A tool for developing javascript in monorepos more documentation here
Mr Builder is a "Mono Repo" build tool. Its designed to be declarative way of quickly creating and managing builds within a mono repo. In particular JS/ES6+ modules.
While lerna concerns itself with the packaging it does not define much in the way of writing apps in a modern javascript pipeline. While Mr Builder is designed to be declarative its also meant to leverage existing knowledge of babel, webpack, mocha and other tools that while all great, may not always work together easily.
Large projects often benefit from being broken into smaller more independent ;more testable modules. However this causes a new pain point trying to test and maintain different builds and build systems. While webpack et al, offer a rich plugin ecosystem, for each project a new configuration must be enabled. Mr Builder allows you to make a plugin for these configuration and reuse them for all the modules; often without having to modify the modules themselves.