Lerna tutorial

Forked from lerna-tutorial

First off, What is lerna? lerna is a tool that allows you to maintain multiple npm packages within one repository.

There's a couple of benefits to this kind of approach, the paradigm is called a monorepo, and more can be read about it from the source of babel, and react.

Here's the gist:

• Single lint, build, test and release process.
• Easy to coordinate changes across modules.
• Single place to report issues.
• Easier to setup a development environment.
• Tests across modules are ran together which finds bugs that touch multiple modules easier.

Getting started.

For this demo I'm going to install lerna is a CLI (command line interface) tool. You're going to want to install it with the --global (-g) flag.

npm i lerna -g

Then once it's done installing your going to want to run the following

lerna init

This will do a couple of things.

• Creating packages folder.
• Updating package.json.
• Creating lerna.json.

The /packages folder is where all of your packages belong. Let's go about making a new package aa-alpha.

cd packages
mkdir aa-alpha
cd aa-alpha
npm init -y
echo "module.exports = 'aa-alpha'" > index.js

Lets go through the same steps for another package aa-beta.

First go up one directory:

cd ..

Now go about creating aa-beta.

mkdir aa-beta
cd aa-beta
npm init -y
echo "module.exports = 'aa-beta'" > index.js

Now we're going to create a usage package that uses both aa-alpha and aa-beta as dependencies.

First go up one directory:

cd ..

Now go about creating \usage.

mkdir usage
cd usage
npm init -y
touch index.js

Open up /packages/usage/index.js in a text editor and paste this in.

var aa-alpha = require('aa-alpha')
var aa-beta = require('aa-beta')
console.log(aa-alpha + " " + aa-beta)

We're almost there. At this point your whole project should look something like this:

.
├── README.md
├── lerna.json
├── package.json
└── packages
    ├── aa-alpha
    │   ├── index.js
    │   └── package.json
    ├── aa-beta
    │   ├── index.js
    │   └── package.json
    └── usage
        ├── index.js
        └── package.json

What you want to do now is go into /packages/usage/package.json and add these lines under dependencies.

{
  "dependencies": {
    "aa-alpha": "1.0.0",
    "aa-beta": "1.0.0"
  }
}

Now you need to wire everything up with this command.

lerna bootstrap

The output from this command should look something like this:

Lerna v2.0.0-aa-beta.20
Linking all dependencies
Successfully bootstrapped 3 packages.

Now using the tree command once more (brew install tree) we can see the folder structure we can see what lerna did.

.
├── README.md
├── lerna.json
├── package.json
└── packages
    ├── aa-alpha
    │   ├── index.js
    │   ├── node_modules
    │   └── package.json
    ├── aa-beta
    │   ├── index.js
    │   ├── node_modules
    │   └── package.json
    └── usage
        ├── index.js
        ├── node_modules
        │   ├── aa-alpha
        │   │   ├── index.js
        │   │   └── package.json
        │   └── aa-beta
        │       ├── index.js
        │       └── package.json
        └── package.json

It added two stubbed (my term not lerna's) modules. If you peak inside /packages/usage/node_modules/aa-alpha/index.js you can see what I mean.

contents of ./packages/usage/node_modules/aa-alpha/index.js

module.exports = require("/Users/user/Desktop/lerna-tutorial/packages/aa-alpha");

Note: This is an absolute path to the module. So if you ever move your lerna project you'll need to rerun lerna bootstrap.

And volia! When we run node ./packages/usage/index.js we get our expected output!

aa-alpha aa-beta