How to build TypeScript mono-repo project

Tools

yarn: NPM client.
Lerna: Multiple packages management tool.

Directory Structure

Put each package under the packages directory.

.
├── README.md
├── lerna.json
├── package.json
├── packages
│   ├── x-cli
│   │   ├── lib
│   │   │   ├── main.d.ts
│   │   │   ├── main.js
│   │   │   └── main.js.map
│   │   ├── package.json
│   │   ├── src
│   │   │   └── main.ts
│   │   ├── tsconfig.build.json
│   │   └── tsconfig.json
│   └── x-core
│       ├── lib
│       │   ├── index.d.ts
│       │   ├── index.js
│       │   └── index.js.map
│       ├── package.json
│       ├── src
│       │   └── index.ts
│       ├── tsconfig.build.json
│       └── tsconfig.json
├── tsconfig.base.json
├── tsconfig.json
└── yarn.lock

Workspaces

Using yarn workspace feature, configure the following files:

/package.json

Append the workspaces key.

{
  "private": true,
  "workspaces": [
    "packages/*"
  ]
}

lerna.json

Set npmClient "yarn" and turn useWorkspaces on.

{
  "lerna": "2.2.0",
  "packages": [
    "packages/*"
  ],
  "npmClient": "yarn",
  "useWorkspaces": true,
  "version": "1.0.0"
}

Exec yarn install(or lerna bootstrap). After successful running, all dependency packages are downloaded under the repository root node_modules directory.

Dependencies between packages

In this example, the x-cli package depends on another package, x-core. So to execute (or test) x-cli, x-core packages should be installed. yarn solve it. This command create sim-link of each packages into the top-level node_modules dir.

Resolve Dependencies as TypeScript Modules

As mentioned above, Lerna resolves dependencies between packages. It's enough for "runtime". However considering writing TypeScript sources, it's not.

For example, the following code depends a module x-core located at other package.

/* packages/x-cli/src/main.ts */
import { awesomeFn } from "@quramy/x-core";

export function cli() {
  awesomeFn();
  return Promise.resolve(true);
}

If you compile this code, TypeScript compiler emits a "Cannot find module" error until building x-core package and creating x-core/index.d.ts. And it's silly to compile dependent packages(e.g. x-core) in the same repository after each editing them.

TypeScript's path mapping is the best solution. Path mappings are declared such as:

/* tsconfig.json */
{
  "extends": "./tsconfig.base.json",
  "compilerOptions": {
    "baseUrl": "./packages",
    "paths": {
      "@quramy/*": ["./*/src"]
    }
  }
}

The above setting means import { awesomeFn } from "@quramy/x-core" is mapped to import { awesomeFn } from "../../x-core/src"(it's relative from "packages/x-cli/src/main.ts"). In other words, path mapping allows to treat developing packages' sources as published(compiled) modules.

alexkrautmann / lerna-yarn-workspaces-example