ESLint

ESLint is the standard linter for JS/TS. v9 (2024/2025) moved to Flat Config (eslint.config.js), a major breaking change that simplifies configuration.

When to Use

• Enforcing Rules: no-unused-vars, react-hooks/rules-of-hooks.
• Code Quality: Catching bugs before they run.

Core Concepts

Flat Config (eslint.config.js)

An array of objects. No more extends string hell.

export default [js.configs.recommended, { rules: { semi: "error" } }];

Plugins

NPM packages that add rules. eslint-plugin-react.

Parsers

@typescript-eslint/parser allows ESLint to understand TS.

Best Practices (2025)

Do:

• Use Flat Config: Use the new format.
• Use typescript-eslint: The typed linting rules are powerful (e.g. no-floating-promises).

Don't:

• Don't configure formatting rules: Disable all formatting rules (use Prettier or Biome for that). use eslint-config-prettier.

References

• ESLint Documentation