node_modules/type-fest/readme.md

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

License

(MIT OR CC0-1.0)