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Configuration Files (New)

Configuration File

The ESLint configuration file is named eslint.config.js and should be placed in the root directory of your project and export an array of configuration objects. Here’s an example:

export default [
{
rules: {
semi: "error",
"prefer-const": "error"
}
}
]

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

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.
  • 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 - Either an object containing a parse() method or a string indicating the name of a parser inside of a plugin (i.e., "pluginName/parserName"). (default: "@/espree")
    • parserOptions - An object specifying additional options that are passed directly to the parser() 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 Boolean value indicating if unused disable directives should be tracked and reported.
  • 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, by default config objects will apply to any JavaScript files passed to ESLint. For example:

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 will be applied. If you pass a non-JavaScript file, like example.txt, the semi rule will not be applied because there are no other configuration objects that match that filename. (ESLint will output 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, like this:

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

Here, only the JavaScript files in the src directory will have the semi rule applied. If you run ESLint on files in another directory, this configuration object will be 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 will still have semi applied.

If ignores is used without files and any other setting, 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 additional global ignores, similar to those found in .eslintignore. Here’s an example:

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 patterns found in .eslintignore.

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:

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:

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

Reporting unused disable directives

Disable directives such as /*eslint-disable*/ 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 will no longer be triggered. You can enable reporting of these unused disable directives by setting the reportUnusedDisableDirectives option to true, as in this example:

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

By default, unused disable directives are reported as warnings. You can change this setting using the --report-unused-disable-directives command line option.

Configuring language options

Options specific to how ESLint evaluates your JavaScript code can be configured using the languageOptions object.

Configuring the JavaScript version

To configure the version of JavaScript (ECMAScript) that ESLint uses to evaluate your JavaScript, use the ecmaVersion property. This property determines which global variables and syntax are valid in your code and can be set to the version number (such as 6), the year number (such as 2022), or "latest" (for the most recent version that ESLint supports). By default, ecmaVersion is set to "latest" and it’s recommended to keep this default unless you need to ensure that your JavaScript code is evaluated as an older version. For example, some older runtimes might only allow ECMAScript 5, in which case you can configure ESLint like this:

export default [
{
files: ["**/*.js"],
languageOptions: {
ecmaVersion: 5
}
}
];

Configuring the JavaScript source type

ESLint can evaluate your code in one of three ways:

  1. ECMAScript module (ESM) - Your code has a module scope and is run in strict mode.
  2. CommonJS - Your code has a top-level function scope and runs in nonstrict mode.
  3. Script - Your code has a shared global scope and runs in nonstrict mode.

You can specify which of these modes your code is intended to run in by specifying the sourceType property. This property can be set to "module", "commonjs", or "script". By default, sourceType is set to "module" for .js and .mjs files and is set to "commonjs" for .cjs files. Here’s an example:

export default [
{
files: ["**/*.js"],
languageOptions: {
sourceType: "script"
}
}
];

Configuring a custom parser and its options

In many cases, you can use the default parser that ESLint ships with for parsing your JavaScript code. You can optionally override the default parser by using the parser property. The parser property can be either a string in the format "pluginName/parserName" (indicating to retrieve the parser from a plugin) or an object containing either a parse() method or a parseForESLint() method. For example, you can use the @babel/eslint-parser package to allow ESLint to parse experimental syntax:

import babelParser from "@babel/eslint-parser";

export default [
{
files: ["**/*.js", "**/*.mjs"],
languageOptions: {
parser: babelParser
}
}
];

This configuration ensures that the Babel parser, rather than the default, will be used to parse all files ending with .js and .mjs.

You can also pass options directly to the custom parser by using the parserOptions property. This property is an object whose name-value pairs are specific to the parser that you are using. For the Babel parser, you might pass in options like this:

import babelParser from "@babel/eslint-parser";

export default [
{
files: ["**/*.js", "**/*.mjs"],
languageOptions: {
parser: babelParser,
parserOptions: {
requireConfigFile: false,
babelOptions: {
babelrc: false,
configFile: false,
// your babel options
presets: ["@babel/preset-env"],
}
}
}
}
];

Configuring global variables

To configure global variables inside of a configuration object, set the globals configuration property to an object containing keys named for each of the global variables you want to use. For each global variable key, set the corresponding value equal to "writable" to allow the variable to be overwritten or "readonly" to disallow overwriting. For example:

export default [
{
files: ["**/*.js"],
languageOptions: {
globals: {
var1: "writable",
var2: "readonly"
}
}
}
];

These examples allow var1 to be overwritten in your code, but disallow it for var2.

Globals can be disabled with the string "off". For example, in an environment where most ES2015 globals are available but Promise is unavailable, you might use this config:

export default [
{
languageOptions: {
globals: {
Promise: "off"
}
}
}
];

For historical reasons, the boolean value false and the string value "readable" are equivalent to "readonly". Similarly, the boolean value true and the string value "writeable" are equivalent to "writable". However, the use of older values is deprecated.

Using plugins in your configuration

Plugins are used to share rules, processors, configurations, parsers, and more across ESLint projects. Plugins are specified in a configuration object using the plugins key, which is an object where the name of the plugin is the property name and the value is the plugin object itself. Here’s an example:

import jsdoc from "eslint-plugin-jsdoc";

export default [
{
files: ["**/*.js"],
plugins: {
jsdoc: jsdoc
}
rules: {
"jsdoc/require-description": "error",
"jsdoc/check-values": "error"
}
}
];

In this configuration, the JSDoc plugin is defined to have the name jsdoc. The prefix jsdoc/ in each rule name indicates that the rule is coming from the plugin with that name rather than from ESLint itself.

Because the name of the plugin and the plugin object are both jsdoc, you can also shorten the configuration to this:

import jsdoc from "eslint-plugin-jsdoc";

export default [
{
files: ["**/*.js"],
plugins: {
jsdoc
}
rules: {
"jsdoc/require-description": "error",
"jsdoc/check-values": "error"
}
}
];

While this is the most common convention, you don’t need to use the same name that the plugin prescribes. You can specify any prefix that you’d like, such as:

import jsdoc from "eslint-plugin-jsdoc";

export default [
{
files: ["**/*.js"],
plugins: {
jsd: jsdoc
}
rules: {
"jsd/require-description": "error",
"jsd/check-values": "error"
}
}
];

This configuration object uses jsd as the prefix plugin instead of jsdoc.

Using processors

Processors allow ESLint to transform text into pieces of code that ESLint can lint. You can specify the processor to use for a given file type by defining a processor property that contains either the processor name in the format "pluginName/processorName" to reference a processor in a plugin or an object containing both a preprocess() and a postprocess() method. For example, to extract JavaScript code blocks from a Markdown file, you might add this to your configuration:

import markdown from "eslint-plugin-markdown";

export default [
{
files: ["**/*.md"],
plugins: {
markdown
},
processor: "markdown/markdown"
settings: {
sharedData: "Hello"
}
}
];

This configuration object specifies that there is a processor called "markdown" contained in the plugin named "markdown" and will apply the processor to all files ending with .md.

Processors may make named code blocks that function as filenames in configuration objects, such as 0.js and 1.js. ESLint handles such a named code block as a child of the original file. You can specify additional configuration objects for named code blocks. For example, the following disables the strict rule for the named code blocks which end with .js in markdown files.

import markdown from "eslint-plugin-markdown";

export default [
{
files: ["**/*.md"],
plugins: {
markdown
},
processor: "markdown/markdown"
settings: {
sharedData: "Hello"
}
},

// applies only to code blocks
{
files: ["**/*.md/*.js"],
rules: {
strict: "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:

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:

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.

Rule severities

There are three possible severities you can specify for a rule

  • "error" (or 2) - the reported problem should be treated as an error. When using the ESLint CLI, errors cause the CLI to exit with a nonzero code.
  • "warn" (or 1) - the reported problem should be treated as a warning. When using the ESLint CLI, warnings are reported but do not change the exit code. If only warnings are reported, the exit code will be 0.
  • "off" (or 0) - the rule should be turned off completely.

Rule configuration cascade

When more than one configuration object specifies the same rule, the rule configuration is merged with the later object taking precedence over any previous objects. For example:

export default [
{
rules: {
semi: ["error", "never"]
}
},
{
rules: {
semi: ["warn", "always"]
}
}
];

Using this configuration, the final rule configuration for semi is ["warn", "always"] because it appears last in the array. The array indicates that the configuration is for the severity and any options. You can change just the severity by defining only a string or number, as in this example:

export default [
{
rules: {
semi: ["error", "never"]
}
},
{
rules: {
semi: "warn"
}
}
];

Here, the second configuration object only overrides the severity, so the final configuration for semi is ["warn", "never"].

Configuring shared settings

ESLint supports adding shared settings into configuration files. Plugins use settings to specify information that should be shared across all of its rules. You can add a settings object to a configuration object and it will be supplied to every rule being executed. This may be useful if you are adding custom rules and want them to have access to the same information. Here’s an example:

export default [
{
settings: {
sharedData: "Hello"
}
}
];

Using predefined configurations

ESLint has two predefined configurations:

  • eslint:recommended - enables the rules that ESLint recommends everyone use to avoid potential errors
  • eslint:all - enables all of the rules shipped with ESLint

To include these predefined configurations, you can insert the string values into the returned array and then make any modifications to other properties in subsequent configuration objects:

export default [
"eslint:recommended",
{
rules: {
semi: ["warn", "always"]
}
}
];

Here, the eslint:recommended predefined configuration is applied first and then another configuration object adds the desired configuration for semi.

Configuration File Resolution

When ESLint is run on the command line, it first checks the current working directory for eslint.config.js, and if not found, will look to the next parent directory for the file. This search continues until either the file is found or the root directory is reached.

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

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

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