typescript

Advanced TypeScript covers variance (how subtyping flows through generics), complex generic constraints (multiple bounds, recursive types), branded types for nominal typing, and declaration merging — these patterns appear in library authoring and senior-level interview questions.

Conditional types use the syntax T extends U ? X : Y to branch at the type level — combined with the infer keyword to extract types from patterns and distributive behavior over unions, they power most of TypeScript's built-in utility types.

Declaration files (.d.ts) provide type information for JavaScript code without implementations — they enable TypeScript to type-check against libraries, ambient declarations describe globals like window extensions, and module augmentation extends third-party package types.

Mapped types transform existing types by iterating over their keys with { [K in keyof T]: NewType } — modifiers add/remove readonly and optional, key remapping (as clause) filters or renames keys, and they power built-in utilities like Partial, Required, Readonly, and Record.

Template literal types use backtick syntax at the type level to construct string types from other types — combined with union distribution they generate combinatorial patterns, and with intrinsic string types (Uppercase, Lowercase, Capitalize) they enable type-safe event handlers, CSS values, and API routes.