sindresorhus / Type Fest

A collection of essential TypeScript types

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---

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.

Help wanted with reviewing proposals and pull requests.

Install

npm install type-fest

Requires TypeScript >=4.2

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 - Matches a class constructor.
• TypedArray - Matches any typed array, like Uint8Array or Float64Array.
• JsonPrimitive - Matches a JSON primitive.
• 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 - Create a type that strips readonly from all or some of an object's keys. The 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 keys.
• RequireAtLeastOne - Create a type that requires at least one of the given keys.
• RequireExactlyOne - Create a type that requires exactly a single key of the given keys and disallows more.
• RequireAllOrNone - Create a type that requires all of the given keys or none of the given keys.
• PartialDeep - Create a deeply optional version of another type. Use Partial<T> if you only need one level deep.
• ReadonlyDeep - Create a deeply immutable version of an object/Map/Set/Array type. Use Readonly<T> if you only need one level deep.
• 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.
• Opaque - Create an opaque type.
• SetOptional - Create a type that makes the given keys optional.
• SetRequired - Create a type that makes the given keys required.
• ValueOf - Create a union of the given object's values, and optionally specify which keys to get the values from.
• ConditionalKeys - Extract keys from a shape where values extend the given Condition type.
• ConditionalPick - Like Pick except it selects properties from a shape where the values extend the given Condition type.
• ConditionalExcept - Like Omit except it removes properties from a shape where the values extend the given Condition type.
• UnionToIntersection - Convert a union type to an intersection type.
• Stringified - Create a type with the keys of the given type changed to string type.
• IterableElement - Get the element type of an Iterable/AsyncIterable. For example, an array or a generator.
• Entry - Create a type that represents the type of an entry of a collection.
• Entries - Create a type that represents the type of the entries of a collection.
• SetReturnType - Create a function type with a return type of your choice and the same parameters as the given function type.
• Simplify - Useful to flatten the type output to improve type hints shown in editors. And also to transform an interface into a type to aide with assignability.
• Jsonify - Transform a type to one that is assignable to the JsonValue type.
• Get - Get a deeply-nested property from an object using a key path, like Lodash's .get() function.

Async

• Promisable - Create a type that represents either the value or the value wrapped in PromiseLike.
• AsyncReturnType - Unwrap the return type of a function that returns a Promise.
• Asyncify - Create an async version of the given function type.

String

• Trim - Remove leading and trailing spaces from a string.
• Split - Represents an array of strings split using a given character or character set.

Array

• Includes - Returns a boolean for whether the given array includes the given item.
• Join - Join an array of strings and/or numbers using the given string as a delimiter.
• LastArrayElement - Extracts the type of the last element of an array.
• FixedLengthArray - Create a type that represents an array of the given type and length.
• MultidimensionalArray - Create a type that represents a multidimensional array of the given type and dimensions.
• MultidimensionalReadonlyArray - Create a type that represents a multidimensional readonly array of the given type and dimensions.

Numeric

• PositiveInfinity - Matches the hidden Infinity type.
• NegativeInfinity - Matches the hidden -Infinity type.
• Finite - A finite number.
• Integer - A number that is an integer.
• Float - A number that is not an integer.
• NegativeFloat - A negative (-∞ < x < 0) number that is not an integer.
• Negative - A negative number/bigint (-∞ < x < 0)
• NonNegative - A non-negative number/bigint (0 <= x < ∞).
• NegativeInteger - A negative (-∞ < x < 0) number that is an integer.
• NonNegativeInteger - A non-negative (0 <= x < ∞) number that is an integer.

Change case

• CamelCase – Convert a string literal to camel-case (fooBar).
• CamelCasedProperties – Convert object properties to camel-case (fooBar).
• CamelCasedPropertiesDeep – Convert object properties to camel-case recursively (fooBar).
• KebabCase – Convert a string literal to kebab-case (foo-bar).
• KebabCasedProperties – Convert a object properties to kebab-case recursively (foo-bar).
• KebabCasedPropertiesDeep – Convert object properties to kebab-case (foo-bar).
• PascalCase – Converts a string literal to pascal-case (FooBar)
• PascalCasedProperties – Converts object properties to pascal-case (FooBar)
• PascalCasedPropertiesDeep – Converts object properties to pascal-case (FooBar)
• SnakeCase – Convert a string literal to snake-case (foo_bar).
• SnakeCasedProperties – Convert object properties to snake-case (foo_bar).
• SnakeCasedPropertiesDeep – Convert object properties to snake-case recursively (foo_bar).
• ScreamingSnakeCase - Convert a string literal to screaming-snake-case (FOO_BAR).
• DelimiterCase – Convert a string literal to a custom string delimiter casing.
• DelimiterCasedProperties – Convert object properties to a custom string delimiter casing.
• DelimiterCasedPropertiesDeep – Convert object properties to a custom string delimiter casing recursively.

Miscellaneous

• PackageJson - Type for npm's package.json file.
• TsConfigJson - Type for TypeScript's tsconfig.json file (TypeScript 4.4).

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.
• Dictionary - You only save a few characters (Dictionary<number> vs Record<string, number>) from Record, which is more flexible and well-known. Also, you shouldn't use an object as a dictionary. We have Map in JavaScript now.
• ExtractProperties and ExtractMethods - The types violate the single responsibility principle. Instead, refine your types into more granular type hierarchies.
• Url2Json - Inferring search parameters from a URL string is a cute idea, but not very useful in practice, since search parameters are usually dynamic and defined separately.
• Nullish - The type only saves a couple of characters, not everyone knows what “nullish” means, and I'm also trying to get away from null.

Alternative type names

If you know one of our types by a different name, add it here for discovery.

• PartialBy - See SetOptional

Tips

Related

• typed-query-selector - Enhances document.querySelector and document.querySelectorAll with a template literal type that matches element types returned from an HTML element query selector.

Built-in types

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

• Partial<T> - Make all properties in T optional.

  Example

  Playground

   interface NodeConfig {
   		appName: string;
   		port: number;
   }
  
   class NodeAppBuilder {
   		private configuration: NodeConfig = {
   				appName: 'NodeApp',
   				port: 3000
   		};
  
   		private updateConfig<Key extends keyof NodeConfig>(key: Key, value: NodeConfig[Key]) {
   				this.configuration[key] = value;
   		}
  
   		config(config: Partial<NodeConfig>) {
   				type NodeConfigKey = keyof NodeConfig;
  
   				for (const key of Object.keys(config) as NodeConfigKey[]) {
   						const updateValue = config[key];
  
   						if (updateValue === undefined) {
   								continue;
   						}
  
   						this.updateConfig(key, updateValue);
   				}
  
   				return this;
   		}
   }
  
   // `Partial<NodeConfig>`` allows us to provide only a part of the
   // NodeConfig interface.
   new NodeAppBuilder().config({appName: 'ToDoApp'});

• Required<T> - Make all properties in T required.

  Example

  Playground

   interface ContactForm {
   		email?: string;
   		message?: string;
   }
  
   function submitContactForm(formData: Required<ContactForm>) {
   		// Send the form data to the server.
   }
  
   submitContactForm({
   		email: '[email protected]',
   		message: 'Hi! Could you tell me more about…',
   });
  
   // TypeScript error: missing property 'message'
   submitContactForm({
   		email: '[email protected]',
   });

• Readonly<T> - Make all properties in T readonly.

  Example

  Playground

   enum LogLevel {
   		Off,
   		Debug,
   		Error,
   		Fatal
   };
  
   interface LoggerConfig {
   		name: string;
   		level: LogLevel;
   }
  
   class Logger {
   		config: Readonly<LoggerConfig>;
  
   		constructor({name, level}: LoggerConfig) {
   				this.config = {name, level};
   				Object.freeze(this.config);
   		}
   }
  
   const config: LoggerConfig = {
   	name: 'MyApp',
   	level: LogLevel.Debug
   };
  
   const logger = new Logger(config);
  
   // TypeScript Error: cannot assign to read-only property.
   logger.config.level = LogLevel.Error;
  
   // We are able to edit config variable as we please.
   config.level = LogLevel.Error;

• Pick<T, K> - From T, pick a set of properties whose keys are in the union K.

  Example

  Playground

   interface Article {
   		title: string;
   		thumbnail: string;
   		content: string;
   }
  
   // Creates new type out of the `Article` interface composed
   // from the Articles' two properties: `title` and `thumbnail`.
   // `ArticlePreview = {title: string; thumbnail: string}`
   type ArticlePreview = Pick<Article, 'title' | 'thumbnail'>;
  
   // Render a list of articles using only title and description.
   function renderArticlePreviews(previews: ArticlePreview[]): HTMLElement {
   		const articles = document.createElement('div');
  
   		for (const preview of previews) {
   				// Append preview to the articles.
   		}
  
   		return articles;
   }
  
   const articles = renderArticlePreviews([
   		{
   			title: 'TypeScript tutorial!',
   			thumbnail: '/assets/ts.jpg'
   		}
   ]);

• Record<K, T> - Construct a type with a set of properties K of type T.

  Example

  Playground

   // Positions of employees in our company.
   type MemberPosition = 'intern' | 'developer' | 'tech-lead';
  
   // Interface describing properties of a single employee.
   interface Employee {
   		firstName: string;
   		lastName: string;
   		yearsOfExperience: number;
   }
  
   // Create an object that has all possible `MemberPosition` values set as keys.
   // Those keys will store a collection of Employees of the same position.
   const team: Record<MemberPosition, Employee[]> = {
   		intern: [],
   		developer: [],
   		'tech-lead': [],
   };
  
   // Our team has decided to help John with his dream of becoming Software Developer.
   team.intern.push({
   	firstName: 'John',
   	lastName: 'Doe',
   	yearsOfExperience: 0
   });
  
   // `Record` forces you to initialize all of the property keys.
   // TypeScript Error: "tech-lead" property is missing
   const teamEmpty: Record<MemberPosition, null> = {
   		intern: null,
   		developer: null,
   };

• Exclude<T, U> - Exclude from T those types that are assignable to U.

  Example

  Playground

   interface ServerConfig {
   	port: null | string | number;
   }
  
   type RequestHandler = (request: Request, response: Response) => void;
  
   // Exclude `null` type from `null | string | number`.
   // In case the port is equal to `null`, we will use default value.
   function getPortValue(port: Exclude<ServerConfig['port'], null>): number {
   	if (typeof port === 'string') {
   		return parseInt(port, 10);
   	}
  
   	return port;
   }
  
   function startServer(handler: RequestHandler, config: ServerConfig): void {
   	const server = require('http').createServer(handler);
  
   	const port = config.port === null ? 3000 : getPortValue(config.port);
   	server.listen(port);
   }

• Extract<T, U> - Extract from T those types that are assignable to U.

  Example

  Playground

   declare function uniqueId(): number;
  
   const ID = Symbol('ID');
  
   interface Person {
   	[ID]: number;
   	name: string;
   	age: number;
   }
  
   // Allows changing the person data as long as the property key is of string type.
   function changePersonData<
   	Obj extends Person,
   	Key extends Extract<keyof Person, string>,
   	Value extends Obj[Key]
   > (obj: Obj, key: Key, value: Value): void {
   	obj[key] = value;
   }
  
   // Tiny Andrew was born.
   const andrew = {
   	[ID]: uniqueId(),
   	name: 'Andrew',
   	age: 0,
   };
  
   // Cool, we're fine with that.
   changePersonData(andrew, 'name', 'Pony');
  
   // Goverment didn't like the fact that you wanted to change your identity.
   changePersonData(andrew, ID, uniqueId());

• NonNullable<T> - Exclude null and undefined from T.

  Example

  Works with strictNullChecks set to true.

  Playground

   type PortNumber = string | number | null;
  
   /** Part of a class definition that is used to build a server */
   class ServerBuilder {
   		portNumber!: NonNullable<PortNumber>;
  
   		port(this: ServerBuilder, port: PortNumber): ServerBuilder {
   				if (port == null) {
   						this.portNumber = 8000;
   				} else {
   						this.portNumber = port;
   				}
  
   				return this;
   		}
   }
  
   const serverBuilder = new ServerBuilder();
  
   serverBuilder
   		.port('8000')   // portNumber = '8000'
   		.port(null)     // portNumber =  8000
   		.port(3000);    // portNumber =  3000
  
   // TypeScript error
   serverBuilder.portNumber = null;

• Parameters<T> - Obtain the parameters of a function type in a tuple.

  Example

  Playground

   function shuffle(input: any[]): void {
   	// Mutate array randomly changing its' elements indexes.
   }
  
   function callNTimes<Fn extends (...args: any[]) => any> (func: Fn, callCount: number) {
   	// Type that represents the type of the received function parameters.
   	type FunctionParameters = Parameters<Fn>;
  
   	return function (...args: FunctionParameters) {
   		for (let i = 0; i < callCount; i++) {
   			func(...args);
   		}
   	}
   }
  
   const shuffleTwice = callNTimes(shuffle, 2);

• ConstructorParameters<T> - Obtain the parameters of a constructor function type in a tuple.

  Example

  Playground

   class ArticleModel {
   	title: string;
   	content?: string;
  
   	constructor(title: string) {
   		this.title = title;
   	}
   }
  
   class InstanceCache<T extends (new (...args: any[]) => any)> {
   	private ClassConstructor: T;
   	private cache: Map<string, InstanceType<T>> = new Map();
  
   	constructor (ctr: T) {
   		this.ClassConstructor = ctr;
   	}
  
   	getInstance (...args: ConstructorParameters<T>): InstanceType<T> {
   		const hash = this.calculateArgumentsHash(...args);
  
   		const existingInstance = this.cache.get(hash);
   		if (existingInstance !== undefined) {
   			return existingInstance;
   		}
  
   		return new this.ClassConstructor(...args);
   	}
  
   	private calculateArgumentsHash(...args: any[]): string {
   		// Calculate hash.
   		return 'hash';
   	}
   }
  
   const articleCache = new InstanceCache(ArticleModel);
   const amazonArticle = articleCache.getInstance('Amazon forests burining!');

• ReturnType<T> – Obtain the return type of a function type.

  Example

  Playground

   /** Provides every element of the iterable `iter` into the `callback` function and stores the results in an array. */
   function mapIter<
   		Elem,
   		Func extends (elem: Elem) => any,
   		Ret extends ReturnType<Func>
   >(iter: Iterable<Elem>, callback: Func): Ret[] {
   		const mapped: Ret[] = [];
  
   		for (const elem of iter) {
   				mapped.push(callback(elem));
   		}
  
   		return mapped;
   }
  
   const setObject: Set<string> = new Set();
   const mapObject: Map<number, string> = new Map();
  
   mapIter(setObject, (value: string) => value.indexOf('Foo')); // number[]
  
   mapIter(mapObject, ([key, value]: [number, string]) => {
   		return key % 2 === 0 ? value : 'Odd';
   }); // string[]

• InstanceType<T> – Obtain the instance type of a constructor function type.

  Example

  Playground

   class IdleService {
   		doNothing (): void {}
   }
  
   class News {
   		title: string;
   		content: string;
  
   		constructor(title: string, content: string) {
   				this.title = title;
   				this.content = content;
   		}
   }
  
   const instanceCounter: Map<Function, number> = new Map();
  
   interface Constructor {
   		new(...args: any[]): any;
   }
  
   // Keep track how many instances of `Constr` constructor have been created.
   function getInstance<
   		Constr extends Constructor,
   		Args extends ConstructorParameters<Constr>
   >(constructor: Constr, ...args: Args): InstanceType<Constr> {
   		let count = instanceCounter.get(constructor) || 0;
  
   		const instance = new constructor(...args);
  
   		instanceCounter.set(constructor, count + 1);
  
   		console.log(`Created ${count + 1} instances of ${Constr.name} class`);
  
   		return instance;
   }
  
  
   const idleService = getInstance(IdleService);
   // Will log: `Created 1 instances of IdleService class`
   const newsEntry = getInstance(News, 'New ECMAScript proposals!', 'Last month...');
   // Will log: `Created 1 instances of News class`

• Omit<T, K> – Constructs a type by picking all properties from T and then removing K.

  Example

  Playground

   interface Animal {
   		imageUrl: string;
   		species: string;
   		images: string[];
   		paragraphs: string[];
   }
  
   // Creates new type with all properties of the `Animal` interface
   // except 'images' and 'paragraphs' properties. We can use this
   // type to render small hover tooltip for a wiki entry list.
   type AnimalShortInfo = Omit<Animal, 'images' | 'paragraphs'>;
  
   function renderAnimalHoverInfo (animals: AnimalShortInfo[]): HTMLElement {
   		const container = document.createElement('div');
   		// Internal implementation.
   		return container;
   }

• Uppercase<S extends string> - Transforms every character in a string into uppercase.

  Example

   type T = Uppercase<'hello'>;  // 'HELLO'
  
   type T2 = Uppercase<'foo' | 'bar'>;  // 'FOO' | 'BAR'
  
   type T3<S extends string> = Uppercase<`aB${S}`>;
   type T4 = T30<'xYz'>;  // 'ABXYZ'
  
   type T5 = Uppercase<string>;  // string
   type T6 = Uppercase<any>;  // any
   type T7 = Uppercase<never>;  // never
   type T8 = Uppercase<42>;  // Error, type 'number' does not satisfy the constraint 'string'

• Lowercase<S extends string> - Transforms every character in a string into lowercase.

  Example

   type T = Lowercase<'HELLO'>;  // 'hello'
  
   type T2 = Lowercase<'FOO' | 'BAR'>;  // 'foo' | 'bar'
  
   type T3<S extends string> = Lowercase<`aB${S}`>;
   type T4 = T32<'xYz'>;  // 'abxyz'
  
   type T5 = Lowercase<string>;  // string
   type T6 = Lowercase<any>;  // any
   type T7 = Lowercase<never>;  // never
   type T8 = Lowercase<42>;  // Error, type 'number' does not satisfy the constraint 'string'

• Capitalize<S extends string> - Transforms the first character in a string into uppercase.

  Example

   type T = Capitalize<'hello'>;  // 'Hello'
  
   type T2 = Capitalize<'foo' | 'bar'>;  // 'Foo' | 'Bar'
  
   type T3<S extends string> = Capitalize<`aB${S}`>;
   type T4 = T32<'xYz'>;  // 'ABxYz'
  
   type T5 = Capitalize<string>;  // string
   type T6 = Capitalize<any>;  // any
   type T7 = Capitalize<never>;  // never
   type T8 = Capitalize<42>;  // Error, type 'number' does not satisfy the constraint 'string'

• Uncapitalize<S extends string> - Transforms the first character in a string into lowercase.

  Example

   type T = Uncapitalize<'Hello'>;  // 'hello'
  
   type T2 = Uncapitalize<'Foo' | 'Bar'>;  // 'foo' | 'bar'
  
   type T3<S extends string> = Uncapitalize<`AB${S}`>;
   type T4 = T30<'xYz'>;  // 'aBxYz'
  
   type T5 = Uncapitalize<string>;  // string
   type T6 = Uncapitalize<any>;  // any
   type T7 = Uncapitalize<never>;  // never
   type T8 = Uncapitalize<42>;  // Error, type 'number' does not satisfy the constraint 'string'

You can find some examples in the TypeScript docs.

Maintainers

• Sindre Sorhus
• Jarek Radosz
• Dimitri Benin
• Pelle Wessman

License

SPDX-License-Identifier: (MIT OR CC0-1.0)