Redux is a predictable state container for JavaScript apps.
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state tree of Redux is readonly.Modification is only possible by dispatching an action. Reducers are the only functions that can change the state tree.
To describe a state mutation requires a function (reducer) is taking the previous state, the action being dispatched and returns the next state.
The function has to be a pure function. A pure function returns a value that is solely referring to the value that it gets as their arguments.
/* pure */
function square(x) {
return x * x;
}
/* impure */
function square(x) {
updateXinSquare(x);
return x * x;
}