What does yarn --prefer-offline do?

Question

I assume when I install npm package say react for the first time with

yarn add react

this will save react file in local cache. I found .y

Answer 1

A quite popular guy here at S.O. said:

"Read the Source, Luke!"

And here is the source of yarn CLI's --prefer-offline flag:

commander.option('--prefer-offline', 'use network only if dependencies are not available in local cache');

Enjoy!