Imagine that instead of a small app, you need to maintain a huge platform consisting of a lot of functional areas. If you are thinking about architecture, you will want to do two main things: Separate concerns and avoid code dupes.

• One place to store all configs and tests.
• Easily refactor global features with atomic commits.
• Simplified package publishing.
• Easier dependency management.
• Re-use code with shared packages while still keeping them isolated.

• No way to restrict access only to some parts of the app.

• Poor Git performance when working on large-scale projects

  Note: Nowadays, Facebook tries to resolve issues with VCS scalability by patching Mercurial and, probably soon, this won’t be such a big issue.

• Higher build time.

• Basel is Google’s monorepo-oriented build system.
• Yarn is a JavaScript dependency management tool that supports monorepos through workspaces.
• Lerna is a tool for managing JavaScript projects with multiple packages, built on Yarn. Lerna is focused on linking multiple packages from the same project and managing npm publishing.
• Nx is an extensible dev tool for monorepos. It's focused on managing development workflow for multiple packages. Nx doesn't rebuild and retest everything on every commit - it only rebuilds what is necessary.

-Guide to Monorepos for Front-end Code