TypeScript + Yarn Workspace Monorepo

When you have serveral highly coupled projects which you want to organize them together, you can consider a monorepo.

Yarn (1.x) provide the workspace feature to help you organize monorepo project.

Yarn workspace has serveral advantages like:

• Hoist same dependecies to top level to avoid duplicate install.
• Upgrade dependencies is much more easier.
• Easy to run a same script for all projects.

A typical monorepo is a backend api and frontend SPA project.

This demo show only the steps to setup the shared part and the backend part.

The source code can find in https://github.com/t7yang/ts-yarn-workspace-demo

File Structure

.
├── backend
│   ├── package.json
│   ├── src
│   │   └── index.ts
├── shared
│   ├── package.json
│   ├── src
│   │   ├── index.ts
├── package.json

How to

Setup yarn workspace in package.json

Yarn workspace must set private to true. In workspaces field you either explicitly list the path of projects or put all projects under a directory (said packages) then write as packages/* in workspaces field.

{
  "private": true,
  "workspaces": [
    "shared",
    "backend"
  ]
}

Add shortcut for projects (optional)

You don't need cd into each project to run a script, simply run yarn workspace 'workspacename' 'scriptname' in project's root.

A shortcut can make this easier:

  ...
  "scripts": {
    "backend": "yarn workspace backend",
    "be": "yarn workspace backend",
    "shared": "yarn workspace shared"
  }

Then you can run script in more shorter way, the scripts below is equivalance.

$ yarn workspace backend start
$ yarn backend start
$ yarn be start

Add TypeScript dependency

Just like the scripts above, adding dependency to a workspace is:

$ yarn workspace shared add -D typescript
$ yarn workspace backend add -D typescript

Run yarn install again in root project, yarn will hoist the same dependencies among sub projects to the top level node_modules.

Setting tsconfig.json

A share tsconfig.json can place in the root project:

{
  "compilerOptions": {
    /* Basic Options */
    "target": "es5",
    "module": "commonjs",
    "lib": ["ESNext"],

    /* Strict Type-Checking Options */
    "strict": true,

    /* Module Resolution Options */
    "moduleResolution": "node",
    "esModuleInterop": true,

    /* Advanced Options */
    "forceConsistentCasingInFileNames": true,
    "skipLibCheck": true
  }
}

For the shared sub project, composite: true field is required, other fields is optional but highly recommended, especially outDir and rootDir.

{
  "compilerOptions": {
    /* Basic Options */
    "declaration": true,
    "declarationMap": true,
    "sourceMap": true,
    "composite": true,
    "outDir": "dist",
    "rootDir": "src"
  },
  "include": ["src"]
}

Due the transpiled code of shared sub project is dist, point the main and types fields in package.json to dist.

{
  ...
  "main": "dist/index.js",
  "types": "dist/index.d.ts",
  ...
}

For the backend sub project, references field is required. The path field should point to the folder contain tsconfig.json or the tsconfig.json itself (here we point to the folder for short).

{
  "compilerOptions": {
    /* Basic Options */
    "outDir": "dist",
    "rootDir": "src"
  },
  "include": ["src"],
  "references": [{ "path": "../shared" }]
}

Start Coding

After the routine setup, now we can start coding. Add some code in shared/src, don't forget to export them to shared/src/index.ts

// shared/src/user.ts
export interface User {
  name: string;
  age: number;
}

export const createUser = (name: string, age: number): User => ({ name, age });

export const showUser = (user: User) => console.log(`${user.name} is ${user.age} years old.`);

// shared/src/index.ts
export * from './user';

Import the code from shared in backend files, if everything is setup correctly, vscode will not complain and recognize the imports properly.

// backend/src/index.ts
import { createUser, showUser, User } from 'shared';

const user: User = createUser('t7yang', 18);

showUser(user);

Add Script and Run

Finally, add the build and start script for backend.

{
  ...
  "scripts": {
    "build": "tsc --build",
    "start": "yarn build && node dist/index.js"
  },
  ...
}

Run yarn backend start in root project, the backend project will transpile and run correctly.

$ yarn backend start
yarn run v1.22.4
$ yarn workspace backend start
$ yarn build && node dist/index.js
$ tsc --build
t7yang is 18 years old.

You may curious why we don't have to build shared before we build backend.

tsc is smart enough to transpile the project which referenced by backend and transpile them if necessary before transpile itself.

Summary

By combining TypeScript project references and Yarn workpsace, we can setup a monorepo fast and easy.

If you need more powerful monorepo features, consider to use lerna or Yarn v2 Workspace.

References:

• Yarn Workspace
• TypeScript Project References