You can install Yarn through the Homebrew package manager. This will also install Node.js if it is not already installed.

brew install yarn  

If you use nvm you should exclude installing Node.js so that nvm’s version of Node.js is used.

brew install yarn --ignore-dependencies

NOTE: Must restart termial before yarn command will work to verify yarn install, after restarting terminal:

yarn --version  

Yarn. Yet Another Resource Negotiator.

Yarn: node special dependency #1408 Exlude Node.js?