What is a Monorepo?

A monorepo is a collection of many different apps and packages in a single codebase.

The alternative setup is called a polyrepo - multiple codebases which are published and versioned separately.

Sharing code

In a polyrepo

In a polyrepo setup, the process for sharing code between applications is relatively lengthy.

Imagine that you have three separate repositories - app, docs, and shared-utils. Both app and docs depend on shared-utils, which is published as a package on npm.

Let's say a bug in shared-utils is causing a critical issue in both app and docs. You'll need to:

1. Make a commit in shared-utils fixing the error
2. Run a publish task inside shared-utils to publish it to npm
3. Make a commit in app bumping the version of the shared-utils dependency
4. Make a commit in docs bumping the version of the shared-utils dependency
5. app and docs are now ready to be deployed.

The more apps you have that depend on shared-utils, the longer this process takes. It can be extremely arduous.

In a monorepo

In a monorepo setup, shared-utils would be in the same codebase as app and docs. This makes the process very simple:

1. Make a commit in shared-utils fixing the error
2. app and docs are now ready to be deployed.

No versioning is required, because app and docs don't depend on the version of shared-utils in npm - they depend on the version that's in the codebase.

This makes it possible to create single commits which fix bugs in multiple apps and packages at once. This can be an enormous gain in speed for teams.

How do monorepos work?

The main building block of the monorepo is the workspace. Each application and package you build will be in its own workspace, with its own package.json. As you'll learn from our guide, workspaces can depend on each other, meaning your docs workspace can depend on shared-utils:

{
  "dependencies": {
    "shared-utils": "*"
  }
}

Workspaces are managed by the same CLI which installs your dependencies.

The root workspace

You'll also have a root workspace - a package.json in the root folder of your codebase. This is a useful place for:

1. Specifying dependencies which are present across your entire monorepo
2. Adding tasks that operate on the whole monorepo, not just individual workspaces
3. Adding documentation on how to use the monorepo