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.
$ npm install type-fest
Requires TypeScript >=3.2
import {Except} from 'type-fest';
type Foo = {
unicorn: string;
rainbow: boolean;
};
type FooWithoutRainbow = Except<Foo, 'rainbow'>;
//=> {unicorn: string}
Click the type names for complete docs.
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.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.PackageJson - Type for npm's package.json file.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.There are many advanced types most users don't know about.
Partial<T> - Make all properties in T optional.
```ts
interface NodeConfig {
appName: string;
port: number;
}
class NodeAppBuilder {
private configuration: NodeConfig = {
appName: 'NodeApp',
port: 3000
};
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.configuration[key] = updateValue;
}
return this;
}
// `Partial`` allows us to provide only a part of the
// NodeConfig interface.
new NodeAppBuilder().config({appName: 'ToDoApp'});
```
Required<T> - Make all properties in T required.
```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<T> - Make all properties in T readonly.
```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<T, K> - From T, pick a set of properties whose keys are in the union K.
```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<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.
```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<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.
```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<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.
```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<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.
strictNullChecks set to true. (Read more here)```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<T> - Obtain the parameters of a function type in a tuple.
```ts
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;
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.
```ts
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> = 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.
``ts /** Provides every element of the iterableiterinto thecallback` 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<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.
```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<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
(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`
```
</details>
Omit<T, K> – Constructs a type by picking all properties from T and then removing K.
```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<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.
(MIT OR CC0-1.0)