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Sample repo using lerna with TypeScript project references
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README.md

Lerna + Project References

This is a "bare minimum" repo that shows one way to configure TypeScript Project References with lerna. There are a lot of different ways you can set things up and this isn't intended to be authoratitive guidance or exclusionary of other ways that might work better in your project.

Setting up this repo

> git clone https://github.com/RyanCavanaugh/learn-a.git
> cd learn-a
> npm install
> lerna bootstrap
> tsc -b packages

Note that you'll need a 3.0 version of tsc (currently available at npm install -g typescript@next).

General Structure

As with a normal lerna repo, there's a packages folder. Inside we have three creatively named packages pkg1, pkg2, and pkg3.

packages/
| tsconfig.settings.json
| tsconfig.json
| pkg1/
  | tsconfig.json
  | src/
  | | (typescript files)        
  | lib/
  | | (javascript files)
  | | (.d.ts files)
| pkg2/
  | (same as pkg1)
| pkg3/
  | (same as pkg1)

Let's review each file in the repo and explain what's going on

tsconfig.settings.json

{
    "compilerOptions": {
        // Always include these settings
        "composite": true,
        "declaration": true,
        "declarationMap": true,
        "sourceMap": true,

        // These settings are totally up to you        
        "esModuleInterop": true,
        "target": "es5",
        "module": "commonjs",
        "strict": true
    }
}

This file contains the "default" settings that all packages will use for compilation. You will definitely want the composite, declaration, declarationMap, and sourceMap settings enabled for all projects, so include those in this file. Other settings, like target and strict, can be specified here if you'd like to enable them by default. You'll also be able to override these settings on a per-package basis if needed.

tsconfig.json

{
    "files": [],
    "references": [
        { "path": "pkg1" },
        { "path": "pkg2" },
        { "path": "pkg3" }
    ]
}

This file is pretty simple - simply list the packages that need to be built with tsc in the references array. You should also include "files": [] in this file - this will prevent an incorrect invocation of tsc without -b from trying to build the entire packages folder source files as one compilation (which will fail, but drop a bunch of .js files in random places as an annoying side effect).

packages/pkg2/tsconfig.json

We'll just cover one of the pkg1 / pkg2 / pkg3 packages since they're basically identical for the purposes of this demo. Here's pkg2's tsconfig.json:

{
  "extends": "../tsconfig.settings.json",
  "compilerOptions": {
    "outDir": "lib",
    "rootDir": "src"
  },
  "references": [
    { "path": "../pkg1" }
  ]
}

The extends property pulls in the settings we wrote in tsconfig.settings.json, so we don't have to duplicate any settings described there.

In compilerOptions, we've set outDir to lib and rootDir to src, then placed all my .ts files in src. This means src/index.ts will build to lib/index.js and lib/index.d.ts. This is also the place where you could override settings like strict or target if you needed to change them on a per-project basis.

In the references array, we list the paths to the other projects' tsconfig.json files (or containing folders, as shown here). This will both ensure that we locate the .d.ts files correctly, and set up a proper build ordering.

packages/pkg2/src/index.ts

import * as p1 from '@ryancavanaugh/pkg1';

export function fn4() {
    p1.fn();
}

Nothing unusual going on here. We import and export with the usual syntax. Notably, if you open this repo in an editor, you can still "Go to Definition (F12)" on p1.fn here and land in pkg1/foo.ts - the original sourcecode - even though "under the covers" it's using the much faster .d.ts file for typechecking.

packages/pkg2/package.json

Here are the relevant excerpts from the package.json:

{
  "main": "lib/index.js",
  "typings": "lib/index.d.ts",
  "scripts": {
    "prepublishOnly": "tsc -b ."
  },
  "devDependencies": {
    "typescript": "^3.0.0-dev.20180626"
  }
}

Because we build to lib, we need to set main to the .js file there and typings to the .d.ts file.

In scripts, we use the local copy of tsc (listed here as a dev dependency) to run a build mode compilation on the project. This will ensure that the lib folder is always built before npm publish, and blocks any publishes that try to push non-compiling code.

packages/pkg2/.npmignore / packages/pkg2/.gitignore

.gitignore

lib/

.npmignore

# Empty, but needs to exist

The .gitignore stops us from checking in build outputs, which is generally a good idea. By default, npm won't publish files that are ignored by git, so we need a separate .npmignore file so that the lib folder still gets published!

Workflow

All your lerna commands and workflow will work as expected here.

To build the TypeScript projects, you can run individual builds with tsc -b:

 > tsc -b packages/pkg3

Or just build everything:

 > tsc -b packages
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