no barrel files
no void for async - use async/effect (exception vscode-window-messages)
no export * - name exports explicitly
prefer undefined over null (unless server requires null)
prefer undefined over empty string
prefer map/filter over loops/conditionals
avoid mutation
avoid any
derive types from the source of truth; don't hand-write or narrow them to satisfy the compiler. ex: use ReturnType<> to match something
no enums or namespaces (enums compile to weird JS; use string union types instead; exception: interfaces defined outside this repo that we can't change)
no runtime errors for developer mistakes (use types to ensure exhaustive switch/case; don't throw for null/undefined when input/consumer is within our control)
.ts filenames: camelCase, no hyphens, no leading capitals
preserve comments when refactoring; remove/fix if wrong/obsolete
exported functions: single-line jsdoc /*_ foo _/ if name unclear; no params/return (TS provides types)
look for uses of (Object|Map).groupBy instead of older patterns
redundant empty-collection guards: if arr.find/some/every/map/filter/reduce already returns the same value for an empty array, drop the if (arr.length === 0) return … guard. e.g. find on [] is undefined, so guarding return undefined is dead code.
no needless const declarations: return directly instead of const x = val; return x (exception: complex expressions where const improves readability)

typescript