They've also got a couple of fully functioning power-ups you can take a look at the code for:
You can store about 4000 characters worth of data on each card (as well as some storage on the board itself for stuff that's not specific to any card) so depending on how simple your power-up is, you might not even need a backend or a database to store anything (which is great for me as a mainly frontend developer).
There's less than 200 Trello Power-Ups available right now so I think it's a great time to get in and build one while there's still so many unexplored opportunities!!
I don't know if I've really answered your question but let me know if there's anything specific you wanted to know 🙂
It's very useful, thank you !
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