Isomorphic App Router is a tiny, framework‑agnostic routing library for single‑page applications that lets you define routes and generate type‑safe URLs using pure JavaScript. It works with any history implementation (e.g., the `history` package) and supports both the native `URLPattern` API and `path‑to‑regexp` for matching, making it suitable for developers who want a dependency‑free, testable router that can be integrated into any front‑end framework.