Mutate.

Second class. An array can grow. .push(x) appends at the end.

Interesting: const amounts = [...] forbids reassigning the name amounts, but the array contents can still change. amounts.push(15) is fine; amounts = [] usually is not.

Mini example:

const list = [1, 2]; list.push(3); console.log(list.length);

Common mistake: using let and replacing the whole array instead of push; or forgetting that push returns the new length.