What problem does it solve? Derived values like filtered lists, counts, and entity lookups often get duplicated across components or stored redundantly in reducer state, causing inconsistency and wasted computation. This Skill guides the creation of cached, composable selectors through store.createSelector so derived state has one canonical, testable implementation. ## Core Features & Use Cases - Store-bound selector factories: Create app-local selectors with store.createSelector(...) that preserve StoreState type inference and return Svelte readables. - Multiple call modes: Use the readable form in component init, .select(state, args) in tests and composition, .effect(args) in sagas, and .withStore(store) for SSR contexts. - Collection-backed lookups: Combine selectors with getItem and getItems collection utilities for O(1) entity access and ordered materialization. - Use Case: When building a todo list view that shows only visible items, define selectVisibleTodos once, compose it from selectAllTodos and selectTodoFilter via .select(state), and reuse it in components, sagas, and tests without duplicating logic. ## Quick Start Ask the AI to create a memoized selector with store.createSelector that derives a filtered todo list from the existing todos collection and filter state.
