Try Nodejs! Here is their PR Guide, it shows how to submit a PR step by step.
My first PR was with Node.js early 2017. I followed the steps in nodetodo.org/ to set up my environment. Then I asked for a first issue from @NodeTodo on twitter. It was an awesome experience because everyone was very helpful.
I remember I couldn't get my build test to pass to start the PR. Several Nodejs members spent several days, helping me fix the issue.
Also when I submitted my solution, they gave me constructive feedback on how I could improve my answer.
If you decide to do a PR for Nodejs or you need help understanding how do submit the PR or work with Github/git, don't hesitate to ask me questions!
Thanks man! I'm gonna go for it
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