JeffJS
Quantum JavaScript Engine
About
A powerful JavaScript REPL for all Apple devices, offering full ES2023 support and a browser-like runtime. Experiment with code, test scripts, and even run JavaScript on your Apple Watch.
What's New in JeffJS
1.0
April 28, 2026
Developer apps
FAQ
What devices does JeffJS support?
JeffJS is available on iPhone, iPad, Apple Watch, Mac, and Vision Pro. The app leverages Swift's cross-platform capabilities to run the same engine across these devices.
What JavaScript features does JeffJS support?
JeffJS offers full ES2023 support, including classes, modules, async/await, destructuring, optional chaining, BigInt, Symbol, Proxy, and dynamic import(). It also includes advanced features like RegExp with named capture groups.
Does JeffJS have ads?
No, JeffJS is ad-free. As a paid application, it provides an uninterrupted experience for users.
How much does JeffJS cost?
JeffJS is a paid application priced at $0.99. This one-time purchase grants access to all features and future updates.
How often is JeffJS updated?
The latest version, 1.0, was released on April 28, 2026, with the initial release on April 27, 2026. Updates are likely to follow based on developer activity and user feedback.
Is JeffJS suitable for beginners learning JavaScript?
Yes, JeffJS is excellent for anyone learning JavaScript. It allows users to experiment with code directly on their devices without needing a full development environment.
Can I run JavaScript on my Apple Watch with JeffJS?
Absolutely. JeffJS includes a dedicated Watch App that runs the same engine as the phone version, allowing you to execute JavaScript directly on your wrist. It even features complications to display recent eval results.
What makes JeffJS's engine unique?
JeffJS uses a pure-Swift engine, a 1:1 port of QuickJS, avoiding JavaScriptCore or FFI. It offers a browser-shaped runtime with features like `window`, `document`, and `fetch` available out-of-the-box.