selectVisibleTodos

selectVisibleTodos is a selector function used in todo list applications to derive the subset of todos that should be displayed based on the current filter and search criteria. It reads the application state and returns a new array containing only the items that are visible to the user, without mutating the original data.

Typically the state includes a todos array and a visibility filter, which can be ALL, ACTIVE, or COMPLETED. Some implementations also track a text search query or other criteria such as tags or priorities. The selector applies the filter: ALL returns all items; ACTIVE returns those with completed set to false; COMPLETED returns those with completed set to true. If a search query is present, it further filters by a text match, usually in a case-insensitive manner.

Performance considerations: selectVisibleTodos is often implemented as a memoized selector to avoid recomputing when inputs have not changed. In Redux-based apps, libraries such as Reselect support memoization. Memoization reduces re-renders by ensuring the derived array changes only when the underlying data or filters change.

Example usage: in a React-Redux component, the visible list is obtained by calling useSelector(selectVisibleTodos) and then rendered as list items. In plain terms, the function acts as a bridge between store data and the UI, exposing only the subset that matches the current view settings.

Variants and extensions: some apps support multiple filters (for example by priority or due date) or add pagination and lazy loading. If the data set is large or changes frequently, separating the filtering logic from data loading can improve maintainability and performance.