getting-started.md

id	title	type
getting-started	Getting Started	tutorial

Getting Started

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You can incrementally adopt Lerna for existing monorepos or create a new Lerna workspace by running:

npx lerna init

All Lerna functionality will work the same way regardless.

This tutorial will give you an introduction to Lerna's features. We will use this repository.

To start the tutorial, clone the repo, check out the prelerna branch and follow along!

git clone https://github.com/lerna/getting-started-example.git
cd getting-started-example
git checkout prelerna
npm install

The repo contains three packages or projects:

• header (a library of React components)
• footer (a library of React components)
• remixapp (an app written using the Remix framework which depends on both header and footer)

packages/
    header/
        src/
            ...
        package.json
        rollup.config.json
        jest.config.js

    footer/
        src/
            ...
        package.json
        rollup.config.json
        jest.config.js

    remixapp/
        app/
            ...
        public/
        package.json
        remix.config.js

package.json

Adding Lerna

To add Lerna run the following command:

npx lerna@latest init

This will generate lerna.json and will add lerna to the root package.json.

{
  "name": "root",
  "private": true,
  "workspaces": ["packages/*"],
  "devDependencies": {
    "lerna": "5.1.6"
  }
}

What makes Lerna 5.1+ so powerful is the task delegation and other features that come with its integration with Nx.

Visualizing Workspace

By having Nx installed alongside Lerna, you can use its capabilities to open an interactive visualization of the workspace project graph.

To open the visualization, run:

npx nx graph

Bootstrapping Projects

Bootstrapping is one of the three main key features of Lerna. It enables different projects in the same repository to import each other without having to be published to a registry.

To see how it works, let for example inspect the package.json file of remixapp.

{
  ...
  "dependencies": {
    "@remix-run/node": "^1.5.1",
    "@remix-run/react": "^1.5.1",
    "@remix-run/serve": "^1.5.1",
    "react": "^17.0.2",
    "react-dom": "^17.0.2",
    "header": "*",
    "footer": "*"
  }
}

The "header": "*" and "footer": "*" tell Lerna to link the contents of the header and footer as if they were published to the registry. To do that, we need to run:

npx lerna bootstrap

Now all the projects in the workspace can properly reference each other so that they can now be built.

Building All Projects

To build all projects, run

npx lerna run build

This builds the three projects in the right order: header and footer will be built first (and in parallel), and remixapp will be built after. The order matters because the remixapp uses the bundles from the compiled header and footer.

    ✔  header:build (501ms)
    ✔  footer:build (503ms)
    ✔  remixapp:build (670ms)

 —————————————————————————————————————————————————————————————————————————————

 >  Lerna (powered by Nx)   Successfully ran target build for 3 projects (1s)

Testing All Projects

Now, let's run the tests.

npx lerna run test

You should see the following output:

    ✔  footer:test (1s)
    ✔  header:test (1s)
    ✔  remixapp:test (236ms)

 ——————————————————————————————————————————————————————————————————————————————

 >  Lerna (powered by Nx)   Successfully ran target test for 3 projects (1s)

Note, lerna will run the three test npm scripts in the topological order as well. Although we had to do it when building, it isn't necessary for tests (and it also makes the command slower). We can change this behavior by configuring caching.

Caching

Running any command right now will execute all the tasks, every time, even when nothing changes. We can fix it by adding a bit of configuration.

First, let's run

npx lerna add-caching

This will take you through a series of prompts to configure your caching:

? Which of the following scripts need to be run in deterministic/topoglogical order?
 (Press <space> to select, <a> to toggle all, <i> to invert selection, and <enter> to proceed)
❯◉ build
 ◯ test
 ◯ dev
 ◯ start

? Which of the following scripts are cacheable? (Produce the same output given the same input, e.g. build, test and lint usually are, serve and
start are not)
 (Press <space> to select, <a> to toggle all, <i> to invert selection, and <enter> to proceed)
 ◉ build
❯◉ test
 ◯ dev
 ◯ start

? Does the "build" script create any outputs? If not, leave blank, otherwise provide a path relative to a project root (e.g. dist, lib, build,
coverage)
 dist
? Does the "test" script create any outputs? If not, leave blank, otherwise provide a path relative to a project root (e.g. dist, lib, build,
coverage)

This generates an nx.json at the root of your workspace:

{
  "tasksRunnerOptions": {
    "default": {
      "runner": "nx/tasks-runners/default",
      "options": {
        "cacheableOperations": ["build", "test"]
      }
    }
  },
  "targetDefaults": {
    "build": {
      "dependsOn": ["^build"],
      "outputs": ["{projectRoot}/dist"]
    }
  }
}

This configuration caches build and test tasks and forces build to run in topological order (but test will not). Also each project's dist folder defaults to being cached as the build output.

Now, let's run tests on the header project twice. The second time the operation will be instant:

npx lerna run test --scope=header

> lerna run test --scope=header

> header:test  [existing outputs match the cache, left as is]

> header@0.1.0 test
> jest

PASS  src/Header.spec.tsx
✓ renders header (12 ms)

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        0.439 s, estimated 1 s
Ran all test suites.

———————————————————————————————————————————————————————————————————————————————————————————————————————————————————————

>  Lerna (powered by Nx)   Successfully ran target test for project header (4ms)

   Nx read the output from the cache instead of running the command for 1 out of 1 tasks.

Lerna (powered by Nx) was able to recognize that the same command has already executed against the same relevant code and environment, so instead running it Lerna restored the necessary files and replayed the terminal output.

We have specified the default build output as the dist folder, which works for header and footer. However, in the case of the Remix app we need to specify the output folder as build by adding the following section to packages/remixapp/package.json.

{
  "nx": {
    "targets": {
      "build": {
        "outputs": ["{projectRoot}/build", "{projectRoot}/public/build"]
      }
    }
  }
}

NOTE: "{projectRoot}" is a special syntax supported by the task-runner, which will be appropriately interpolated internally when the command runs. You should therefore not replace "{projectRoot}" with a fixed path as this makes your configuration less flexible.

Caching not only restores the terminal output logs, but also artifacts that might have been produced.

Build all the projects, then remove the remix build folder and run the build command again.

npx lerna run build
rm -rf packages/remixapp/public/build

You will see all the files restored from cache and the command executing instantly.

    ✔  header:build  [existing outputs match the cache, left as is]
    ✔  footer:build  [existing outputs match the cache, left as is]
    ✔  remixapp:build  [local cache]

 ———————————————————————————————————————————————————————————————————————————————————————————————————————————————————————

 >  Lerna (powered by Nx)   Successfully ran target build for 3 projects (19ms)

    Nx read the output from the cache instead of running the command for 3 out of 3 tasks.

Lerna also supports distributed caching and config-free distributed task execution.

Target Dependencies (aka task pipelines)

We have made good progress, but there is one problem left to be solved. The following configuration in nx.json is incomplete:

{
  "targetDefaults": {
    "build": {
      "dependsOn": ["^build"]
    }
  }
}

This ensures that build dependencies are run before any build command, but we also need to remember to build header and footer before we run lerna run dev --scope=remixapp. We can fix that by defining dependencies between targets (also known as task pipelines) in the nx.json:

{
  ...
  "targetDefaults": {
    "build": {
      "dependsOn": [
        "^build"
      ]
    },
    "dev": {
      "dependsOn": [
        "^build"
      ]
    }
  }
}

Note, older versions of Nx used targetDependencies instead of targetDefaults. Both still work, but targetDefaults is recommended.

With this change:

• npx lerna run build will run the build targets in the right order.
• npx lerna run dev --scope=remixapp will run the build targets for header and footer first and then run the dev target for remixapp.
• npx lerna run test will run all the three test targets in parallel.

If you are wondering whether it is slow to run lerna run dev --scope=remixapp given that you have to rebuild all the dependencies all the time, the answer is "no". The dependencies will be rebuilt only when they change. Otherwise, their dist folders will be kept as is.

Publishing

Finally, let's talk about the third key Lerna feature: publishing to npm. Lerna comes already with a publish command built-in. To publish our packages header and footer, all we need to do is to run:

npx lerna publish --no-private

This will

• determine the current version of the packages
• detect which packages have changed since the last publishing & then update its version in package.json accordingly
• create a commit of the changed package.json files, tag the commit and push the tag & commit to the remote
• publish the packages to NPM

Read more about the publishing and versioning process in the corresponding docs page.