type-fest

A collection of essential TypeScript types

Usage no npm install needed!

<script type="module">
  import typeFest from 'https://cdn.skypack.dev/type-fest';
</script>

README



npm dependents npm downloads Docs

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.

Help wanted with reviewing proposals and pull requests.

Install

npm install type-fest

Requires TypeScript >=4.2

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.
  • RequireAllOrNone - Create a type that requires all of the given keys or none of the given keys.
  • RemoveIndexSignature - Create a type that only has explicitly defined properties, absent of any index signatures.
  • 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.
  • Opaque - Create an opaque type.
  • InvariantOf - Create an invariant type, which is a type that does not accept supertypes and subtypes.
  • 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.
  • 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.
  • LiteralToPrimitive - Convert a literal type to the primitive type it belongs to.
  • Stringified - Create a type with the keys of the given type changed to string type.
  • 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.
  • Simplify - Useful to flatten the type output to improve type hints shown in editors. And also to transform an interface into a type to aide with assignability.
  • Get - Get a deeply-nested property from an object using a key path, like Lodash's .get() function.
  • StringKeyOf - Get keys of the given type as strings.
  • Schema - Create a deep version of another object type where property values are recursively replaced into a given value type.

JSON

Async

  • Promisable - Create a type that represents either the value or the value wrapped in PromiseLike.
  • AsyncReturnType - Unwrap the return type of a function that returns a Promise.
  • Asyncify - Create an async version of the given function type.

String

  • Trim - Remove leading and trailing spaces from a string.
  • Split - Represents an array of strings split using a given character or character set.

Array

  • Includes - Returns a boolean for whether the given array includes the given item.
  • Join - Join an array of strings and/or numbers using the given string as a delimiter.
  • LastArrayElement - Extracts the type of the last element of an array.
  • FixedLengthArray - Create a type that represents an array of the given type and length.
  • MultidimensionalArray - Create a type that represents a multidimensional array of the given type and dimensions.
  • MultidimensionalReadonlyArray - Create a type that represents a multidimensional readonly array of the given type and dimensions.

Numeric

Change case

Miscellaneous

Declined types

If we decline a type addition, we will make sure to document the better solution here.

  • Diff and Spread - The pull request 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.
  • Url2Json - Inferring search parameters from a URL string is a cute idea, but not very useful in practice, since search parameters are usually dynamic and defined separately.
  • Nullish - The type only saves a couple of characters, not everyone knows what "nullish" means, and I'm also trying to get away from null.
  • TitleCase - It's not solving a common need and is a better fit for a separate package.
  • ExtendOr and ExtendAnd - The benefits don't outweigh having to learn what they mean.

Alternative type names

If you know one of our types by a different name, add it here for discovery.

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.

    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

SPDX-License-Identifier: (MIT OR CC0-1.0)

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