While trying to install yarn, you might encounter a SyntaxError: Unexpected token issue referring to /usr/local/lib/node_modules/yarn/lib/cli.js:45726.
According to the related GitHub issue, the problem occurs when your environment meets both the below conditions:
yarn version is v1.13.0
Node.js is lower than v6
Solution
Upgrade the version of Node.js
Upgrade the version of yarn
You should no longer see the issue when building.