functional-patterns

Use spread syntax to create new objects instead of mutating in place, such as `{ ...user, name: newName }`. For nesting deeper than two levels, use Immer's `produce` function, and mark immutable data with `as const` or `readonly` modifiers.

Replace `for` loops with declarative array methods like `map`, `filter`, `find`, and `flatMap` that state intent directly. Chain the methods instead of creating intermediate variables, and reserve `reduce` for accumulating into a different shape like an object or Map.

Prefer exported functions and module-level composition for most logic, since they avoid hidden state and are easier to test. Use classes only when you genuinely need encapsulated mutable state with a lifecycle, such as database connections or WebSocket managers.

Prefer `filter` and `map` chains when selecting and transforming elements, since the intent is self-documenting. Use `reduce` only when accumulating a sequence into a genuinely different shape like a grouped object, Map, or Set.

The `??` operator only replaces `null` and `undefined`, while `||` replaces all falsy values including `0`, empty strings, and `false`. Use `??` when falsy values are legitimate data, and combine it with optional chaining (`?.`) for safe nested access.