# TypeScript ESLint Documentation
# Source: https://raw.githubusercontent.com/typescript-eslint/typescript-eslint/main/docs/getting-started/Typed_Linting.mdx
# Path: docs/getting-started/Typed_Linting.mdx

---
id: typed-linting
title: Linting with Type Information
---

import Tabs from '@theme/Tabs';
import TabItem from '@theme/TabItem';

Some typescript-eslint rules utilize TypeScript's type checking APIs to provide much deeper insights into your code.
This requires TypeScript to analyze your entire project instead of just the file being linted.
As a result, these rules are slower than traditional lint rules but are much more powerful.

To enable typed linting, there are two small changes you need to make to your config file:

<Tabs groupId="eslint-config">
<TabItem value="Flat Config">

1. Add `TypeChecked` to the name of any preset configs you're using, namely `recommended`, `strict`, and `stylistic`.
2. Add `languageOptions.parserOptions` to tell our parser how to find the TSConfig for each source file.

```js title="eslint.config.mjs"
import eslint from '@eslint/js';
import { defineConfig } from 'eslint/config';
import tseslint from 'typescript-eslint';

export default defineConfig(
  eslint.configs.recommended,
  // Remove this line
  tseslint.configs.recommended,
  // Added lines start
  tseslint.configs.recommendedTypeChecked,
  {
    languageOptions: {
      parserOptions: {
        projectService: true,
      },
    },
  },
  // Added lines end
);
```

In more detail:

- `tseslint.configs.recommendedTypeChecked` is a [shared configuration](../users/Shared_Configurations.mdx). It contains recommended rules that additionally require type information.
- `parserOptions.projectService: true` indicates to ask TypeScript's type checking service for each source file's type information (see [Parser > Project Service](../packages/Parser.mdx#projectservice)).
- `parserOptions.tsconfigRootDir` tells our parser the absolute path of your project's root directory (see [Parser > tsconfigRootDir](../packages/Parser.mdx#tsconfigrootdir)).

</TabItem>
<TabItem value="Legacy Config">

1. Add `-type-checked` to the name of any preset configs you're using, namely `recommended`, `strict`, and `stylistic`.
2. Add `parserOptions` to tell our parser how to find the TSConfig for each source file.

```js title=".eslintrc.cjs"
/* eslint-env node */
module.exports = {
  extends: [
    'eslint:recommended',
    // Remove this line
    'plugin:@typescript-eslint/recommended',
    // Add this line
    'plugin:@typescript-eslint/recommended-type-checked',
  ],
  plugins: ['@typescript-eslint'],
  parser: '@typescript-eslint/parser',
  // Added lines start
  parserOptions: {
    projectService: true,
    tsconfigRootDir: __dirname,
  },
  // Added lines end
  root: true,
};
```

In more detail:

- `plugin:@typescript-eslint/recommended-type-checked` is a [shared configuration](../users/Shared_Configurations.mdx). It contains recommended rules that additionally require type information.
- `parserOptions.projectService: true` indicates to ask TypeScript's type checking service for each source file's type information (see [Parser > `projectService`](../packages/Parser.mdx#projectservice)).
- `parserOptions.tsconfigRootDir` tells our parser the absolute path of your project's root directory (see [Parser > `tsconfigRootDir`](../packages/Parser.mdx#tsconfigrootdir)).

</TabItem>
</Tabs>

:::caution
Your ESLint config file may start receiving a parsing error about type information.
See [our TSConfig inclusion FAQ](../../troubleshooting/typed-linting#i-get-errors-telling-me-eslint-was-configured-to-run--however-that-tsconfig-does-not--none-of-those-tsconfigs-include-this-file).
:::

With that done, run the same lint command you ran before.
You may see new rules reporting errors based on type information!

## Shared Configurations

If you enabled the [`strict` shared config](../users/Shared_Configurations.mdx#strict) and/or [`stylistic` shared config](../users/Shared_Configurations.mdx#stylistic) in a previous step, be sure to replace them with [`strictTypeChecked`](../users/Shared_Configurations.mdx#strict-type-checked) and [`stylisticTypeChecked`](../users/Shared_Configurations.mdx#stylistic-type-checked) respectively to add their type-checked rules.

<Tabs groupId="eslint-config">
<TabItem value="Flat Config">

```js title="eslint.config.mjs"
export default defineConfig(
  eslint.configs.recommended,
  // Removed lines start
  tseslint.configs.strict,
  tseslint.configs.stylistic,
  // Removed lines end
  // Added lines start
  tseslint.configs.strictTypeChecked,
  tseslint.configs.stylisticTypeChecked,
  // Added lines end
  // ...
);
```

</TabItem>
<TabItem value="Legacy Config">

```js title=".eslintrc.cjs"
/* eslint-env node */
module.exports = {
  extends: [
    'eslint:recommended',
    // Removed lines start
    'plugin:@typescript-eslint/strict',
    'plugin:@typescript-eslint/stylistic',
    // Removed lines end
    // Added lines start
    'plugin:@typescript-eslint/strict-type-checked',
    'plugin:@typescript-eslint/stylistic-type-checked',
    // Added lines end
  ],
  // ...
};
```

</TabItem>
</Tabs>

You can read more about the rules provided by typescript-eslint in our [rules docs](/rules) and [shared configurations docs](../users/Shared_Configurations.mdx).

## Performance

Typed rules come with a catch.
By using typed linting in your config, you incur the performance penalty of asking TypeScript to do a build of your project before ESLint can do its linting.
For small projects this takes a negligible amount of time (a few seconds or less); for large projects, it can take longer.

Most of our users do not mind this cost as the power and safety of type-aware static analysis rules is worth the tradeoff.
Additionally, most users primarily consume lint errors via IDE plugins which, through caching, do not suffer the same penalties.
This means that generally they usually only run a complete lint before a push, or via their CI, where the extra time often doesn't matter.

**We strongly recommend you do use type-aware linting**, but the above information is included so that you can make your own, informed decision.
See [Troubleshooting > Typed Linting > Performance](../troubleshooting/typed-linting/Performance.mdx) for more information.

## Troubleshooting

If you're having problems with typed linting, please see our [Troubleshooting FAQs](../troubleshooting/faqs/General.mdx) and in particular [Troubleshooting > Typed Linting](../troubleshooting/typed-linting/index.mdx).

For details on the parser options that enable typed linting, see:

- [Parser > `projectService`](../packages/Parser.mdx#projectservice): our recommended option, with settings to customize TypeScript project information
- [Parser > `project`](../packages/Parser.mdx#project): an older option that can be used as an alternative