type-fest

What is type-fest?

The type-fest package provides a collection of essential TypeScript types for use in a wide range of applications. It includes utility types, primitive types, and complex object types to enhance TypeScript's static typing capabilities.

What are type-fest's main functionalities?

Primitive types

Includes types like LiteralUnion, Primitive, and Promisable, which help with defining literals, primitive values, and promise-like structures.

{"isPrimitive": true}

Basic utilities

Provides utility types such as Except, Mutable, ReadonlyDeep, and many others that help manipulate and transform types in various ways.

{"isEmptyArray": array.length === 0}

Object types

Contains types for working with objects, such as Merge and RequireAtLeastOne, which assist in merging types and ensuring at least one property is present, respectively.

{"isObjectClean": Object.keys(object).length === 0 && object.constructor === Object}

Conditional types

Includes conditional types like ConditionalExcept and ConditionalKeys, which apply conditions to filter keys or properties of types.

{"isTruthy": T extends false | '' | 0 | null | undefined ? false : true}

Other packages similar to type-fest

utility-types

The utility-types package provides a collection of utility types for TypeScript, similar to type-fest. It includes types for operations like picking, omitting, and readonly transformations. It is comparable to type-fest but may have a different set of utilities and slightly different implementations.

ts-essentials

ts-essentials offers a wide range of TypeScript types, including deep readonly and writable types, as well as utility types for better type inference. It is similar to type-fest in its goal to enhance TypeScript's type system but may differ in the specific types offered and its API design.

typesafe-actions

While typesafe-actions is more focused on Redux action creators, it also provides utility types for better type safety in Redux. It is similar to type-fest in that it enhances TypeScript's type system but is more specialized for Redux-related typing.

README

A collection of essential TypeScript types

---

Many of the types here should have been built-in. You can help by suggesting some of them to the TypeScript project.

Either add this package as a dependency or copy-paste the needed types. No credit required. 👌

PR welcome for additional commonly needed types and docs improvements. Read the contributing guidelines first.

Install

$ npm install type-fest

Requires TypeScript >=3.2

Usage

import {Except} from 'type-fest';

type Foo = {
	unicorn: string;
	rainbow: boolean;
};

type FooWithoutRainbow = Except<Foo, 'rainbow'>;
//=> {unicorn: string}

API

Click the type names for complete docs.

Basic

• Primitive - Matches any primitive value.
• Class - Matches a class constructor.
• TypedArray - Matches any typed array, like Uint8Array or Float64Array.
• JsonObject - Matches a JSON object.
• JsonArray - Matches a JSON array.
• JsonValue - Matches any valid JSON value.
• ObservableLike - Matches a value that is like an Observable.

Utilities

• Except - Create a type from an object type without certain keys. This is a stricter version of Omit.
• Mutable - Convert an object with readonly properties into a mutable object. Inverse of Readonly<T>.
• Merge - Merge two types into a new type. Keys of the second type overrides keys of the first type.
• MergeExclusive - Create a type that has mutually exclusive properties.
• RequireAtLeastOne - Create a type that requires at least one of the given properties.
• ReadonlyDeep - Create a deeply immutable version of a object/Map/Set/Array type.
• LiteralUnion - Create a union type by combining primitive types and literal types without sacrificing auto-completion in IDEs for the literal type part of the union. Workaround for Microsoft/TypeScript#29729.
• Promisable - Create a type that represents either the value or the value wrapped in PromiseLike.
• Opaque - Create an opaque type.

Miscellaneous

• PackageJson - Type for npm's package.json file.

Declined types

If we decline a type addition, we will make sure to document the better solution here.

• Diff and Spread - The PR author didn't provide any real-world use-cases and the PR went stale. If you think this type is useful, provide some real-world use-cases and we might reconsider.

Tips

Built-in types

There are many advanced types most users don't know about.

• Partial<T> - Make all properties in T optional.
• Required<T> - Make all properties in T required.
• Readonly<T> - Make all properties in T readonly.
• Pick<T, K> - From T, pick a set of properties whose keys are in the union K.
• Record<K, T> - Construct a type with a set of properties K of type T.
• Exclude<T, U> - Exclude from T those types that are assignable to U.
• Extract<T, U> - Extract from T those types that are assignable to U.
• NonNullable<T> - Exclude null and undefined from T.
• Parameters<T> - Obtain the parameters of a function type in a tuple.
• ConstructorParameters<T> - Obtain the parameters of a constructor function type in a tuple.
• ReturnType<T> – Obtain the return type of a function type.
• InstanceType<T> – Obtain the instance type of a constructor function type.

You can find some examples in the TypeScript docs.

Maintainers

• Sindre Sorhus
• Jarek Radosz
• Dimitri Benin

License

(MIT OR CC0-1.0)