type-fest is a TypeScript utility types library that offers a curated, battle-tested suite of type definitions and type transformations that aren’t included in the TypeScript standard library. 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.
Features
- Deep types for making nested objects optional or required (e.g. PartialDeep, RequiredDeep)
- Utility types for merging / overriding properties (Merge, MergeDeep, OverrideProperties)
- Types to enforce key constraints: RequireAtLeastOne, RequireExactlyOne, MergeExclusive etc.
- Types for readability and manipulation: turning readonly fields into mutable (Writable, WritableDeep), converting property names (like CamelCasedProperties, etc.), trimming strings, dealing with unions etc.
- JSON-related types: JsonValue, JsonObject, JsonArray etc. for better typing of JSON data
- Conditional / utility types: Extracting nested paths, handling index signatures, checking assignability, etc.
Project Samples
