Learn TypeScript w/ Mike North

Local Packages

October 27, 2023

Table of Contents

A need for modular architecture

As a monolithic project starts to get larger and larger, builds get slower, and the need for some sort of modular architecture starts to become more important. Thankfully, it’s easier than ever to do this with TypeScript and yarn!

We’ve already taken the first steps by creating the chat-stdlib project at the beginning of this course. This package exports the Deferred class and the stringifyError function.

Introducing the dependency

Yarn 2+ includes support for workspaces — individual packages that are part of the same project. The root level package.json in our course repo contains the following configuration

"workspaces": [

This indicates that every folder in packages/ is to be treated as a workspace.

Next, go to packages/chat/package.json, and add the following to the dependencies object

"chat-stdlib": "workspace:*"

The * here isn’t really important for our purposes, since we’re not publishing this chat package to npm.

Once you’ve added this line to chat’s package.json, be sure to save the file and then run yarn

Consuming the dependency

chat has a couple of modules that we won’t need anymore

  • packages/chat/src/utils/deferred.ts
  • packages/chat/src/utils/error.cjs
  • packages/chat/tests/utils/deferred.test.ts
  • packages/chat/tests/utils/error.test.ts

Feel free to delete these, as they’re all represented by code we already have in chat-stdlib. You’ll have to update a couple of import statements as a result of this

in packages/chat/src/utils/networking.ts

- import { stringifyError } from './error'
+ import { stringifyError } from 'chat-stdlib'

in packages/chat/src/ui/components/api.ts

- import { Deferred } from './deferred'
+ import { Deferred } from 'chat-stdlib'

Using “beta” types

You may notice that stringifyError types don’t seem to be available.

Earlier, when we wrote chat-stdlib, we added a @beta JSDoc comment tag to stringifyError

You’ll need to add this entry to the top of your tsconf.json compilerOptions.paths property

"chat-stdlib": ["../../node_modules/chat-stdlib/dist/chat-stdlib-beta.d.ts"],

You should now be able to run

yarn typecheck && echo "passed type checking"

and see it return successfully.

Focusing on the small stuff

One of the advantages of this kind of modular architecture, is we can focus on a particular sub-part of our repo, installing only the dependencies necessary to run that part.

If you want to test this, start by going to the root of the project and removing your node_modules folder

Next, let’s run

yarn workspaces focus chat-stdlib

Check out the root node_modules folder now. You should see a very small number of dependencies. You have just enough in there to build, test and run the one package.

If you want to go back to installing all of the dependencies needed by the whole repo, just run


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