# Migrating Between ESLint Versions — ESLint and Prettier: Code Quality and Formatting

Source: https://www.skillbyai.com/en/eslint-prettier/eslint-prettier-19

> Upgrade to ESLint 9 safely by migrating .eslintrc files to flat config and rolling out changes gradually.

## Migration strategy

ESLint 9 made flat config the default and v10 removes the old format. Migrate with `npx @eslint/migrate-config .eslintrc.json`, test locally, then roll out gradually, using `--max-warnings` for a transition period.

## Safe upgrade process

Steps for upgrading ESLint without breaking the team.

```bash
# 1. Upgrade
npm install --save-dev eslint@latest @eslint/js

# 2. Convert the old config
npx @eslint/migrate-config .eslintrc.json   # creates eslint.config.mjs

# 3. Test locally and fix what you can
npm run lint
npx eslint . --fix

# 4. Transition period: cap warnings and reduce over time
npx eslint . --max-warnings 50

# 5. Commit and open a PR
git commit -am "chore: migrate to ESLint 9 flat config"
```

Output:

```
ESLint upgraded safely
```

## Read release notes

ESLint releases often include breaking changes and new rules. Read the release notes to understand what changed and plan your migration.

Quick check

**Quiz:** How should you upgrade ESLint in a team?

- [x] Test locally, fix issues gradually, use --max-warnings during transition.
- [ ] Upgrade immediately on all machines.
- [ ] Never upgrade, stay on old versions.
- [ ] Delete eslint.config.js

*Answer:* Test locally, fix issues gradually, use --max-warnings during transition.. Gradual migration prevents disruption and gives the team time to adapt to new rules.