A collection of essential TypeScript types

[![Build Status](https://travis-ci.com/sindresorhus/type-fest.svg?branch=master)](https://travis-ci.com/sindresorhus/type-fest) [![](https://img.shields.io/badge/unicorn-approved-ff69b4.svg)](https://www.youtube.com/watch?v=9auOCbH5Ns4) Many of the types here should have been built-in. You can help by suggesting some of them to the [TypeScript project](https://github.com/Microsoft/TypeScript/blob/master/CONTRIBUTING.md). 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](.github/contributing.md) first. ## Install ``` $ npm install type-fest ``` *Requires TypeScript >=3.2* ## Usage ```ts import {Except} from 'type-fest'; type Foo = { unicorn: string; rainbow: boolean; }; type FooWithoutRainbow = Except; //=> {unicorn: string} ``` ## API Click the type names for complete docs. ### Basic - [`Primitive`](source/basic.d.ts) - Matches any [primitive value](https://developer.mozilla.org/en-US/docs/Glossary/Primitive). - [`Class`](source/basic.d.ts) - Matches a [`class` constructor](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes). - [`TypedArray`](source/basic.d.ts) - Matches any [typed array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray), like `Uint8Array` or `Float64Array`. - [`JsonObject`](source/basic.d.ts) - Matches a JSON object. - [`JsonArray`](source/basic.d.ts) - Matches a JSON array. - [`JsonValue`](source/basic.d.ts) - Matches any valid JSON value. - [`ObservableLike`](source/basic.d.ts) - Matches a value that is like an [Observable](https://github.com/tc39/proposal-observable). ### Utilities - [`Except`](source/except.d.ts) - Create a type from an object type without certain keys. This is a stricter version of [`Omit`](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-5.html#the-omit-helper-type). - [`Mutable`](source/mutable.d.ts) - Convert an object with `readonly` keys into a mutable object. The inverse of `Readonly`. - [`Merge`](source/merge.d.ts) - Merge two types into a new type. Keys of the second type overrides keys of the first type. - [`MergeExclusive`](source/merge-exclusive.d.ts) - Create a type that has mutually exclusive keys. - [`RequireAtLeastOne`](source/require-at-least-one.d.ts) - Create a type that requires at least one of the given keys. - [`RequireExactlyOne`](source/require-exactly-one.d.ts) - Create a type that requires exactly a single key of the given keys and disallows more. - [`PartialDeep`](source/partial-deep.d.ts) - Create a deeply optional version of another type. Use [`Partial`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1401-L1406) if you only need one level deep. - [`ReadonlyDeep`](source/readonly-deep.d.ts) - Create a deeply immutable version of an `object`/`Map`/`Set`/`Array` type. Use [`Readonly`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1415-L1420) if you only need one level deep. - [`LiteralUnion`](source/literal-union.d.ts) - 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](https://github.com/Microsoft/TypeScript/issues/29729). - [`Promisable`](source/promisable.d.ts) - Create a type that represents either the value or the value wrapped in `PromiseLike`. - [`Opaque`](source/opaque.d.ts) - Create an [opaque type](https://codemix.com/opaque-types-in-javascript/). - [`SetOptional`](source/set-optional.d.ts) - Create a type that makes the given keys optional. - [`SetRequired`](source/set-required.d.ts) - Create a type that makes the given keys required. - [`ValueOf`](source/value-of.d.ts) - Create a union of the given object's values, and optionally specify which keys to get the values from. - [`PromiseValue`](source/promise-value.d.ts) - Returns the type that is wrapped inside a `Promise`. - [`AsyncReturnType`](source/async-return-type.d.ts) - Unwrap the return type of a function that returns a `Promise`. - [`ConditionalKeys`](source/conditional-keys.d.ts) - Extract keys from a shape where values extend the given `Condition` type. - [`ConditionalPick`](source/conditional-pick.d.ts) - Like `Pick` except it selects properties from a shape where the values extend the given `Condition` type. - [`ConditionalExcept`](source/conditional-except.d.ts) - Like `Omit` except it removes properties from a shape where the values extend the given `Condition` type. - [`UnionToIntersection`](source/union-to-intersection.d.ts) - Convert a union type to an intersection type. - [`Stringified`](source/stringified.d.ts) - Create a type with the keys of the given type changed to `string` type. ### Miscellaneous - [`PackageJson`](source/package-json.d.ts) - Type for [npm's `package.json` file](https://docs.npmjs.com/creating-a-package-json-file). - [`TsConfigJson`](source/tsconfig-json.d.ts) - Type for [TypeScript's `tsconfig.json` file](https://www.typescriptlang.org/docs/handbook/tsconfig-json.html) (TypeScript 3.7). ## Declined types *If we decline a type addition, we will make sure to document the better solution here.* - [`Diff` and `Spread`](https://github.com/sindresorhus/type-fest/pull/7) - 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`](https://github.com/sindresorhus/type-fest/issues/33) - You only save a few characters (`Dictionary` vs `Record`) from [`Record`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1429-L1434), which is more flexible and well-known. Also, you shouldn't use an object as a dictionary. We have `Map` in JavaScript now. - [`SubType`](https://github.com/sindresorhus/type-fest/issues/22) - The type is powerful, but lacks good use-cases and is prone to misuse. - [`ExtractProperties` and `ExtractMethods`](https://github.com/sindresorhus/type-fest/pull/4) - 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`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1401-L1406) - Make all properties in `T` optional.
Example
[Playground](https://www.typescriptlang.org/play/# ) ```ts interface NodeConfig { appName: string; port: number; } class NodeAppBuilder { private configuration: NodeConfig = { appName: 'NodeApp', port: 3000 }; private updateConfig(key: Key, value: NodeConfig[Key]) { this.configuration[key] = value; } config(config: Partial) { 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`` allows us to provide only a part of the // NodeConfig interface. new NodeAppBuilder().config({appName: 'ToDoApp'}); ```
- [`Required`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1408-L1413) - Make all properties in `T` required.
Example
[Playground](https://typescript-play.js.org/?target=6#code/AQ4SwOwFwUwJwGYEMDGNgGED21VQGJZwC2wA3gFCjXAzFJgA2A-AFzADOUckA5gNxUaIYjA4ckvGG07c+g6gF8KQkAgCuEFFDA5O6gEbEwUbLm2ESwABQIixACJIoSdgCUYAR3Vg4MACYAPGYuFvYAfACU5Ko0APRxwADKMBD+wFAAFuh2Vv7OSBlYGdmc8ABu8LHKsRyGxqY4oQT21pTCIHQMjOwA5DAAHgACxAAOjDAAdChYxL0ANLHUouKSMH0AEmAAhJhY6ozpAJ77GTCMjMCiV0ToSAb7UJPPC9WRgrEJwAAqR6MwSRQPFGUFocDgRHYxnEfGAowh-zgUCOwF6KwkUl6tXqJhCeEsxDaS1AXSYfUGI3GUxmc0WSneQA) ```ts interface ContactForm { email?: string; message?: string; } function submitContactForm(formData: Required) { // 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`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1415-L1420) - Make all properties in `T` readonly.
Example
[Playground](https://typescript-play.js.org/?target=6# ) ```ts enum LogLevel { Off, Debug, Error, Fatal }; interface LoggerConfig { name: string; level: LogLevel; } class Logger { config: Readonly; 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`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1422-L1427) - From `T`, pick a set of properties whose keys are in the union `K`.
Example
[Playground](https://typescript-play.js.org/?target=6# ) ```ts 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; // 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`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1429-L1434) - Construct a type with a set of properties `K` of type `T`.
Example
[Playground](https://typescript-play.js.org/?target=6# ) ```ts // 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 = { 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 = { intern: null, developer: null, }; ```
- [`Exclude`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1436-L1439) - Exclude from `T` those types that are assignable to `U`.
Example
[Playground](https://typescript-play.js.org/?target=6# ) ```ts 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): 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`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1441-L1444) - Extract from `T` those types that are assignable to `U`.
Example
[Playground](https://typescript-play.js.org/?target=6# ) ```ts 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, 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`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1446-L1449) - Exclude `null` and `undefined` from `T`.
Example
Works with `strictNullChecks` set to `true`. (Read more here) [Playground](https://typescript-play.js.org/?target=6#code/C4TwDgpgBACg9gJ2AOQK4FsBGEFQLxQDOwCAlgHYDmUAPlORtrnQwDasDcAUFwPQBU-WAEMkUOADMowqAGNWwwoSgATCBIqlgpOOSjAAFsOBRSy1IQgr9cKJlSlW1mZYQA3HFH68u8xcoBlHA8EACEHJ08Aby4oKDBUTFZSWXjEFEYcAEIALihkXTR2YSSIAB54JDQsHAA+blj4xOTUsHSACkMzPKD3HHDHNQQAGjSkPMqMmoQASh7g-oihqBi4uNIpdraxPAI2VhmVxrX9AzMAOm2ppnwoAA4ABifuE4BfKAhWSyOTuK7CS7pao3AhXF5rV48E4ICDAVAIPT-cGQyG+XTEIgLMJLTx7CAAdygvRCA0iCHaMwarhJOIQjUBSHaACJHk8mYdeLwxtdcVAAOSsh58+lXdr7Dlcq7A3n3J4PEUdADMcspUE53OluAIUGVTx46oAKuAIAFZGQwCYAKIIBCILjUxaDHAMnla+iodjcIA) ```ts type PortNumber = string | number | null; /** Part of a class definition that is used to build a server */ class ServerBuilder { portNumber!: NonNullable; 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`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1451-L1454) - Obtain the parameters of a function type in a tuple.
Example
[Playground](https://typescript-play.js.org/?target=6#code/GYVwdgxgLglg9mABAZwBYmMANgUwBQxgAOIUAXIgIZgCeA2gLoCUFAbnDACaIDeAUIkQB6IYgCypSlBxUATrMo1ECsJzgBbLEoipqAc0J7EMKMgDkiHLnU4wp46pwAPHMgB0fAL58+oSLARECEosLAA5ABUYG2QAHgAxJGdpVWREPDdMylk9ZApqemZEAF4APipacrw-CApEgBogkKwAYThwckQwEHUAIxxZJl4BYVEImiIZKF0oZRwiWVdbeygJmThgOYgcGFYcbhqApCJsyhtpWXcR1cnEePBoeDAABVPzgbTixFeFd8uEsClADcIxGiygIFkSEOT3SmTc2VydQeRx+ZxwF2QQ34gkEwDgsnSuFmMBKiAADEDjIhYk1Qm0OlSYABqZnYka4xA1DJZHJYkGc7yCbyeRA+CAIZCzNAYbA4CIAdxg2zJwVCkWirjwMswuEaACYmCCgA) ```ts function shuffle(input: any[]): void { // Mutate array randomly changing its' elements indexes. } function callNTimes any> (func: Fn, callCount: number) { // Type that represents the type of the received function parameters. type FunctionParameters = Parameters; return function (...args: FunctionParameters) { for (let i = 0; i < callCount; i++) { func(...args); } } } const shuffleTwice = callNTimes(shuffle, 2); ```
- [`ConstructorParameters`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1456-L1459) - Obtain the parameters of a constructor function type in a tuple.
Example
[Playground](https://typescript-play.js.org/?target=6# ) ```ts class ArticleModel { title: string; content?: string; constructor(title: string) { this.title = title; } } class InstanceCache any)> { private ClassConstructor: T; private cache: Map> = new Map(); constructor (ctr: T) { this.ClassConstructor = ctr; } getInstance (...args: ConstructorParameters): InstanceType { 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`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1461-L1464) – Obtain the return type of a function type.
Example
[Playground](https://typescript-play.js.org/?target=6# ) ```ts /** 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 >(iter: Iterable, callback: Func): Ret[] { const mapped: Ret[] = []; for (const elem of iter) { mapped.push(callback(elem)); } return mapped; } const setObject: Set = new Set(); const mapObject: Map = new Map(); mapIter(setObject, (value: string) => value.indexOf('Foo')); // number[] mapIter(mapObject, ([key, value]: [number, string]) => { return key % 2 === 0 ? value : 'Odd'; }); // string[] ```
- [`InstanceType`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1466-L1469) – Obtain the instance type of a constructor function type.
Example
[Playground](https://typescript-play.js.org/?target=6# ) ```ts class IdleService { doNothing (): void {} } class News { title: string; content: string; constructor(title: string, content: string) { this.title = title; this.content = content; } } const instanceCounter: Map = 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 >(constructor: Constr, ...args: Args): InstanceType { 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`](https://github.com/microsoft/TypeScript/blob/71af02f7459dc812e85ac31365bfe23daf14b4e4/src/lib/es5.d.ts#L1446) – Constructs a type by picking all properties from T and then removing K.
Example
[Playground](https://typescript-play.js.org/?target=6# ) ```ts 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; function renderAnimalHoverInfo (animals: AnimalShortInfo[]): HTMLElement { const container = document.createElement('div'); // Internal implementation. return container; } ```
You can find some examples in the [TypeScript docs](https://www.typescriptlang.org/docs/handbook/advanced-types.html#predefined-conditional-types). ## Maintainers - [Sindre Sorhus](https://github.com/sindresorhus) - [Jarek Radosz](https://github.com/CvX) - [Dimitri Benin](https://github.com/BendingBender) ## License (MIT OR CC0-1.0) ---
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