Yarn is another package manager for JavaScript that Facebook developed as a more efficient and reliable alternative to npm. It was released in 2016, several years after npm.

Here are a few features of Yarn:

1. Speed and Efficiency. Yarn is known for its speed and efficiency. It uses a deterministic installation algorithm that ensures the exact dependencies are installed in the same structure every time.
2. Offline Mode. Yarn has a feature where it can install packages from a local cache if they have been downloaded before, making it possible to work offline.
3. Security. When installing packages, Yarn first suggestions their checksums for integrity before actual code execution takes place.
4. Workspaces. Yarn has a native feature called "workspaces" to manage mono repo (mono repository) architectures, which can make it easier to manage projects with multiple sub-packages more accessible.
5. Compatibility. Yarn supports the npm registry, so you can access the many libraries that npm hosts.

Comparison between npm and Yarn

• Performance. When Yarn was first released, one of its main advantages over npm was its performance. Yarn offered faster package installation. However, since npm version 5, the performance gap has significantly reduced.
• Security. Yarn has the edge over npm because it uses checksums for every package before they are installed, ensuring its integrity.
• Offline mode. Yarn can reuse previously downloaded, making it possible to install packages without an internet connection, which npm can't do.
• Workspaces. While both package managers support managing multiple packages within the same top-level root package, Yarn's workspace feature is more advanced and configurable.
• Lockfile. Both npm and Yarn generate a lock file (package-lock.json for npm, yarn. lock for Yarn), which helps lock dependencies down to a specific version, making installations more reliable between different systems.
• Documentation. npm tends to have better and more comprehensive documentation than Yarn.