How do I get yarn installed on elastic beanstalk?

Currently you can't install yarn using yum so there doesn't seem to be an easy way to create a config that installs it before asset pre-compilation.

Solution 1:

You can customize packages that are installed and commands that are run on deploy with .ebextensions

For yarn, I created a file with the following commands which install a recent node version and yarn:

# .ebextensions/yarn.config
commands:
  01_install_node:
    command: |
      sudo curl --silent --location https://rpm.nodesource.com/setup_8.x | sudo bash -
      sudo yum -y install nodejs

  02_install_yarn:
    # don't run the command if yarn is already installed (file /usr/bin/yarn exists)
    test: '[ ! -f /usr/bin/yarn ] && echo "Yarn not found, installing..."'
    command: |
      sudo wget https://dl.yarnpkg.com/rpm/yarn.repo -O /etc/yum.repos.d/yarn.repo
      sudo yum -y install yarn

More Documentation: https://docs.aws.amazon.com/elasticbeanstalk/latest/dg/customize-containers-ec2.html

Solution 2:

As stated in the accepted answer, you can create .ebextensions/some.config to order EB instances to install yarn on the instance creation. In current EB environment, I find following config to be more succinct and understandable:

# .ebextensions/some.config
commands:
  01_install_yarn:
    command: |
      set -e
      npm i -g yarn
      ln -s "$(npm bin --global)"/yarn /usr/bin/yarn
    test: "! yarn -v"

In fact, current EB ruby environment includes node binary available at /usr/bin/node, you can use that to install yarn via npm.