type-fest
A collection of essential TypeScript types
...It provides types like Except, Merge, LiteralUnion, Writable, Promisable, PartialDeep, JsonObject, and many others that solve everyday typing needs in complex TypeScript codebases. Developers pull in just the types they need, which makes code more expressive and safer without reinventing tricky type logic. Because many of the types in type-fest are things “that should have been built-in,” it’s widely adopted across TypeScript projects that need more advanced static typing capabilities. The package is modular, well documented, and regularly maintained to stay compatible with evolving TypeScript versions and features. It plays a central role in type-heavy codebases (frameworks, utilities, libraries) by reducing repetition and giving reliable tools for type-level programming.