Why does the state need to be immutable?

Question

redux

Why the state can only be replaced with a new one and not change it?

Answer 1

if the reducer changes the state and returns it, then subscribers (components) will not be notified about its change, since redux will think that the state has not changed, because the comparison of the previous and next state is not deep, but only the reference is compared, and in general it is recommended in FP use pure functions and work with everything immutably, this makes it easier to debug and simplifies testing.