Yeah immer.js allows you to mutate arrays and then it works out an immutable version from it. That's very cool, but still has a cost as I replied in the comments to that article with a sandbox example showing (on my machine at least) significant frame drops in some use cases.
It so happened the example I had given and then implemented in immer is a kind of kryptonite to immer.js (because it happens over multiple cycles) but yeah, I see things on DEV all the time that use spread in loops - I just think, hope no one calls that example with 1 million entries not the example 10!
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