A menagerie of useful type operators for TypeScript
Last updated a year ago by pelotom .
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TypeScript ships with some handy user-definable type operators: Partial, Readonly, Pick and Record. However many other useful operators have been demonstrated in GitHub issue comments and elsewhere. This repository is intended to collect all this folklore in one place, so you can stop copying and pasting these solutions into project after project.

PRs more than welcome! Please note that this library is intended to be fully static, i.e. it has no runtime component, only a type definition file. The idea is that these could all potentially make their way into lib.d.ts at some point.


yarn add type-zoo


ExcludeStrict<T, U extends T>

Exclude from T those types that are assignable to U, where U must exist in T.

Similar to Exclude but requires the exclusion list to be composed of valid members of T.

See: https://github.com/pelotom/type-zoo/issues/37

ExtractStrict<T, U extends T>

Extract from T those types that are assignable to U, where U must exist in T.

Similar to Extract but requires the extraction list to be composed of valid members of T.

See: https://github.com/pelotom/type-zoo/issues/37


Use to prevent a usage of type T from being inferred in other generics.


declare function assertEqual<T>(actual: T, expected: NoInfer<T>): boolean;

Type T will now only be inferred based on the type of the actual param, and the expected param is required to be assignable to the type of actual. This allows you to give one particular usage of type T full control over how the compiler infers type T.

See: https://github.com/Microsoft/TypeScript/issues/14829#issuecomment-322267089

Omit<T, K extends keyof any>

Drop keys K from T if they are present.

See: https://github.com/Microsoft/TypeScript/issues/12215#issuecomment-377567046

OmitStrict<T, K extends keyof T>

Drop keys K from T, where K must exist in T.

See: https://github.com/Microsoft/TypeScript/issues/12215#issuecomment-377567046

Overwrite<T, U>

Like T & U, but where there are overlapping properties using the type from U only.

See: https://github.com/pelotom/type-zoo/pull/14#discussion_r183527882

Param#<T extends Function> and ParamTypes<T extends Function>

These helpers extract the Parameter-types from Functions.

See: https://github.com/pelotom/type-zoo/issues/22

Pick#<T, K1 in keyof T, K2 in ...>

Like Pick<> but for # of nested levels!

See https://gist.github.com/staltz/368866ea6b8a167fbdac58cddf79c1bf


Get only the public members of a type or class. When applied to a class T with private members, Public<T> can be implemented.

See https://github.com/Microsoft/TypeScript/issues/18499#issuecomment-429272545

Related Projects

typelevel-ts and typical are two projects with similar goals to this one. The main difference is that those libraries are more focused on advanced type-level computation, whereas Type Zoo is meant to capture more basic type operators which have been proposed as candidates for inclusion in lib.d.ts, or even as first-class language primitives. The idea is that these types will hopefully make their way into the language proper, at which point you can simply stop importing them from type-zoo and be on your merry way.

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