type-fest

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 >=3.4

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 keys into a mutable object. 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.
• 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.
• Promisable - Create a type that represents either the value or the value wrapped in PromiseLike.
• 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.
• PromiseValue - Returns the type that is wrapped inside a Promise.
• AsyncReturnType - Unwrap the return type of a function that returns a Promise.
• 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.
• FixedLengthArray - Create a type that represents an array of the given type and length.
• 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.
• Asyncify - Create an async version of the given function type.

Template literal types

Note: These require TypeScript 4.1 or newer.

• CamelCase – Convert a string literal to camel-case (fooBar).
• KebabCase – Convert a string literal to kebab-case (foo-bar).
• PascalCase – Converts a string literal to pascal-case (FooBar)
• SnakeCase – Convert a string literal to snake-case (foo_bar).
• DelimiterCase – Convert a string literal to a custom string delimiter casing.

Miscellaneous

• PackageJson - Type for npm's package.json file.
• TsConfigJson - Type for TypeScript's tsconfig.json file (TypeScript 3.7).

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.
• SubType - The type is powerful, but lacks good use-cases and is prone to misuse.
• ExtractProperties and ExtractMethods - The types violate the single responsibility principle. Instead, refine your types into more granular type hierarchies.

Tips

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: 'ex@mple.com',
  		message: 'Hi! Could you tell me more about…',
  });
  
  // TypeScript error: missing property 'message'
  submitContactForm({
  		email: 'ex@mple.com',
  });
  

• 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. (Read more here)

  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;
  }
  

You can find some examples in the TypeScript docs.

Maintainers

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

License

(MIT OR CC0-1.0)

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