CTjs is a pure‑ES6 JavaScript library that implements all the data structures and verification algorithms required to interact with Certificate Transparency (CT) logs, supporting both the original RFC 6962 (v1) and the updated RFC 6962‑bis (v2). It provides tools for building CT monitors/auditors—such as inclusion‑proof, consistency‑proof, Signed Tree Head and SCT verification—and includes ready‑to‑run examples, making it useful for security engineers, developers, and researchers who need to validate or audit CT logs in Node.js or browser environments.