type-fest

type-fest is a large collection of TypeScript utility types that fill gaps in TypeScript’s built-in type toolbox — types like PartialDeep, CamelCase, Merge, Simplify, Except, and Jsonify that are commonly needed but not shipped with the language itself. Rather than every project reinventing the same conditional and mapped types, type-fest centralizes over 200 well-tested, documented type definitions covering objects, arrays, strings, JSON, async utilities, and more.

Because the package is types-only — it has zero runtime code or JavaScript output — adding it as a dependency has no effect on bundle size or runtime behavior; it exists purely to be imported in .d.ts/.ts files. Types can also be copy-pasted individually without adding the dependency at all, per the project’s own stated policy.

What You Get

• Object utilities like PartialDeep, Merge, Except, Simplify, and SetRequired for common type transformations TypeScript doesn’t ship natively
• String literal utilities such as CamelCase, KebabCase, SnakeCase, and Delimiter-based case conversions at the type level
• JSON-focused types like Jsonify and JsonValue for accurately typing values that survive JSON.stringify/parse round-trips
• Array utilities including ArrayElement, ArraySlice, ArrayReverse, and fixed-length tuple helpers
• Async and function types such as AsyncReturnType, Asyncify, and Promisable for working with sync/async interop
• Zero runtime footprint — the package ships only .d.ts declaration files, so it never appears in a compiled JavaScript bundle

Common Use Cases

• Typing configuration objects that need deep partials or deep readonly variants without hand-writing recursive mapped types
• Converting between naming conventions (snake_case API responses to camelCase application models) at the type level
• Typing values that pass through JSON serialization boundaries, such as API payloads or localStorage-persisted state
• Building strongly-typed utility functions in library code that need generic helpers like Merge or Except for parameter shaping
• Reducing duplicated one-off utility type definitions across a large TypeScript codebase or monorepo

Under The Hood

Architecture - Each utility type lives in its own file under source/ (215 files as of this analysis), individually exported and re-exported from the root index.d.ts, so the package has no shared runtime module graph to reason about — just a flat, well-organized collection of independent type definitions, many of which are documented with JSDoc comments and usage examples directly in the .d.ts file. This one-type-per-file structure keeps additions and reviews isolated and makes it straightforward for maintainers to accept community-contributed types without touching unrelated code.

Tech Stack - The package is TypeScript-only with no JavaScript runtime output; tsconfig.json and xo.config.js (plus custom lint-rules/lint-processors) enforce style and type-checking across all 215+ definition files. It has zero runtime dependencies, and the entire distribution consists of declaration files plus documentation.

Code Quality - The test-d/ directory contains 197 type-level test files (using tsd or an equivalent type-assertion tool), one per utility type in most cases, verifying that each type resolves to the expected shape for representative inputs — a testing discipline appropriate for a types-only package where “correctness” means type inference behaves as documented. Dual CC0-1.0 and MIT licensing files are both present, and the project maintains an active CLAUDE.md contributor guide alongside its human-facing contributing guidelines.

API Design - Every exported type is self-contained and named descriptively (PartialDeep, CamelCasedProperties, SetRequired), so most usages require nothing more than a single named import; there is no configuration, initialization, or class instantiation anywhere in the package. The README explicitly invites copy-pasting individual type definitions instead of adding the dependency, which signals a design philosophy prioritizing zero lock-in over API surface control.