createSelector

createSelector is a factory function, most commonly associated with the Reselect library for JavaScript, that produces memoized selector functions used to derive computed data from application state. A selector created by createSelector receives the application state (and optionally props), runs one or more input selector functions to extract pieces of state, and then applies a result function to those inputs to produce a derived value. The result is recomputed only when an input value changes, using a shallow equality check by default.

Typical usage is in Redux-based applications to compute filtered, aggregated, or transformed data for components without redoing expensive calculations on every render. A simple pattern is createSelector(inputSelectorA, inputSelectorB, (a, b) => compute(a, b)). Reselect also exposes createSelectorCreator to customize memoization strategy or equality checks.

Benefits include avoiding unnecessary recomputation, keeping selectors pure and testable, and improving rendering performance when used with connect or useSelector. Important caveats: default memoization is shared across calls to the created selector, so per-instance selectors are recommended when selectors depend on component props or when multiple independent instances are needed (e.g., via selector factories). Alternatives and related tools include memoization helpers, React's useMemo, re-reselect (per-parameter memoization), and Redux Toolkit integrations that re-export or wrap createSelector.