ESLint v9.0.0-rc.0 released

We just pushed ESLint v9.0.0-rc.0, which is a major release upgrade of ESLint. This release adds some new features and fixes several bugs found in the previous release. This release also has some breaking changes, so please read the following closely.

Highlights

This is a summary of the major changes you need to know about for this version of ESLint.

Installing

Since this is a pre-release version, you will not automatically be upgraded by npm. You must specify the next tag when installing:

npm i eslint@next --save-dev

You can also specify the version directly:

npm i eslint@9.0.0-rc.0 --save-dev

Migration Guide

As there are a lot of changes, we’ve created a migration guide describing the breaking changes in great detail along with the steps you should take to address them. We expect that most users should be able to upgrade without any build changes, but the migration guide should be a useful resource if you encounter problems.

Precalculated Code Paths

ESLint v9.0.0 now precalculates code path information before the traversal used by rules. As a result, the code path information is now complete regardless of where it is accessed inside of a rule.

Disallow Unused // fallthrough Comments

In ESLint v9.0.0-rc.0, the rule no-fallthrough has a new option reportUnusedFallthroughComment. When this option is set to true, the rule will prohibit a fallthrough comment if a case can never fallthrough. This is intended to help avoiding misleading comments that can occur as a result of refactoring.

Breaking Changes

  • 09bd7fe feat!: move AST traversal into SourceCode (#18167) (Nicholas C. Zakas)

Features

Bug Fixes

  • ae8103d fix: load plugins in the CLI in flat config mode (#18185) (Francesco Trotta)

Documentation

  • b91f9dc build: fix TypeError in prism-eslint-hooks.js (#18209) (Francesco Trotta)

Chores

  • 297416d chore: package.json update for eslint-9.0.0-rc.0 (#18223) (Francesco Trotta)
  • d363c51 chore: package.json update for @eslint/js release (Jenkins)
  • 1b841bb chore: fix some comments (#18213) (avoidaway)
  • 29c3595 chore: remove repetitive words (#18193) (cuithon)
  • acc2e06 chore: Introduce Knip (#18005) (Lars Kappert)

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