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readme.md

A collection of essential TypeScript types

^{Sindre Sorhus' open source work is supported by the community}

^{Special thanks to:}

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 - 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.
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.
Get - Get a deeply-nested property from an object using a key path, like Lodash's .get() function.

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;
}

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

License

(MIT OR CC0-1.0)

Get professional support for this package with a Tidelift subscription
_{Tidelift helps make open source sustainable for maintainers while giving companies
assurances about security, maintenance, and licensing for their dependencies.}

readme.md Unescape Escape

Install

Usage

API

Basic

Utilities

Template literal types

Miscellaneous

Declined types

Tips

Built-in types

Maintainers

License

readme.md