### Basic Selector Concepts Selectors are primarily used to encapsulate logic for looking up specific values from state, logic for actually deriving values, and improving performance by avoiding unnecessary recalculations. **A "selector function" is any function that accepts the Redux store state (or part of the state) as an argument, and returns data that is based on that state.** Whenever possible, **keep the actual data in the Redux store as minimal as possible, and _derive_ additional values from that state as needed**. This includes things like calculating filtered lists or summing up values. As an example, a todo app would keep an original list of todo objects in state, but derive a filtered list of todos outside the state whenever the state is updated. Similarly, a check for whether all todos have been completed, or number of todos remaining, can be calculated outside the store as well. This has several benefits: - The actual state is easier to read - Less logic is needed to calculate those additional values and keep them in sync with the rest of the data - The original state is still there as a reference and isn't being replaced Deriving data is often done in "selector" functions, which can encapsulate the logic for doing the derived data calculations. In order to improve performance, these selectors can be _memoized_ to cache previous results, using libraries like `reselect` and `proxy-memoize`. #### More info [Deriving Data with Selectors](https://redux.js.org/usage/deriving-data-selectors) ___ **Tags**: #redux #selectors s