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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.

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

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.

Miscellaneous

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.

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: '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)


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