For me the worst is slice/splice.

Kind of stupid, but I imagine the 'p' in 'splice' being an axe that chops the array :D Works for me...

I think OP was saying slice returns a mutated copy while splice mutates in place.

Yes, it stems from arrays. But extends from there to any collection, built in or custom. Kind of bridges the built in type system but it's really about the expressivity of the language.

It becomes especially important in React where you share objects up and down an immutable structure of objects.

Yep. If you've bitten the apple of JavaScript tooling you can kinda sorta rig up compiler-enforced immutable data structures with TypeScript. But IMO if you're going that far it's much easier/well-documented to just use Elm or something.