Yalc is a simple local JavaScript package repository and a tool for publishing and using packages across a local development environment. It's a more reliable alternative to the npm link command, which has some constraints. Yalc is useful when working with multiple packages, especially when some use yarn and some use npm. It's also helpful for testing the packages locally before publishing them to a remote registry. In our experience, Yalc is valuable in a multi-package setup and speeds up front-end and other JavaScript app development workflows.