Hi, I am Bruno Paulino.
Software is my craft.

The TypeScript Monorepo Series

August 28, 2022

Welcome to the TypeScript monorepo article series. In this series, you will learn about what a monorepo is, why it’s becoming so popular today and how you can build your own production-ready monorepo with a Web application, a documentation website and a component library, all built with TypeScript.

If you are working on a small and isolated app, chances are that you probably don’t need a monorepo, but if you are building several different apps and small libraries that somehow are related or dependent on each other, specially if you are working on a large team, a monorepo is most probably the best strategy to organize your code. It will make your code much closer to the engineers working on them and will give your team a great productivity boost in the short and long-run by sharing common standards.

So bear with me and let’s do a deep dive into monorepos by creating a rock-solid version of it that you and your team is going to love to use. By the end of this series, you will have a complete TypeScript monorepo template to start your projects with the right foot, including dependency management, CI/CD with parallel task execution and caching to avoid unnecessary work, testing pipelines, commit strategy that drives automated releases with automatic changelog generation.

But what is a monorepo anyways?

Before jumping to definitions, let’s take a step back and understand how teams usually go about creating new software projects and how they manage their codebases.

Teams across the globe need a way to share code. And the de facto standard is Git. Along this series, we will create a git repo for a hypothetical use-case where we need a Web app while we discuss the challenges that a team might face along the way.

When starting out, you usually create a git repository offline on your machine and eventually you share your code with your team using the platform of your choice, the most popular being Github. This repository is usually the house of your app. It’s where it lives your business logic, build and deployment pipelines, automated tests (hopefully) and the history of your changes.

When starting a new project, you usually do the following:

  • Create a new Git repo
  • Setup your project from scratch or based on a template with build, test and deployment pipelines
  • Configure tools to help you with your development workflow, for instance a dev server.

Once the initial setup is done, you go about your work building your Web app for your users.

Now we need a component library

Things are all good and fine, but now your team needs to build a component library with common standards like buttons, colors, spacing, fonts and all the visual identity of your brand because your product team needs a new Web app to manage its customers and one of the requirements is that it must look and feel like your current one.

This component library uses the same technologies you are using to build your Web app like TypeScript and a very similar, but different, build pipeline. So to start working on this new component library, you usually have two choices:

  1. Build the component library as a subdirectory of your app, just like any other shared module.
  2. Decouple your component library from your app in a separate repository so engineers can work independently on both the App and the Component library.

Let’s break these options down and explore their trade-offs next.

Option 1: Building as a subdirectory

Building the new component library within our existing Web app has clear advantages:

  • Your entire setup is already in place.

    • You can start creating your component library right away and make use of your testing and build pipelines. Your Web app can use these components with no ceremony.
  • Team collaboration is high

    • Having the team working on the Web app and Component library all together under the same repo will be a lot easier for collaboration. They can review each other’s PRs, quickly test changes locally without extra setup and share the same standards, significantly reducing mental overhead.

But there are a few drawbacks with this approach:

  • You cannot version your components.
    • Which means that if you make a breaking change on your Button component, your entire Web app needs to be changed in one go, otherwise you will not be able to build your app.
  • nobody else can use your component library.
    • Eventually, you might need to build an internal Web app to manage your customers and ideally this internal Web app could have the same visual identity as your public app. But given the Component library is tightly coupled with your Web app, you can’t easily share your components outside of your repo. Copy/paste is no fun.

Option 2: Creating a new repository

If the trade-offs of option 1 are not acceptable, you need an alternative way, so the usual route taken by most teams is to go and create a new repository and make your component library an independent package. This approach has several benefits:

  • You can version your components

    • You can now make breaking changes to your components and as long as you publish different versions of your library to a registry like npm, consumers of your component library can remain safe until they decide to upgrade to the newest version of your component library.
  • More than one app can use your components

    • Since you now publish your component library as an independent package, other apps can now use it just as any other library out there. They can quickly run npm install @acme/components and be ready to go.

But like any architecture choices, there are trade-offs we must consider:

  • You can no longer make sure that your app is using the latest version of your components

    • This might be a desired side-effect, so you can actually let you team adopt the new versions of your component library in an incremental way. On the other hand, if you don’t prioritize library upgrades, the change drift of what you have on your Web app and the newest version of your component library might be so large that upgrading can become a nightmare.
  • You now have two codebases to worry about

    • Let’s face it, setting up a project from the ground up, with all it’s build, test and deployment pipelines in a way that is ready for production and with great developer experience takes time. Now you have two repositories to keep dependencies and pipelines all updated and secure. This can become a major mental overhead for your team, specially if you cannot afford to have a dedicated team member or a platform team to support you with these tasks. Your Web app and Component library repositories will grow apart and they will eventually have divergent standards. Questions like “Which version of TypeScript we are using?” or “Which Node version we are using now?” will get harder to answer and even harder to make sure that everyone is at the same page.
  • Engineers cannot collaborate in the same codebase anymore

    • You now have two independent codebases where engineers need to implement new features, make changes, review pull-requests and push code to production. But now they are far apart in two different repositories, which means that engineers cannot review changes in a single place anymore. If they want to checkout these changes locally, they will need to have the repositories all setup locally and if it’s their first time working on the component library, they might need to prepare their local environment from scratch to be able to run and explore the code.

But what is the alternative?

With all the being said, there are plenty of trade-offs here that might be hard to reach to a compromise. Both options 1 and 2 mentioned above are perfectly reasonable, but it will really depend on the context you are working with.

The example given here is small in context, we are only considering a Web app and a component library. What if you are part of a larger organization, with dozens of engineers working on several different apps, component libraries and other smaller libraries that need to be shared across all these apps?

A monorepo might be your best choice

What if I told you that there was a way to keep all your apps and libraries under the same git repository and still keep them isolated from each other, behaving just like libraries, but with all the benefits of working under the same repo?

This is what a monorepo is. A set of tools and practices that allow you to work under the same repository, but still be able to leverage module isolation and team autonomy.

In the JavaScript/TypeScript ecosystem, this is possible due to a native feature called workspaces which allows us to manage independent packages nested under a top-level root package. You will see that applied in practice during this series, but this subtle difference has a huge difference:

  • Your team can now work together in the same repo

    • Which means that working across different apps and libraries is just transparent. No need to clone a different repo, check Node and package manager version or anything that could drift apart in your tech stack and standards. With a monorepo, we can guarantee that all apps and libraries are using the exact same version of every dependency.
  • Module-level isolation with libraries versioned and published individually

    • Given that packages are isolated, you can still work on them individually, making changes and publishing new versions to npm as you need. For packages that contain applications that require a deployment step to some cloud environment, it can be handled in the exact same way libraries are published, just with a different deployment workflow. You will see that being applied during this series.

With that being said, a monorepo isn’t a silver bullet. It does come with challenges and we will talk about them as we start building our monorepo example.

What is next

In the next article, we will start by creating our monorepo from scratch, setting up a Web app, a Component library and a Documentation website. All of them will be fully independent within their own packages and we will be able to build, test and deploy them independently.

Stay tuned for the next blogpost.