Twitter communities can be amorphous and the community at an office can be great until you switch jobs, what's a community that you rely on to give...
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I haven't found any Slack things (places, communities, rooms? I don't know the term) that are worth hanging around. They're harder to find and proprietary and generally harder to work with than plain old IRC anyway.
As far as reddit goes, well, r/badcode is worth a laugh. r/programming is good, r/programmerhumor isn't.
At work, I rely, if anything, on r/listentothis (via Music Player For Reddit while working and on r/nosleep between tickets.
/r/reactjs
/r/javascript
threejs.slack.com/ Three.js slack group.
It is reasonably busy and the people there are really helpful.
/r/nim
This is a good one mostly for career/jobs related advice r/cscareerquestions/
Don't really rely on it, but vue.js has a pretty nice discord server for discussions.