SQLite JS is an extension that integrates JavaScript functionality into SQLite, allowing users to create custom functions, aggregates, and collations directly within their databases. It supports various types of functions including scalar, aggregate, window functions, and enables JavaScript evaluation, enhancing data manipulation capabilities. Installation instructions and examples for using the extension across different platforms are provided for developers.

Antoine shares his experiences developing a local-first app that syncs a SQLite database with a remote server, focusing on challenges and solutions regarding reactivity and performance. After experimenting with various tools, he outlines a minimal sync engine tailored for a single-player notes application.