ESLint Class

Primary Node.js API for linting files and text

The ESLint class is the primary interface for integrating ESLint into Node.js applications. It supports flat config and provides high-level methods for linting files, formatting results, and managing configurations.

Constructor

const { ESLint } = require("eslint");
const eslint = new ESLint(options);

options · object

Configuration options for the ESLint instance.

Show properties

allowInlineConfig · boolean · default: "true"

Enable or disable inline configuration comments.

baseConfig · Config | Config[]

Base config, extended by all configs used with this instance.

cache · boolean · default: "false"

Enable result caching to improve performance.

cacheLocation · string · default: ".eslintcache"

Path to the cache file.

cacheStrategy · 'metadata' | 'content' · default: "metadata"

Strategy used to detect changed files.

concurrency · number | 'auto' | 'off' · default: "off"

Maximum number of linting threads. Use "auto" to choose automatically.

cwd · string

Current working directory. Defaults to process.cwd().

errorOnUnmatchedPattern · boolean · default: "true"

If false, lintFiles() doesn’t throw when no target files are found.

fix · boolean | function · default: "false"

Execute in autofix mode. If a function, should return a boolean.

fixTypes · string[]

Array of rule types to apply fixes for (e.g., ["problem", "suggestion"]).

flags · string[]

Array of feature flags to enable.

globInputPaths · boolean · default: "true"

Set to false to skip glob resolution of input file paths.

ignore · boolean · default: "true"

False disables all ignore patterns except for the default ones.

ignorePatterns · string[]

Ignore file patterns in addition to config ignores. Relative to cwd.

overrideConfig · Config | Config[]

Override config, overrides all configs used with this instance.

overrideConfigFile · boolean | string

Config file path, or true to disable config file lookup.

passOnNoPatterns · boolean · default: "false"

When true, missing patterns don’t cause the operation to fail.

plugins · Record<string, Plugin>

Preloaded plugins to use.

stats · boolean · default: "false"

Enable statistics collection on lint results.

warnIgnored · boolean · default: "false"

Show warnings when the file list includes ignored files.

Static Properties

ESLint.version

const version = ESLint.version;

version · string

The ESLint version string (e.g., “9.0.0”).

ESLint.configType

const configType = ESLint.configType; // "flat"

configType · string

The type of configuration used by this class. Always returns "flat".

ESLint.defaultConfig

const defaultConfig = ESLint.defaultConfig;

defaultConfig · FlatConfigArray

The default configuration that ESLint uses internally.

Instance Methods

lintFiles()

Lints files matching the given patterns.

const results = await eslint.lintFiles(patterns);

patterns · string | string[] · required

File patterns to lint. Can be file paths, directory paths, or glob patterns.

results · LintResult[]

Array of linting results for each file.

Show LintResult properties

filePath · string

The absolute path to the file.

messages · LintMessage[]

Array of lint messages.

suppressedMessages · SuppressedLintMessage[]

Array of suppressed lint messages.

errorCount · number

Number of errors.

warningCount · number

Number of warnings.

fixableErrorCount · number

Number of fixable errors.

fixableWarningCount · number

Number of fixable warnings.

output · string

The fixed code (only present if fixes were applied).

usedDeprecatedRules · DeprecatedRuleInfo[]

Information about deprecated rules used.

lintText()

Lints the provided source code text.

const results = await eslint.lintText(code, options);

code · string · required

The source code to lint.

options · object

Show properties

filePath · string

The path to the file. Used for configuration matching.

warnIgnored · boolean

When true, warns if the filePath is ignored.

results · LintResult[]

Array containing a single linting result.

loadFormatter()

Loads a formatter to format lint results.

const formatter = await eslint.loadFormatter(name);
const formatted = formatter.format(results, resultsMeta);

name · string · default: "stylish"

Name of the formatter. Can be:

• A built-in formatter name (e.g., "stylish", "json")
• A third-party formatter (e.g., "eslint-formatter-pretty")
• A file path to a custom formatter

formatter · object

Show properties

format · function

Function that formats results: (results, resultsMeta) => string

calculateConfigForFile()

Calculates the configuration for a given file.

const config = await eslint.calculateConfigForFile(filePath);

filePath · string · required

The path to the file.

config · object | undefined

The calculated configuration object, or undefined if no configuration applies.

findConfigFile()

Finds the config file being used.

const configPath = await eslint.findConfigFile(filePath);

filePath · string

The path to find the config file for. Defaults to cwd.

configPath · string | undefined

The path to the config file, or undefined if not found.

isPathIgnored()

Checks if a path is ignored by ESLint.

const ignored = await eslint.isPathIgnored(filePath);

filePath · string · required

The path to check.

ignored · boolean

true if the path is ignored, false otherwise.

getRulesMetaForResults()

Returns metadata for rules used in the results.

const rulesMeta = eslint.getRulesMetaForResults(results);

results · LintResult[] · required

The lint results to get metadata for.

rulesMeta · Record<string, RulesMeta>

Object mapping rule IDs to their metadata.

hasFlag()

Checks if a feature flag is enabled.

const enabled = eslint.hasFlag(flag);

flag · string · required

The feature flag name to check.

enabled · boolean

true if the flag is enabled, false otherwise.

Static Methods

ESLint.outputFixes()

Writes fixes to files.

await ESLint.outputFixes(results);

results · LintResult[] · required

The lint results containing fixes to apply.

ESLint.getErrorResults()

Filters results to only include errors.

const errorResults = ESLint.getErrorResults(results);

results · LintResult[] · required

The lint results to filter.

errorResults · LintResult[]

Results containing only error messages.

ESLint.fromOptionsModule()

Creates an ESLint instance from an options module URL.

const eslint = await ESLint.fromOptionsModule(optionsURL);

optionsURL · URL · required

URL to a module that exports ESLint options.

eslint · ESLint

A new ESLint instance.

Examples

Basic Linting

const { ESLint } = require("eslint");

async function main() {
  const eslint = new ESLint();
  const results = await eslint.lintFiles(["src/**/*.js"]);
  
  const formatter = await eslint.loadFormatter("stylish");
  const resultText = formatter.format(results);
  console.log(resultText);
}

main();

Autofix

const { ESLint } = require("eslint");

async function fixFiles() {
  const eslint = new ESLint({ fix: true });
  const results = await eslint.lintFiles(["src/**/*.js"]);
  
  await ESLint.outputFixes(results);
  
  const hasErrors = results.some(
    result => result.errorCount > 0
  );
  
  if (hasErrors) {
    const formatter = await eslint.loadFormatter("stylish");
    console.log(formatter.format(results));
    process.exit(1);
  }
}

fixFiles();

Lint Text

const { ESLint } = require("eslint");

async function lintCode() {
  const eslint = new ESLint({
    overrideConfig: {
      rules: {
        "no-console": "error",
        "no-var": "error"
      }
    }
  });
  
  const code = "var x = 1; console.log(x);";
  const results = await eslint.lintText(code, {
    filePath: "example.js"
  });
  
  console.log(results[0].messages);
}

lintCode();