Nikolay Grozev

# Introduction

In a previous post we looked at Node.js and its package manager NPM. However, NPM has several shortcomings in terms of performance and reproducibility.

Facebook’s Yarn is a JavaScript package manager, which resolves NPM’s problems and is a bit more user-friendly. It parallelises better and caches all installed modules. Most importantly, it works with NPM’s repositories and preserves the structure of the package.json file which makes it easy to migrate existing NPM projects.

In this post, we will review Yarn’s basic commands and use cases.

# Setup

Yarn can be installed as a global NPM module:

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npm install -g yarn


This will give us the yarn command on our terminal path. Starting a new NPM/Yarn project is as easy as:

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# Very similar to "npm init"
# Will create a package.json
yarn init


If you already have an existing NPM project with a package.json, there is no need to initialise Yarn. Yarn commands will work right away.

# Install and Remove Packages

Given an existing NPM/Yarn project, you can install all packages as:

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# Similar to "npm install"
yarn install


This is similar to npm install but achieves two additional things. Firstly, it caches the new modules outside of /node_modules so subsequent installations can be faster. Secondly, it creates the yarn.lock file with the exact versions of all used packages. In other words, even if a module has a range of version in its package.json, subsequent yarn commands will only use the version specified in yarn.lock. This is similar to the role of Gemfile.lock in Ruby’s package manager Bundler.

Adding and removing new packages/modules is similar to NPM, but by default Yarn saves all new modules in the package.json. This is equivalent to the using the --save flag in NPM:

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# Adds the latest Express to package.json"
# Similar to "npm install express --save"

# The same, but with a version

# Adds the latest Express as a "dev" dependency
# Similar to "npm install express --save --dev"

# Adds the latest Express to package.json"
# Similar to "npm install express --save"

# Remove a package
yarn remove express


Each of thes commands modifies the yarn.lock and package.json files and populates the Yarn cache.

Working with global packages is pretty easy as well:

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# Add a package globally
yarn globall add nodemon
yarn globall add nodemon@1.11.0

# Remove a global package
yarn globall remove nodemon


Yarn allows you to easily upgrade and downgrade packages. It has a special command for inspecting your dependencies and identifying if any of them is outdated:

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# Prints if any of your dependencies is outdated
yarn outdated


To upgrade/downgrade a package, we can simply use:

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# Upgrades Express to the newest version

# Upgrades/Downgrades Express to a specific version


The upgrade command modifies both the yarn.lock and package.json files.

# Cache

Yarn keeps all modules it installed in cache outside of the /node_modules folder. When needed, these modules can be quickly retrieved from the cache. For the most parts, we should not be concerned with the caching at all. However, in some rare occasions (e.g. an internal module changed without increasing its version) we may need to clean the cache:

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# Force Yarn to clean its cache
yarn clean cache


# Scripts

We can specify scripts in the package.json file like this:

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"scripts" : {
"start": "node server.js",
"test": "echo MISSING TESTS"
}


With NPM, we could run them with the npm run command. Yarn allows for this as well:

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# Run a script from "package.json"
yarn run start
yarn run test

# Lists all scripts and lets you choose
yarn run


# Resources

Here are some nice resources on Yarn: