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Configuration Files

You can put your ESLint project configuration in a configuration file. You can include built-in rules, how you want them enforced, plugins with custom rules, shareable configurations, which files you want rules to apply to, and more.

Configuration File

The ESLint configuration file may be named any of the following:

  • eslint.config.js
  • eslint.config.mjs
  • eslint.config.cjs

It should be placed in the root directory of your project and export an array of configuration objects. Here’s an example:

// eslint.config.js
export default [
    {
        rules: {
            semi: "error",
            "prefer-const": "error"
        }
    }
];

In this example, the configuration array contains just one configuration object. The configuration object enables two rules: semi and prefer-const. These rules are applied to all of the files ESLint processes using this config file.

If your project does not specify "type":"module" in its package.json file, then eslint.config.js must be in CommonJS format, such as:

// eslint.config.js
module.exports = [
    {
        rules: {
            semi: "error",
            "prefer-const": "error"
        }
    }
];

Configuration Objects

Each configuration object contains all of the information ESLint needs to execute on a set of files. Each configuration object is made up of these properties:

  • files - An array of glob patterns indicating the files that the configuration object should apply to. If not specified, the configuration object applies to all files matched by any other configuration object.
  • ignores - An array of glob patterns indicating the files that the configuration object should not apply to. If not specified, the configuration object applies to all files matched by files.
  • languageOptions - An object containing settings related to how JavaScript is configured for linting.
    • ecmaVersion - The version of ECMAScript to support. May be any year (i.e., 2022) or version (i.e., 5). Set to "latest" for the most recent supported version. (default: "latest")
    • sourceType - The type of JavaScript source code. Possible values are "script" for traditional script files, "module" for ECMAScript modules (ESM), and "commonjs" for CommonJS files. (default: "module" for .js and .mjs files; "commonjs" for .cjs files)
    • globals - An object specifying additional objects that should be added to the global scope during linting.
    • parser - An object containing a parse() method or a parseForESLint() method. (default: espree)
    • parserOptions - An object specifying additional options that are passed directly to the parse() or parseForESLint() method on the parser. The available options are parser-dependent.
  • linterOptions - An object containing settings related to the linting process.
    • noInlineConfig - A Boolean value indicating if inline configuration is allowed.
    • reportUnusedDisableDirectives - A severity string indicating if and how unused disable and enable directives should be tracked and reported. For legacy compatibility, true is equivalent to "warn" and false is equivalent to "off". (default: "warn").
  • processor - Either an object containing preprocess() and postprocess() methods or a string indicating the name of a processor inside of a plugin (i.e., "pluginName/processorName").
  • plugins - An object containing a name-value mapping of plugin names to plugin objects. When files is specified, these plugins are only available to the matching files.
  • rules - An object containing the configured rules. When files or ignores are specified, these rule configurations are only available to the matching files.
  • settings - An object containing name-value pairs of information that should be available to all rules.

Specifying files and ignores

You can use a combination of files and ignores to determine which files should apply the configuration object and which should not. By default, ESLint matches **/*.js, **/*.cjs, and **/*.mjs. Because config objects that don’t specify files or ignores apply to all files that have been matched by any other configuration object, those config objects apply to any JavaScript files passed to ESLint by default. For example:

// eslint.config.js
export default [
    {
        rules: {
            semi: "error"
        }
    }
];

With this configuration, the semi rule is enabled for all files that match the default files in ESLint. So if you pass example.js to ESLint, the semi rule is applied. If you pass a non-JavaScript file, like example.txt, the semi rule is not applied because there are no other configuration objects that match that filename. (ESLint outputs an error message letting you know that the file was ignored due to missing configuration.)

Excluding files with ignores

You can limit which files a configuration object applies to by specifying a combination of files and ignores patterns. For example, you may want certain rules to apply only to files in your src directory:

// eslint.config.js
export default [
    {
        files: ["src/**/*.js"],
        rules: {
            semi: "error"
        }
    }
];

Here, only the JavaScript files in the src directory have the semi rule applied. If you run ESLint on files in another directory, this configuration object is skipped. By adding ignores, you can also remove some of the files in src from this configuration object:

export default [
    {
        files: ["src/**/*.js"],
        ignores: ["**/*.config.js"],
        rules: {
            semi: "error"
        }
    }
];

This configuration object matches all JavaScript files in the src directory except those that end with .config.js. You can also use negation patterns in ignores to exclude files from the ignore patterns, such as:

export default [
    {
        files: ["src/**/*.js"],
        ignores: ["**/*.config.js", "!**/eslint.config.js"],
        rules: {
            semi: "error"
        }
    }
];

Here, the configuration object excludes files ending with .config.js except for eslint.config.js. That file still has semi applied.

Non-global ignores patterns can only match file names. A pattern like "dir-to-exclude/" will not ignore anything. To ignore everything in a particular directory, a pattern like "dir-to-exclude/**" should be used instead.

If ignores is used without files and there are other keys (such as rules), then the configuration object applies to all files except the ones specified in ignores, for example:

export default [
    {
        ignores: ["**/*.config.js"],
        rules: {
            semi: "error"
        }
    }
];

This configuration object applies to all files except those ending with .config.js. Effectively, this is like having files set to **/*. In general, it’s a good idea to always include files if you are specifying ignores.

Globally ignoring files with ignores

If ignores is used without any other keys in the configuration object, then the patterns act as global ignores. Here’s an example:

// eslint.config.js
export default [
    {
        ignores: [".config/*"]
    }
];

This configuration specifies that all of the files in the .config directory should be ignored. This pattern is added after the default patterns, which are ["**/node_modules/", ".git/"].

For more information on configuring rules, see Ignore Files.

Cascading Configuration Objects

When more than one configuration object matches a given filename, the configuration objects are merged with later objects overriding previous objects when there is a conflict. For example:

// eslint.config.js
export default [
    {
        files: ["**/*.js"],
        languageOptions: {
            globals: {
                MY_CUSTOM_GLOBAL: "readonly"
            }
        }
    },
    {
        files: ["tests/**/*.js"],
        languageOptions: {
            globals: {
                it: "readonly",
                describe: "readonly"
            }
        }
    }
];

Using this configuration, all JavaScript files define a custom global object defined called MY_CUSTOM_GLOBAL while those JavaScript files in the tests directory have it and describe defined as global objects in addition to MY_CUSTOM_GLOBAL. For any JavaScript file in the tests directory, both configuration objects are applied, so languageOptions.globals are merged to create a final result.

Configuring Linter Options

Options specific to the linting process can be configured using the linterOptions object. These effect how linting proceeds and does not affect how the source code of the file is interpreted.

Disabling Inline Configuration

Inline configuration is implemented using an /*eslint*/ comment, such as /*eslint semi: error*/. You can disallow inline configuration by setting noInlineConfig to true. When enabled, all inline configuration is ignored. Here’s an example:

// eslint.config.js
export default [
    {
        files: ["**/*.js"],
        linterOptions: {
            noInlineConfig: true
        }
    }
];

Reporting Unused Disable Directives

Disable and enable directives such as /*eslint-disable*/, /*eslint-enable*/ and /*eslint-disable-next-line*/ are used to disable ESLint rules around certain portions of code. As code changes, it’s possible for these directives to no longer be needed because the code has changed in such a way that the rule is no longer triggered. You can enable reporting of these unused disable directives by setting the reportUnusedDisableDirectives option to a severity string, as in this example:

// eslint.config.js
export default [
    {
        files: ["**/*.js"],
        linterOptions: {
            reportUnusedDisableDirectives: "error"
        }
    }
];

This setting defaults to "warn".

You can override this setting using the --report-unused-disable-directives or the --report-unused-disable-directives-severity command line options.

For legacy compatibility, true is equivalent to "warn" and false is equivalent to "off".

Configuring Rules

You can configure any number of rules in a configuration object by add a rules property containing an object with your rule configurations. The names in this object are the names of the rules and the values are the configurations for each of those rules. Here’s an example:

// eslint.config.js
export default [
    {
        rules: {
            semi: "error"
        }
    }
];

This configuration object specifies that the semi rule should be enabled with a severity of "error". You can also provide options to a rule by specifying an array where the first item is the severity and each item after that is an option for the rule. For example, you can switch the semi rule to disallow semicolons by passing "never" as an option:

// eslint.config.js
export default [
    {
        rules: {
            semi: ["error", "never"]
        }
    }
];

Each rule specifies its own options and can be any valid JSON data type. Please check the documentation for the rule you want to configure for more information about its available options.

For more information on configuring rules, see Configure Rules.

Configuring Shared Settings

ESLint supports adding shared settings into configuration files. When you add a settings object to a configuration object, it is supplied to every rule. By convention, plugins namespace the settings they are interested in to avoid collisions with others. Plugins can use settings to specify the information that should be shared across all of their rules. This may be useful if you are adding custom rules and want them to have access to the same information. Here’s an example:

// eslint.config.js
export default [
    {
        settings: {
            sharedData: "Hello"
        }
    }
];

Using Predefined Configurations

ESLint has two predefined configurations for JavaScript:

  • js.configs.recommended - enables the rules that ESLint recommends everyone use to avoid potential errors
  • js.configs.all - enables all of the rules shipped with ESLint

To include these predefined configurations, install the @eslint/js package and then make any modifications to other properties in subsequent configuration objects:

// eslint.config.js
import js from "@eslint/js";

export default [
    js.configs.recommended,
    {
        rules: {
            "no-unused-vars": "warn"
        }
    }
];

Here, the js.configs.recommended predefined configuration is applied first and then another configuration object adds the desired configuration for no-unused-vars.

For more information on how to combine predefined configs with your preferences, please see Combine Configs.

Using a Shareable Configuration Package

A sharable configuration is an npm package that exports a configuration object or array. This package should be installed as a dependency in your project and then referenced from inside of your eslint.config.js file. For example, to use a shareable configuration named eslint-config-example, your configuration file would look like this:

// eslint.config.js
import exampleConfig from "eslint-config-example";

export default [
    exampleConfig,

    // your modifications
    {
        rules: {
            "no-unused-vars": "warn"
        }
    }
];

In this example, exampleConfig is an object, so you insert it directly into the configuration array.

Some shareable configurations will export an array instead, in which case you’ll need to use the spread operator to insert those items into the configuration array. For example:

// eslint.config.js
import exampleConfigs from "eslint-config-example";

export default [
    ...exampleConfigs,

    // your modifications
    {
        rules: {
            "no-unused-vars": "warn"
        }
    }
];

Please refer to the documentation for the shareable configuration package you’re using to determine whether it is exporting an object or an array.

For more information on how to combine shareable configs with your preferences, please see Combine Configs.

Configuration File Resolution

When ESLint is run on the command line, it first checks the current working directory for eslint.config.js. If that file is found, then the search stops, otherwise it checks for eslint.config.mjs. If that file is found, then the search stops, otherwise it checks for eslint.config.cjs. If none of the files are not found, it checks the parent directory for each file. This search continues until either a config file is found or the root directory is reached.

You can prevent this search for eslint.config.js by using the -c or --config option on the command line to specify an alternate configuration file, such as:

npx eslint --config some-other-file.js **/*.js

In this case, ESLint does not search for eslint.config.js and instead uses some-other-file.js.

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