ESLint Major Version Migration (v8 to v9)

ESLint v8 to v9 Migration

ESLint 9 introduced the flat config system, which fundamentally changed how ESLint is configured. This migration involves not just updating ESLint itself but replacing many plugins and config packages.

Typical ESLint 8 to 9 diff

devDependencies:
  ~ eslint: ^8.50.0 -> ^9.0.0                    [MAJOR]
  - eslint-config-next: ^14.0.0                   [REMOVED]
  - @typescript-eslint/parser: ^6.0.0             [REMOVED]
  - @typescript-eslint/eslint-plugin: ^6.0.0      [REMOVED]
  + @eslint/js: ^9.0.0                            [ADDED]
  + typescript-eslint: ^8.0.0                     [ADDED]
  + eslint-config-next: ^15.0.0                   [ADDED - new version]
  + globals: ^15.0.0                              [ADDED]

Key changes in ESLint 9

Old (v8)	New (v9)
.eslintrc.json	eslint.config.mjs (flat config)
@typescript-eslint/parser	typescript-eslint (unified package)
@typescript-eslint/eslint-plugin	typescript-eslint
eslint-plugin-import	Often replaced with eslint-plugin-import-x
extends array	...configs spread syntax

Migration checklist

Use the diff to verify:

1. ESLint core updated to v9+
2. Old packages removed: @typescript-eslint/parser, @typescript-eslint/eslint-plugin
3. New packages added: typescript-eslint, @eslint/js, globals
4. Config file renamed: .eslintrc.* to eslint.config.*
5. Scripts updated: eslint commands may need new flags

Why the diff tool helps here

This migration involves simultaneous additions and removals. Without a structured diff, it is easy to miss a removed package or forget to add a replacement. The summary panel shows you the total count of changes at a glance.