ESLint 9 is the latest major version of the popular JavaScript linter, ESLint, released to bring significant improvements, new features, and some breaking changes. With each major version, ESLint enhances performance, provides new configuration options, and updates rule behaviors to help developers catch bugs, enforce consistent code style, and maintain code quality.

In version 9, ESLint has introduced the Flat Config system as the default way to configure ESLint, moving away from the older .eslintrc file format. Flat Config aims to streamline configuration by providing a single configuration file format, eslint.config.js, and simplifying rule, plugin, and parser management. Other improvements include native support for ECMAScript modules (ESM), optimized file matching, and improved defaults to support modern JavaScript development patterns.

Key changes in ESLint 9:

Flat Config as the default: Simplifies configuration with eslint.config.js as the single format.
Better default settings: Supports newer JavaScript syntax by default, like ECMAScript latest version and ESM.

Improved plugin and rule management: Makes it easier to set up and use ESLint with various plugins and rule sets.

Performance improvements: Faster linting, especially in large projects.

These updates make ESLint 9 a more streamlined tool for developers who work with modern JavaScript, improving both ease of setup and performance.