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. The no-redeclare rule is now more strict by default
6. The comma-dangle rule is now more strict by default
7. The no-confusing-arrow rule is now more lenient by default
8. Overrides in a config file can now match dotfiles
9. The deprecated experimentalObjectRestSpread option has been removed

Breaking changes for plugin/custom rule developers

1. Plugin authors may need to update installation instructions
2. 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 2018, 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:

• Node.js 8 (8.10.0 and above)
• Node.js 10 (10.13.0 and above)
• Anything above Node.js 11.10.1

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

Note: This update is planned, but has not been implemented in the latest alpha release yet.

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

• no-async-promise-executor disallows using an async function as the argument to the Promise constructor, which is usually a bug.
• no-misleading-character-class reports character classes in regular expressions that might not behave as expected.
• no-shadow-restricted-names disallows shadowing variables like undefined (e.g. with code like let undefined = 5;), since is likely to confuse readers.
• no-useless-catch reports catch clauses that are redundant and can be removed from the code without changing its behavior.
• no-with disallows use of the with statement, which can make code difficult to understand and cause compatibility problems.
• require-atomic-updates reports race condition bugs that can occur when reassigning variables in async functions.

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

• 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).

Finally, in ESLint v5 eslint:recommended would explicitly disable all core rules that were not considered “recommended”. This could cause confusion 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 always resolves plugins relative to the end user’s project, 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.

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:

• 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.)
• 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.)
• Setting sourceType to anything other than "script" or "module" now causes an error to be thrown.

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

The no-redeclare rule is now more strict by default

Note: This update is planned, but has not been implemented in the latest alpha release yet.

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

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

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

The eslintExplicitGlobalComment scope analysis property has been removed

Note: This update is planned, but has not been implemented in the latest alpha release yet.

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