Linter Class

Low-level API for verifying code with explicit configuration

The Linter class is a low-level API for verifying JavaScript code. Unlike the ESLint class, Linter does not handle file I/O or configuration file loading. It works directly with code strings and configuration objects.

Constructor

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

options · object

Configuration options for the Linter instance.

Show properties

cwd · string

Path to a directory that should be considered as the current working directory. Defaults to process.cwd().

flags · string[]

Array of feature flags to enable.

configType · 'flat' · default: "flat"

The type of config used. Must be "flat". Retained for backwards compatibility.

Static Properties

Linter.version

const version = Linter.version;

version · string

The ESLint version string.

Instance Methods

verify()

Verifies text against the specified rules.

const messages = linter.verify(textOrSourceCode, config, filenameOrOptions);

textOrSourceCode · string | SourceCode · required

The JavaScript code to verify, or a SourceCode object.

config · Config | Config[] · required

The configuration object or array of configuration objects.

filenameOrOptions · string | object

The filename or options object.

Show options properties

filename · string · default: "<input>"

The filename of the source code.

allowInlineConfig · boolean · default: "true"

Allow/disallow inline comments’ ability to change config.

disableFixes · boolean · default: "false"

If true, the linter doesn’t make fix properties in the result.

reportUnusedDisableDirectives · boolean | 'off' | 'warn' | 'error'

Adds reported errors for unused eslint-disable directives.

ruleFilter · function

A predicate function that determines whether a given rule should run.

messages · LintMessage[]

Array of lint messages.

Show LintMessage properties

ruleId · string | null

The rule ID that generated this message.

severity · number

The severity of the message: 1 for warning, 2 for error.

message · string

The error message.

line · number

The 1-based line number where the issue occurs.

column · number

The 1-based column number where the issue occurs.

endLine · number

The 1-based line number where the issue ends.

endColumn · number

The 1-based column number where the issue ends.

fix · object

The fix object if the rule provides a fix.

suggestions · object[]

Array of suggestion objects if the rule provides suggestions.

verifyAndFix()

Performs multiple autofix passes over the text.

const result = linter.verifyAndFix(text, config, filenameOrOptions);

text · string · required

The source text to apply fixes to.

config · Config | Config[] · required

The ESLint config object or array.

filenameOrOptions · string | object

The filename or options object.

Show options properties

filename · string

The filename of the source code.

fix · boolean | function · default: "true"

Whether fixes should be applied. Can be a predicate function.

allowInlineConfig · boolean

Allow/disallow inline config.

result · object

Show properties

fixed · boolean

Whether any fixes were applied.

messages · LintMessage[]

Array of remaining lint messages.

output · string

The fixed code text.

getSourceCode()

Gets the SourceCode object from the last verification.

const sourceCode = linter.getSourceCode();

sourceCode · SourceCode | null

The SourceCode object from the last verify() call, or null.

getSuppressedMessages()

Gets suppressed messages from the last verification.

const suppressedMessages = linter.getSuppressedMessages();

suppressedMessages · SuppressedLintMessage[]

Array of messages that were suppressed by directives.

getTimes()

Gets timing information from the last verification (when stats are enabled).

const times = linter.getTimes();

times · object

Show properties

passes · TimePass[]

Array of timing information for each pass.

getFixPassCount()

Gets the number of autofix passes made in the last run.

const count = linter.getFixPassCount();

count · number

The number of autofix passes.

hasFlag()

Checks if a feature flag is enabled.

const enabled = linter.hasFlag(flag);

flag · string · required

The feature flag name to check.

enabled · boolean

true if the flag is enabled, false otherwise.

Examples

Basic Verification

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

const linter = new Linter();

const code = "var foo = 'bar';";
const config = {
  rules: {
    "no-var": "error",
    "quotes": ["error", "double"]
  }
};

const messages = linter.verify(code, config);
console.log(messages);
// [
//   {
//     ruleId: 'no-var',
//     severity: 2,
//     message: 'Unexpected var, use let or const instead.',
//     line: 1,
//     column: 1,
//     ...
//   },
//   ...
// ]

Auto-fixing

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

const linter = new Linter();

const code = "var foo = 'bar';";
const config = {
  rules: {
    "quotes": ["error", "double"]
  }
};

const result = linter.verifyAndFix(code, config);
console.log(result);
// {
//   fixed: true,
//   output: 'var foo = "bar";',
//   messages: []
// }

With Filename

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

const linter = new Linter();

const code = "console.log('test');";
const config = {
  rules: {
    "no-console": "error"
  }
};

const messages = linter.verify(code, config, {
  filename: "test.js",
  allowInlineConfig: false
});

console.log(messages);

Custom CWD

const { Linter } = require("eslint");
const path = require("path");

const linter = new Linter({
  cwd: path.join(__dirname, "src")
});

const code = "var x = 1;";
const config = {
  rules: { "no-var": "error" }
};

const messages = linter.verify(code, config);
console.log(messages);

Differences from ESLint Class

The Linter class differs from the ESLint class in several ways:

Feature	Linter	ESLint
File I/O	No	Yes
Config file loading	No	Yes
Caching	No	Yes
Formatters	No	Yes
Glob patterns	No	Yes
Synchronous API	Yes	No
Use case	Low-level, in-memory linting	Full linting workflow

Use Linter when you need fine-grained control and already have the code and configuration in memory. Use ESLint for a complete linting workflow with file handling.