Quick recall
useEffect runs side effects after rendering and replaces many class lifecycle use cases in function components.
useEffect behavior depends heavily on its dependency array, and many common bugs come from missing dependencies or stale captured values.
These hooks and helpers solve timing, refs, deferred rendering, and stable identity concerns that basic hooks do not cover.
React memoization tools help avoid unnecessary work, but they only help when they stabilize expensive calculations or props meaningfully.
useRef holds mutable values that do not trigger re-renders and is commonly used for DOM access or persistent instance-like values.
useState stores local component state, and updates should be based on immutable patterns and previous state when needed.
Object and array state require immutable updates, and using multiple state variables can sometimes be clearer than one large state object.
React compares virtual tree snapshots, reconciles differences, and updates the real DOM only where needed.
`agg` reduces groups, `transform` returns group-shaped output, and `apply` handles custom logic but is usually slower.
Inspect null patterns first, then choose to drop, fill, or flag them based on meaning.
`loc` selects by labels and `iloc` selects by integer position.
`merge` combines rows by key columns, `join` is index-oriented sugar, and `concat` stacks objects along an axis.