Since then, we have continued to show our support for Yarn: Yarn is now pre-installed in our CircleCI Ubuntu 14.04 (trusty) image. We first wrote about Yarn in December where we showed the best way to download and install Yarn for use in your builds.

Switching from npm to Yarn is painless since they’re compatible. Committing this lockfile to your VCS ensures that all developers working on the project, if they are using Yarn, will be using the same versions of every package. Borrowing from package managers such as Bundler, Yarn creates a yarn.lock file that records the exact version of every package you are using for your project. Any CI-trained engineer knows that the ability to reproduce an environment, with the exact same dependencies, is crucial for efficient debugging and onboarding of new team members. This can cause an issue where different developers on the same team are using different versions of the same package. In package.json, package versions could be specified as a range, or without a version at all. This is possibly the most useful feature of Yarn for developers working in teams. Yarn introduces a lockfile, yarn.lock, to manage JavaScript packages. yarn.lock - keeps dependencies locked to specific versions similar to Gemfile.lock in the Ruby world.fully compatible with npm - switching from npm to Yarn is a no friction process.parallel downloads - Yarn can download packages in parallel, reducing the time builds take to run.Requests are retried upon failure, reducing red builds due to temporary network issues automatic retries - a single network request failing won’t cause an install to fail.

flat dependency structure - simpler dependency resolution means Yarn finishes faster and can be told to use a single version of certain packages, which uses less disk space.

In the event one goes down, your project can continue to be built in CI without issue

multiple registries - Yarn reads and installs packages from both as well as Bower.

indeterminate package state (you can’t be sure all copies of the project will be using the same package versions).

allows packages to run code upon installation (not good for security).

requires network to install packages (though we can create a makeshift cache).

single package registry ( ever go down for you?).

Large projects–like the ones Facebook and Google have–magnify the issues developers might face when using npm. Yarn isn’t a fork of npm but rather a reimagining of it. In October 2016, Facebook, Exponent, Google, and Tilde released something unexpected, an npm replacement they dubbed Yarn.

Among those changes was that Angular2 was released, Node.js 7.0.0 was released, and the feature set for ECMAScript 2016 (ES7) was finalized. To say a lot has happened in the past year would be an understatement. The JavaScript world changes every second.

Yarn add save dev