eslint-compat-utils

Provides a backward-compatible API for ESLint custom rules that works with both ESLint v9 (flat config) and older ESLint versions (v6–v8). Current stable version is 0.6.5, released in 2024. The package is experimental but actively maintained with monthly patches. Key differentiator: it offers polyfills for new ESLint APIs (like getSourceCode, getScope, RuleTester shims) so rule authors can write modern code without dropping support for older ESLint installations. Ships TypeScript definitions.