typescript

Conditional types select types based on conditions using the syntax T extends U ? X : Y. They enable type-level programming, allowing different types based on input. Combined with infer, they can extract and transform types dynamically.

Mapped types transform existing types by iterating over their keys using 'in keyof' syntax. They can modify properties (make optional, readonly), rename keys, and filter properties. Built-in utilities like Partial, Required, and Pick use mapped types.

Template literal types use the same syntax as template strings but operate on types. They can create union types from string combinations, validate string patterns, and transform string types. Useful for type-safe event names, CSS properties, and API routes.

Complex constraints use multiple extends, conditional types, and recursive patterns. Variance determines how type parameters relate in subtyping: covariant (out), contravariant (in), invariant (both), and bivariant (neither). Understanding variance helps design safe generic APIs.

Declaration files (.d.ts) provide type information for JavaScript code without implementation. They use 'declare' keyword for ambient declarations, support module augmentation for extending existing types, and global augmentation for adding to global scope.