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Yarn is an open-source package manager for JavaScript. CircleCI can cache both Yarn and the packages it installs. This can potentially speed up builds but, more importantly, can reduce errors related to network connectivity.
To learn more about how to install Yarn in your CircleCI builds–including a full example–read more in our documentation.
Yarn was a the result of a collaboration between Google, Facebook, Exponent and Tilde and was released in October 2016. To learn more about Yarn in general, you can check out their repo or the announcement on Facebook.