Migrating to v6.0.0

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.

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.

Breaking changes for users

  1. Node.js 6 is no longer supported
  2. eslint:recommended has been updated
  3. Plugins and shareable configs are no longer affected by ESLint’s location
  4. The default parser now validates options more strictly
  5. Rule configuration are validated more strictly
  6. The no-redeclare rule is now more strict by default
  7. The comma-dangle rule is now more strict by default
  8. The no-confusing-arrow rule is now more lenient by default
  9. Overrides in a config file can now match dotfiles
  10. Overrides in an extended config file can now be overridden by a parent config file
  11. Configuration values for globals are now validated
  12. The deprecated experimentalObjectRestSpread option has been removed
  13. User-provided regular expressions in rule options are parsed with the unicode flag

Breaking changes for plugin/custom rule developers

  1. Plugin authors may need to update installation instructions
  2. RuleTester now validates against invalid default keywords in rule schemas
  3. RuleTester now requires an absolute path on parser option
  4. The eslintExplicitGlobalComment scope analysis property has been removed

Breaking changes for integration developers

  1. Plugins and shareable configs are no longer affected by ESLint’s location
  2. Linter no longer tries to load missing parsers from the filesystem

Node.js 6 is no longer supported

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:

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.

Related issue(s): eslint/eslint#11546

eslint:recommended has been updated

The following rules have been added to the eslint:recommended config:

Additionally, the following rule has been removed from eslint:recommended:

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.

To address: To mimic the eslint:recommended behavior from 5.x, you can explicitly disable/enable rules in a config file as follows:

{
  "extends": "eslint:recommended",

  "rules": {
    "no-async-promise-executor": "off",
    "no-misleading-character-class": "off",
    "no-prototype-builtins": "off",
    "no-shadow-restricted-names": "off",
    "no-useless-catch": "off",
    "no-with": "off",
    "require-atomic-updates": "off",

    "no-console": "error"
  }
}

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.

Related issue(s): eslint/eslint#10768, eslint/eslint#10873

Plugins and shareable configs are no longer affected by ESLint’s location

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 and Yarn Plug n’ Play.

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.

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.

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 flag to the location of the config file.

Related issue(s): eslint/eslint#10125, eslint/rfcs#7

The default parser now validates options more strictly

espree, the default parser used by ESLint, will now raise an error in the following cases:

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.

Related issue(s): eslint/eslint#9687, eslint/espree#384

Rule configuration are validated more strictly

To catch config errors earlier, ESLint v6 will report a linting error if you are trying to configure a non-existent rule.

config ESLint v5 ESLint v6
/*eslint-enable foo*/ no error linting error
/*eslint-disable(-line) foo*/ no error linting error
/*eslint foo: 0*/ no error linting error
{rules: {foo: 0}} no error no error
{rules: {foo: 1} linting warning linting error

To address: You can remove the non-existent rule in your (inline) config.

Related issue(s): eslint/eslint#9505

The no-redeclare rule is now more strict by default

The default options for the 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.

To address:

To restore the previous options for the rule, you can configure it as follows:

{
  "rules": {
    "no-redeclare": ["error", { "builtinGlobals": false }]
  }
}

Additionally, if you see new errors for global comments in your code, you should remove those comments.

Related issue(s): eslint/eslint#11370, eslint/eslint#11405

The comma-dangle rule is now more strict by default

Previously, the 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.

To address: You can restore the previous default behavior of the rule with:

{
  "rules": {
    "comma-dangle": ["error", {
        "arrays": "never",
        "objects": "never",
        "imports": "never",
        "exports": "never",
        "functions": "ignore"
    }]
  }
}

To restore the previous behavior of a string option like "always-multiline", replace "never" with "always-multiline" in the example above.

Related issue(s): eslint/eslint#11502

The no-confusing-arrow rule is now more lenient by default

The default options for the no-confusing-arrow rule have changed from { allowParens: false } to { allowParens: true }.

To address: You can restore the previous default behavior of the rule with:

{
  "rules": {
    "no-confusing-arrow": ["error", { "allowParens": false }]
  }
}

Related issue(s): eslint/eslint#11503

Overrides in a config file can now match dotfiles

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.

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 for more details.

Related issue(s): eslint/eslint#11201

Overrides in an extended config file can now be overridden by a parent config file

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:

// .eslintrc.js
module.exports = {
  extends: ["foo"],
  rules: {
    semi: "off"
  }
};
// eslint-config-foo/index.js
module.exports = {
  overrides: {
    files: ["*.js"],
    rules: {
      semi: "error"
    }
  }
};

In ESLint v6.0.0, a parent config always has precedence over extended configs, even with overrides blocks.

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.)

Related issue(s): eslint/eslint#11510

Configuration values for globals are now validated

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".

// .eslintrc.js
module.exports = {
  globals: {
    foo: "readonly",
    bar: "writable",
    baz: "hello!" // ???
  }
};

With this change, any unknown values in a globals object result in a config validation error.

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.)

The depreacted experimentalObjectRestSpread option has been removed

Previously, when using the default parser, a config could use the experimentalObjectRestSpread option to enable parsing support for object rest/spread properties:

{
  "parserOptions": {
    "ecmaFeatures": {
      "experimentalObjectRestSpread": true
    }
  }
}

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.

To address: If you use the experimentalObjectRestSpread option, you should change your config to contain this instead:

{
  "parserOptions": {
    "ecmaVersion": 2018
  }
}

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.

Related issue(s): eslint/eslint#9990

User-provided regular expressions in rule options are parsed with the unicode flag

Rules like 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, which should exhibit more reasonable behavior when matching characters like astral symbols. Unicode regexes also validate escape sequences more strictly than non-unicode regexes.

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.

Related issue(s): eslint/eslint#11423


Plugin authors may need to update installation instructions

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. In particular, if your plugin was generated with the generator-eslint package, it likely contains outdated instructions for how to use the plugin with global ESLint installations.

Related issue(s): eslint/rfcs#7

RuleTester now validates against invalid default keywords in rule schemas

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.

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.)

Related issue(s): eslint/eslint#11473

RuleTester now requires an absolute path on parser option

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.

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.

Related issue(s): eslint/eslint#11728, eslint/eslint#10125, eslint/rfcs#7

The eslintExplicitGlobalComment scope analysis property has been removed

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.

To address: If you maintain a rule that uses the eslintExplicitGlobalComment property, update it to use the eslintExplicitGlobalComments property as a list instead.

Related issue(s): eslint/rfcs#17


Linter no longer tries to load missing parsers from the filesystem

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.

In ESLint v6, Linter will no longer perform any filesystem operations, including loading parsers.

To address: If you’re using Linter with a custom parser, use Linter#defineParser to explicitly define the parser before linting any code.

Related issue(s): eslint/rfcs#7