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Announcing typescript-eslint v6 Beta

ยท 15 min read
Josh Goldberg
Josh Goldberg
typescript-eslint Maintainer
Newer Information Available

This blog post is now out of date, as we've released typescript-eslint v6! ๐Ÿš€ Please see Announcing typescript-eslint v6 for the latest information.

typescript-eslint is the tooling that enables standard JavaScript tools such as ESLint and Prettier to support TypeScript code. We've been working on a set of breaking changes and general features that we're excited to get in front of users soon. And now, after over two years of development, we're excited to say that typescript-eslint v6 is ready for public beta testing! ๐ŸŽ‰

Our plan for typescript-eslint v6 is to:

  1. Have users try out betas starting in early March of 2023
  2. Respond to user feedback for the next 1-3 months
  3. Release a stable version summer of 2023

Nothing mentioned in this blog post is set in stone. If you feel passionately about any of the choices we've made here -positively or negatively- then do let us know on the typescript-eslint Discord's #v6 channel!

Trying Out v6โ€‹

Please do try out the typescript-eslint v6 beta!

As A New Userโ€‹

If you don't yet use typescript-eslint, you can go through our configuration steps on the v6 Getting Started docs. It'll walk you through setting up typescript-eslint in a project.

To use v6 specifically, see the following section for an updated install command.

As An Existing Userโ€‹

If you already use typescript-eslint, you'll need to first replace your package's previous versions of @typescript-eslint/eslint-plugin and @typescript-eslint/parser with @rc-v6 versions:

npm i @typescript-eslint/eslint-plugin@rc-v6 @typescript-eslint/parser@rc-v6 --save-dev

We highly recommend then basing your ESLint configuration on the reworked typescript-eslint recommended configurations mentioned later in this post โ€” especially if it's been a while since you've reworked your linter config.

User-Facing Breaking Changesโ€‹

These are the changes that users of typescript-eslint -generally, any developer running ESLint on TypeScript code- should pay attention to when upgrading typescript-eslint from v5 to v6.

โณ indicates a change that has been scheduled for v6 but not yet released. We'll update this blog post as the corresponding pull requests land.

Reworked Configuration Namesโ€‹

The biggest configuration change in typescript-eslint v6 is that we've reworked the names of our provided user configuration files. typescript-eslint v5 provided three recommended configurations:

  • recommended: Recommended rules for code correctness that you can drop in without additional configuration.
  • recommended-requiring-type-checking: Additional recommended rules that require type information.
  • strict: Additional strict rules that can also catch bugs but are more opinionated than recommended rules.

Those configurations worked well for most projects. However, some users correctly noted two flaws in that approach:

  • Strict rules that didn't require type checking were lumped in with those that did.
  • Stylistic best practices were lumped in with rules that actually find bugs.

As a result, we've reworked the configurations provided by typescript-eslint into these two groups:

  • Functional rule configurations, for best best practices and code correctness:
    • recommended: Recommended rules that you can drop in without additional configuration.
    • recommended-type-checked:ย Additional recommended rules that require type information.
    • strict: Additional strict rules that can also catch bugs but are more opinionated than recommended rules (without type information).
    • strict-type-checked: Additional strict rules that do require type information.
  • Stylistic rule configurations, for consistent and predictable syntax usage:
    • stylistic: Stylistic rules you can drop in without additional configuration.
    • stylistic-type-checked: Additional stylistic rules that require type information.

recommended-requiring-type-checking is now an alias for recommended-type-checked. The alias will be removed in a future major version.

As of v6, we recommend that projects enable two configs from the above:

  • If you are not using typed linting, enable stylistic and either recommended or strict, depending on how intensely you'd like your lint rules to scrutinize your code.
  • If you are using typed linting, enable stylistic-type-checked and either recommended-type-checked or strict-type-checked, depending on how intensely you'd like your lint rules to scrutinize your code.

For example, a typical project that enables typed linting might have an ESLint configuration file that changes like:

.eslintrc.cjs
module.exports = {
  extends: [
    'eslint:recommended',
    'plugin:@typescript-eslint/recommended',
    'plugin:@typescript-eslint/recommended-requiring-type-checking',
    'plugin:@typescript-eslint/recommended-type-checked',
    'plugin:@typescript-eslint/stylistic-type-checked',
  ],
  plugins: ['@typescript-eslint'],
  parser: '@typescript-eslint/parser',
  parserOptions: {
    project: './tsconfig.json',
    project: true,
    tsconfigRootDir: __dirname,
  },
  root: true,
};

See Configurations on the v6 docs site preview for the updated documentation on configurations provided by typescript-eslint.

For more information on these changes, see:

Updated Configuration Rulesโ€‹

Every new major version of typescript-eslint comes with changes to which rules are enabled in the preset configurations - and with which options. Because this release also includes a reworking of the configurations themselves, the list of changes is too large to put in this blog post. Instead see the table in Changes to configurations for 6.0.0 for a full list of the changes.

Please do try out the new rule configurations presets and let us know in that discussion!

tip

If your ESLint configuration contains many rules configurations, we suggest the following strategy to start anew:

  1. Remove all your rules configurations
  2. Extend from the preset configs that make sense for you
  3. Run ESLint on your project
  4. In your ESLint configuration, turn off any rules creating errors that don't make sense for your project - with comments explaining why
  5. In your ESLint configuration and/or with inline eslint-disable comments, turn off any rules creating too many errors for you to fix - with "TODO" comments linking to tracking issues/tickets to re-enable them

Miscellaneous changes to all shared configurations include:

Rule Breaking Changesโ€‹

Newer Information Available

This section is now out of date, as we've released typescript-eslint v6! ๐Ÿš€ Please see Announcing typescript-eslint v6 > Rule Breaking Changes for the latest information.

Several rules were changed in significant enough ways to be considered breaking changes:

Tooling Breaking Changesโ€‹

Newer Information Available

This section is now out of date, as we've released typescript-eslint v6! ๐Ÿš€ Please see Announcing typescript-eslint v6 > Tooling Breaking Changes for the latest information.

Developer-Facing Changesโ€‹

typescript-eslint v6 comes with a suite of cleanups and improvements for developers as well. If you author any ESLint plugins or other tools that interact with TypeScript syntax, then we recommend you try out typescript-eslint v6 soon. It includes some breaking changes that you may need to accommodate for.

tip

If you're having trouble working with the changes, please let us know on the typescript-eslint Discord's #v6 channel!

Type Checker Wrapper APIsโ€‹

As described in our ASTs and typescript-eslint post, ESLint rules don't natively work with AST nodes compatible with TypeScript's API. Retrieving type information for an ESLint AST node in a custom rule requires code somewhat like:

custom-rule-with-v5.ts
{
  // ...
  create() {
    const services = util.getParserServices(context);
    const checker = services.program.getTypeChecker();
    const tsNode = services.esTreeNodeToTSNodeMap.get(esNode);
    const type = checker.getTypeAtLocation(node);

    // ...
  }
  // ...
}

How cumbersome, just to call to a single method (getTypeAtLocation) on the TypeScript API!

In typescript-eslint v6, we've added a set of wrapper APIs on the services: ParserServices object that act as shortcuts for commonly used TypeScript APIs including getTypeAtLocation:

custom-rule-with-v6.ts
{
  // ...
  create() {
    const services = util.getParserServices(context);
    const type = services.getTypeAtLocation(node);

    // ...
  }
  // ...
}

For now, the available wrapper APIs are:

  • getSymbolAtLocation: passes an ESTree's equivalent TypeScript node to checker.getSymbolAtLocation
  • getTypeAtLocation: passes an ESTree node's equivalent TypeScript node to checker.getTypeAtLocation

We hope these wrapper APIs make it more convenient to write lint rules that rely on the awesome power of TypeScript's type checking. In the future, we may add more wrapper APIs, and may even add internal caching to those APIs to improve performance.

Newer Information Available

Rules can still retrieve their full backing TypeScript type checker with services.program.getTypeChecker(). This can be necessary for TypeScript APIs not wrapped by the parser services.

See Custom Rules on the v6 docs site preview for the updated documentation on creating custom rules with typescript-eslint.

AST Breaking Changesโ€‹

These PRs changed the AST shapes generated by typescript-eslint when parsing code. If you author any ESLint rules that refer to the syntax mentioned by them, these are relevant to you.

Errors on Invalid AST Parsingโ€‹

Newer Information Available

These changes only impact API consumers of typescript-eslint that work at parsing level. If the extent of your API usage is writing custom rules, these changes don't impact you.

The @typescript-eslint/typescript-estree parser is by default very forgiving of invalid ASTs. If it encounters invalid syntax, it will still attempt create an AST if possible: even if required properties of nodes don't exist.

For example, this snippet of TypeScript code creates a ClassDeclaration whose id is null:

export class {}

Invalid parsed ASTs can cause problems for downstream tools expecting AST nodes to adhere to the ESTree spec. ESLint rules in particular tend to crash when given invalid ASTs.

@typescript-eslint/typescript-estree will now throw an error when it encounters a known invalid AST such as the export class {} example. This is generally the correct behavior for most parsing contexts so downstream tools don't have to work with a potentially invalid AST.

For consumers that don't want the updated behavior of throwing on invalid ASTs, a new allowInvalidAST option exists to disable the throwing behavior. Keep in mind that with it enabled, ASTs produced by typescript-eslint might not match their TSESTree type definitions.

For more information, see:

Standalone RuleTester packageโ€‹

Previously we provided a version of ESLint's RuleTester class from @typescript-eslint/utils/eslint-utils. This version was a sub-class of the original version and was implemented in a very fragile way that made it hard to test, maintain and build new features into.

This was also reasonably cumbersome for users to access as users had to do deep imports in order to access the class without a namespace.

In v6 we have extracted this into its own package - @typescript-eslint/rule-tester. Additionally instead of being a hacky subclass it's now a complete fork of the original tooling. For the most part you should be able to update your tests as follows:

import { TSESLint } from '@typescript-eslint/utils';
import { RuleTester } from '@typescript-eslint/rule-tester';

import rule from '../src/rules/my-rule';

const ruleTester = new TSESLint.RuleTester({
const ruleTester = new RuleTester({
  parser: '@typescript-eslint/parser',
});

ruleTester.run('my-rule', rule, { /* ... */ });

Breaking changes:

  • Previously if you set parserOptions.ecmaFeatures.jsx = true the rule tester would attempt to look for a fixture named file.tsx. Now instead the rule tester will look for a file named react.tsx.
    • The previous behavior was incorrect because it would encourage you to have both file.ts and file.tsx and TypeScript would ignore one of those files, causing weird breakages in tests.
    • You can control the default filenames by passing defaultFilenames to the RuleTester constructor.

New features:

  • skip: boolean - the inverse option of only: boolean. When true we will use your test framework's test skip functionality (it.skip) to mark the test as skipped. This is useful during development as it enables you to control which tests run without needing to comment blocks out.
  • Dependency version filtering. It's useful to test your rule against multiple versions of your dependencies to ensure it doesn't break on older versions. However in some cases certain tests will not work on older versions of some dependencies due to features that didn't exist until recently - for example a test might use newer syntax that didn't exist in an older version of TypeScript. Our rule tester includes options that allow you to declare the allowed version ranges for a test so that it is automatically skipped when necessary.

For more information on the package, see the rule-tester package documentation.

Other Developer-Facing Breaking Changesโ€‹

Newer Information Available

This section is now out of date, as we've released typescript-eslint v6! ๐Ÿš€ Please see Announcing typescript-eslint v6 > Other Developer-Facing Breaking Changes for the latest information.

Appreciationโ€‹

We'd like to extend a sincere thank you to everybody who pitched in to make typescript-eslint v6 possible.

See the v6.0.0 milestone for the list of issues and associated merged pull requests.

Supporting typescript-eslintโ€‹

If you enjoyed this blog post and/or or use typescript-eslint, please consider supporting us on Open Collective. We're a small volunteer team and could use your support to make the ESLint experience on TypeScript great. Thanks! ๐Ÿ’–