install plugins locally. However, due to a design bug, this strategy caused ESLint to randomly fail to load plugins and shareable configs under certain circumstances, particularly when using package management tools like [`lerna`](https://github.com/lerna/lerna) and [Yarn Plug n' Play](https://yarnpkg.com/lang/en/docs/pnp/).
  97
  98 As a rule of thumb: With ESLint v6, plugins should always be installed locally, even if ESLint was installed globally. More precisely, ESLint v6 resolves plugins relative to the end user's project by default, and always resolves shareable configs and parsers relative to the location of the config file that imports them.
  99
 100 **To address:** If you use a global installation of ESLint (e.g. installed with `npm install eslint --global`) along with plugins, you should install those plugins locally in the projects where you run ESLint. If your config file extends shareable configs and/or parsers, you should ensure that those packages are installed as dependencies of the project containing the config file.
 101
 102 If you use a config file located outside of a local project (with the `--config` flag), consider installing the plugins as dependencies of that config file, and setting the [`--resolve-plugins-relative-to`](./command-line-interface#--resolve-plugins-relative-to) flag to the location of the config file.
 103
 104 **Related issue(s):** [eslint/eslint#10125](https://github.com/eslint/eslint/issues/10125), [eslint/rfcs#7](https://github.com/eslint/rfcs/pull/7)
 105
 106 ## <a name="espree-validation"></a> The default parser now validates options more strictly
 107
 108 `espree`, the default parser used by ESLint, will now raise an error in the following cases:
 109
 110 * The `ecmaVersion` parser option is set to something other than a number, such as the string `"2015"`. (Previously, a non-number option would simply be ignored.)
 111 * The `sourceType: "module"` parser option is set while `ecmaVersion` is set to `5` or left unspecified. (Previously, setting `sourceType: "module"` would implicitly cause `ecmaVersion` to be set to a minimum of 2015, which could be surprising.)
 112 * The `sourceType` is set to anything other than `"script"` or `"module"`.
 113
 114 **To address:** If your config sets `ecmaVersion` to something other than a number, you can restore the previous behavior by removing `ecmaVersion`. (However, you may want to double-check that your config is actually working as expected.) If your config sets `parserOptions: { sourceType: "module" }` without also setting `parserOptions.ecmaVersion`, you should add `parserOptions: { ecmaVersion: 2015 }` to restore the previous behavior.
 115
 116 **Related issue(s):** [eslint/eslint#9687](https://github.com/eslint/eslint/issues/9687), [eslint/espree#384](https://github.com/eslint/espree/issues/384)
 117
 118 ## <a name="rule-config-validating"></a> Rule configuration are validated more strictly
 119
 120 To catch config errors earlier, ESLint v6 will report a linting error if you are trying to configure a non-existent rule.
 121
 122 config | ESLint v5 | ESLint v6
 123 ------------- | ------------- | -------------
 124 `/*eslint-enable foo*/`  | no error | linting error
 125 `/*eslint-disable(-line) foo*/`  | no error | linting error
 126 `/*eslint foo: 0*/` | no error | linting error
 127 `{rules: {foo: 0}}` | no error | no error
 128 `{rules: {foo: 1}` | linting warning | linting error
 129
 130 **To address:** You can remove the non-existent rule in your (inline) config.
 131
 132 **Related issue(s):** [eslint/eslint#9505](https://github.com/eslint/eslint/issues/9505)
 133
 134 ## <a name="no-redeclare-updates"></a> The `no-redeclare` rule is now more strict by default
 135
 136 The default options for the [`no-redeclare`](../rules/no-redeclare) rule have changed from `{ builtinGlobals: false }` to `{ builtinGlobals: true }`. Additionally, the `no-redeclare` rule will now report an error for globals enabled by comments like `/* global foo */` if those globals were already enabled through configuration anyway.
 137
 138 **To address:**
 139
 140 To restore the previous options for the rule, you can configure it as follows:
 141
 142 ```json
 143 {
 144   "rules": {
 145     "no-redeclare": ["error", { "builtinGlobals": false }]
 146   }
 147 }
 148 ```
 149
 150 Additionally, if you see new errors for `global` comments in your code, you should remove those comments.
 151
 152 **Related issue(s):** [eslint/eslint#11370](https://github.com/eslint/eslint/issues/11370), [eslint/eslint#11405](https://github.com/eslint/eslint/issues/11405)
 153
 154 ## <a name="comma-dangle-updates"></a> The `comma-dangle` rule is now more strict by default
 155
 156 Previously, the [`comma-dangle`](../rules/comma-dangle) rule would ignore trailing function arguments and parameters, unless explicitly configured to check for function commas. In ESLint v6, function commas are treated the same way as other types of trailing commas.
 157
 158 **To address:** You can restore the previous default behavior of the rule with:
 159
 160 ```json
 161 {
 162   "rules": {
 163     "comma-dangle": ["error", {
 164         "arrays": "never",
 165         "objects": "never",
 166         "imports": "never",
 167         "exports": "never",
 168         "functions": "ignore"
 169     }]
 170   }
 171 }
 172 ```
 173
 174 To restore the previous behavior of a string option like `"always-multiline"`, replace `"never"` with `"always-multiline"` in the example above.
 175
 176 **Related issue(s):** [eslint/eslint#11502](https://github.com/eslint/eslint/issues/11502)
 177
 178 ## <a name="no-confusing-arrow-updates"></a> The `no-confusing-arrow` rule is now more lenient by default
 179
 180 The default options for the [`no-confusing-arrow`](../rules/no-confusing-arrow) rule have changed from `{ allowParens: false }` to `{ allowParens: true }`.
 181
 182 **To address:** You can restore the previous default behavior of the rule with:
 183
 184 ```json
 185 {
 186   "rules": {
 187     "no-confusing-arrow": ["error", { "allowParens": false }]
 188   }
 189 }
 190 ```
 191
 192 **Related issue(s):** [eslint/eslint#11503](https://github.com/eslint/eslint/issues/11503)
 193
 194 ## <a name="overrides-dotfiles"></a> Overrides in a config file can now match dotfiles
 195
 196 Due to a bug, the glob patterns in a `files` list in an `overrides` section of a config file would never match dotfiles, making it impossible to have overrides apply to files starting with a dot. This bug has been fixed in ESLint v6.
 197
 198 **To address:** If you don't want dotfiles to be matched by an override, consider adding something like `excludedFiles: [".*"]` to that `overrides` section. See the [documentation](../use/configure#configuration-based-on-glob-patterns) for more details.
 199
 200 **Related issue(s):** [eslint/eslint#11201](https://github.com/eslint/eslint/issues/11201)
 201
 202 ## <a name="overrides-precedence"></a> Overrides in an extended config file can now be overridden by a parent config file
 203
 204 Due to a bug, it was previously the case that an `overrides` block in a shareable config had precedence over the top level of a parent config. For example, with the following config setup, the `semi` rule would end up enabled even though it was explicitly disabled in the end user's config:
 205
 206 ```js
 207 // .eslintrc.js
 208 module.exports = {
 209   extends: ["foo"],
 210   rules: {
 211     semi: "off"
 212   }
 213 };
 214 ```
 215
 216 ```js
 217 // eslint-config-foo/index.js
 218 module.exports = {
 219   overrides: {
 220     files: ["*.js"],
 221     rules: {
 222       semi: "error"
 223     }
 224   }
 225 };
 226 ```
 227
 228 In ESLint v6.0.0, a parent config always has precedence over extended configs, even with `overrides` blocks.
 229
 230 **To address:** We expect the impact of this issue to be very low because most shareable configs don't use `overrides` blocks. However, if you use a shareable config with `overrides` blocks, you might encounter a change in behavior due to something that is explicitly specified in your config but was inactive until now. If you would rather inherit the behavior from the shareable config, simply remove the corresponding entry from your own config. (In the example above, the previous behavior could be restored by removing `semi: "off"` from `.eslintrc.js`.)
 231
 232 **Related issue(s):** [eslint/eslint#11510](https://github.com/eslint/eslint/issues/11510)
 233
 234 ## <a name="globals-validation"></a> Configuration values for globals are now validated
 235
 236 Previously, when configuring a set of global variables with an object, it was possible to use anything as the values of the object. An unknown value would be treated the same as `"writable"`.
 237
 238 ```js
 239 // .eslintrc.js
 240 module.exports = {
 241   globals: {
 242     foo: "readonly",
 243     bar: "writable",
 244     baz: "hello!" // ???
 245   }
 246 };
 247 ```
 248
 249 With this change, any unknown values in a `globals` object result in a config validation error.
 250
 251 **To address:** If you see config validation errors related to globals after updating, ensure that all values configured for globals are either `readonly`, `writable`, or `off`. (ESLint also accepts some alternate spellings and variants for compatibility.)
 252
 253 ## <a name="experimental-object-rest-spread"></a> The deprecated `experimentalObjectRestSpread` option has been removed
 254
 255 Previously, when using the default parser, a config could use the `experimentalObjectRestSpread` option to enable parsing support for object rest/spread properties:
 256
 257 ```json
 258 {
 259   "parserOptions": {
 260     "ecmaFeatures": {
 261       "experimentalObjectRestSpread": true
 262     }
 263   }
 264 }
 265 ```
 266
 267 Since ESLint v5, `ecmaFeatures: { experimentalObjectRestSpread: true }` has been equivalent to `ecmaVersion: 2018`, and has also emitted a deprecation warning. In ESLint v6, the `experimentalObjectRestSpread` feature has been removed entirely and has no effect. If your config was relying on `experimentalObjectRestSpread` to enable ES2018 parsing, you might start seeing parsing errors for recent syntax.
 268
 269 **To address:** If you use the `experimentalObjectRestSpread` option, you should change your config to contain this instead:
 270
 271 ```json
 272 {
 273   "parserOptions": {
 274     "ecmaVersion": 2018
 275   }
 276 }
 277 ```
 278
 279 If you're not sure which config file needs to be updated, it may be useful to run ESLint v5 and look at what config file is mentioned in the deprecation warning.
 280
 281 **Related issue(s):** [eslint/eslint#9990](https://github.com/eslint/eslint/issues/9990)
 282
 283 ## <a name="unicode-regexes"></a> User-provided regular expressions in rule options are parsed with the unicode flag
 284
 285 Rules like [`max-len`](../rules/max-len) accept a string option which is interpreted as a regular expression. In ESLint v6.0.0, these regular expressions are interpreted with the [unicode flag](https://mathiasbynens.be/notes/es6-unicode-regex), which should exhibit more reasonable behavior when matching characters like astral symbols. Unicode regexes also validate escape sequences more strictly than non-unicode regexes.
 286
 287 **To address:** If you get rule option validation errors after upgrading, ensure that any regular expressions in your rule options have no invalid escape sequences.
 288
 289 **Related issue(s):** [eslint/eslint#11423](https://github.com/eslint/eslint/issues/11423)
 290
 291 ---
 292
 293 ## <a name="plugin-documentation"></a> Plugin authors may need to update installation instructions
 294
 295 If you maintain a plugin and provide installation instructions, you should ensure that the installation instructions are up to date with the [user-facing changes to how plugins are loaded](#package-loading-simplification). In particular, if your plugin was generated with the [`generator-eslint`](https://github.com/eslint/generator-eslint) package, it likely contains outdated instructions for how to use the plugin with global ESLint installations.
 296
 297 **Related issue(s):** [eslint/rfcs#7](https://github.com/eslint/rfcs/pull/7)
 298
 299 ## <a name="rule-tester-defaults"></a> `RuleTester` now validates against invalid `default` keywords in rule schemas
 300
 301 In some cases, rule schemas can use the `default` keyword to automatically specify default values for rule options. However, the `default` keyword is only effective in certain schema locations, and is ignored elsewhere, which creates a risk of bugs if a rule incorrectly expects a default value to be provided as a rule option. In ESLint v6.0.0, `RuleTester` will raise an error if a rule has an invalid `default` keyword in its schema.
 302
 303 **To address:** If `RuleTester` starts reporting an error about an invalid default, you can remove the `default` property at the indicated location in your rule schema, and the rule will behave the same way. (If this happens, you might also want to verify that the rule behaves correctly when no option value is provided in that location.)
 304
 305 **Related issue(s):** [eslint/eslint#11473](https://github.com/eslint/eslint/issues/11473)
 306
 307 ## <a name="rule-tester-parser"></a> `RuleTester` now requires an absolute path on `parser` option
 308
 309 To use custom parsers in tests, we could use `parser` property with a package name or file path. However, if a package name was given, it's unclear where the tester should load the parser package from because the tester doesn't know which files are running the tester. In ESLint v6.0.0, `RuleTester` disallows `parser` property with a package name.
 310
 311 **To address:** If you use `parser` property with package names in test cases, update it with `require.resolve()` function to resolve the package name to the absolute path to the package.
 312
 313 **Related issue(s):** [eslint/eslint#11728](https://github.com/eslint/eslint/issues/11728), [eslint/eslint#10125](https://github.com/eslint/eslint/issues/10125), [eslint/rfcs#7](https://github.com/eslint/rfcs/pull/7)
 314
 315 ## <a name="eslintExplicitGlobalComment"></a> The `eslintExplicitGlobalComment` scope analysis property has been removed
 316
 317 Previously, ESLint would add an `eslintExplicitGlobalComment` property to `Variable` objects in scope analysis to indicate that a variable was introduced as a result of a `/* global */` comment. This property was undocumented, and the ESLint team was unable to find any usage of the property outside of ESLint core. The property has been removed in ESLint v6, and replaced with the `eslintExplicitGlobalComments` property, which can contain a list of all `/* global */` comments if a variable was declared with more than one of them.
 318
 319 **To address:** If you maintain a rule that uses the `eslintExplicitGlobalComment` property, update it to use the `eslintExplicitGlobalComments` property as a list instead.
 320
 321 **Related issue(s):** [eslint/rfcs#17](https://github.com/eslint/rfcs/pull/17)
 322
 323 ---
 324
 325 ## <a name="linter-parsers"></a> `Linter` no longer tries to load missing parsers from the filesystem
 326
 327 Previously, when linting code with a parser that had not been previously defined, the `Linter` API would attempt to load the parser from the filesystem. However, this behavior was confusing because `Linter` never access the filesystem in any other cases, and it was difficult to ensure that the correct parser would be found when loading the parser from the filesystem.
 328
 329 In ESLint v6, `Linter` will no longer perform any filesystem operations, including loading parsers.
 330
 331 **To address:** If you're using `Linter` with a custom parser, use [`Linter#defineParser`](../integrate/nodejs-api#linterdefineparser) to explicitly define the parser before linting any code.
 332
 333 **Related issue(s):** [eslint/rfcs#7](https://github.com/eslint/rfcs/pull/7)