What is reselect and how it works?

Reselect is a selector library (for Redux) which uses memoization concept. It was originally written to compute derived data from Redux-like applications state, but it can't be tied to any architecture or library.

Reselect keeps a copy of the last inputs/outputs of the last call, and recomputes the result only if one of the inputs changes. If the same inputs are provided twice in a row, Reselect returns the cached output. It's memoization and cache are fully customizable.

Practical Code Example

// Reselect: Memoized selector library preventing unnecessary re-computations
import { createSelector } from 'reselect';

const selectItems = (state) => state.cart.items;

export const selectCartTotal = createSelector(
  [selectItems],
  (items) => items.reduce((sum, item) => sum + item.price, 0) // Memoized!
);