My first recommendation is to take a look at an article series that I wrote just for this question.
webpack has 100 repo's across two github orgs and we have so much to offer for anyone from a Technical Writer, to someone who wants to tweak graph traversal algorythms, even to someone who wants to rework some design on webpack.js.org.
So read the contributors guide, and then pick something that interests you! Then, as always you can ping me on Twitter/DM, or Tag me in a github issue and we can help answer any questions along the way.
In the end, Open Source is about people who share the same love for working on things they enjoy, and making it better for the greater good. If you can't find something you like with webpack, then we have lots of connections to other maintainers like Henry Zhu of babel who also would love to have you contribute to Babel as well.
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