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 >=3.4*
## Usage
```ts
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`](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/typed-array.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).
- [`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<T>`](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<T>`](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.
- [`FixedLengthArray`](source/fixed-length-array.d.ts) - Create a type that represents an array of the given type and length.
- [`IterableElement`](source/iterable-element.d.ts) - Get the element type of an `Iterable`/`AsyncIterable`. For example, an array or a generator.
- [`Entry`](source/entry.d.ts) - Create a type that represents the type of an entry of a collection.
- [`Entries`](source/entries.d.ts) - Create a type that represents the type of the entries of a collection.
- [`SetReturnType`](source/set-return-type.d.ts) - Create a function type with a return type of your choice and the same parameters as the given function type.
- [`Asyncify`](source/asyncify.d.ts) - Create an async version of the given function type.
### Template literal types
*Note:* These require [TypeScript 4.1 or newer](https://devblogs.microsoft.com/typescript/announcing-typescript-4-1/#template-literal-types).
- [`CamelCase`](ts41/camel-case.d.ts) – Convert a string literal to camel-case (`fooBar`).
- [`KebabCase`](ts41/kebab-case.d.ts) – Convert a string literal to kebab-case (`foo-bar`).
- [`PascalCase`](ts41/pascal-case.d.ts) – Converts a string literal to pascal-case (`FooBar`)
- [`SnakeCase`](ts41/snake-case.d.ts) – Convert a string literal to snake-case (`foo_bar`).
- [`DelimiterCase`](ts41/delimiter-case.d.ts) – Convert a string literal to a custom string delimiter casing.
- [`Get`](ts41/get.d.ts) - Get a deeply-nested property from an object using a key path, like [Lodash's `.get()`](https://lodash.com/docs/latest#get) function.
- [`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<number>` vs `Record<string, number>`) 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<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1401-L1406) - Make all properties in `T` optional.
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'});
```
</details>
- [`Required<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1408-L1413) - Make all properties in `T` required.
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',
});
```
</details>
- [`Readonly<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1415-L1420) - Make all properties in `T` readonly.
// 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;
```
</details>
- [`Pick<T, K>`](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`.
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'
}
]);
```
</details>
- [`Record<K, T>`](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`.
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,
};
```
</details>
- [`Exclude<T, U>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1436-L1439) - Exclude from `T` those types that are assignable to `U`.
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);
}
```
</details>
- [`Extract<T, U>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1441-L1444) - Extract from `T` those types that are assignable to `U`.
// Allows changing the person data as long as the property key is of string type.
function changePersonData<
Obj extends Person,
Key extends Extract<keyofPerson,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());
```
</details>
- [`NonNullable<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1446-L1449) - Exclude `null` and `undefined` from `T`.
<details>
<summary>
Example
</summary>
Works with <code>strictNullChecks</code> set to <code>true</code>. (Read more <ahref="https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-0.html">here</a>)
- [`Parameters<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1451-L1454) - Obtain the parameters of a function type in a tuple.
// Mutate array randomly changing its' elements indexes.
}
function callNTimes<Fnextends(...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);
```
</details>
- [`ConstructorParameters<T>`](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.
- [`ReturnType<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1461-L1464) – Obtain the return type of a function type.
- [`InstanceType<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1466-L1469) – Obtain the instance type of a constructor function type.
- [`Omit<T, K>`](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.
- [`Uppercase<S extends string>`](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-1.html#template-literal-types) - Transforms every character in a string into uppercase.
<details>
<summary>
Example
</summary>
```ts
type T = Uppercase<'hello'>; // 'HELLO'
type T2 = Uppercase<'foo' | 'bar'>; // 'FOO' | 'BAR'
type T3<Sextendsstring> = Uppercase<`aB${S}`>;
type T4 = T30<'xYz'>; // 'ABXYZ'
type T5 = Uppercase<string>; // string
type T6 = Uppercase<any>; // any
type T7 = Uppercase<never>; // never
type T8 = Uppercase<42>; // Error, type 'number' does not satisfy the constraint 'string'
```
</details>
- [`Lowercase<S extends string>`](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-1.html#template-literal-types) - Transforms every character in a string into lowercase.
<details>
<summary>
Example
</summary>
```ts
type T = Lowercase<'HELLO'>; // 'hello'
type T2 = Lowercase<'FOO' | 'BAR'>; // 'foo' | 'bar'
type T3<Sextendsstring> = Lowercase<`aB${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'
```
</details>
- [`Capitalize<S extends string>`](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-1.html#template-literal-types) - Transforms the first character in a string into uppercase.
<details>
<summary>
Example
</summary>
```ts
type T = Capitalize<'hello'>; // 'Hello'
type T2 = Capitalize<'foo' | 'bar'>; // 'Foo' | 'Bar'
type T3<Sextendsstring> = Capitalize<`aB${S}`>;
type T4 = T32<'xYz'>; // 'ABxYz'
type T5 = Capitalize<string>; // string
type T6 = Capitalize<any>; // any
type T7 = Capitalize<never>; // never
type T8 = Capitalize<42>; // Error, type 'number' does not satisfy the constraint 'string'
```
</details>
- [`Uncapitalize<S extends string>`](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-1.html#template-literal-types) - Transforms the first character in a string into lowercase.
<details>
<summary>
Example
</summary>
```ts
type T = Uncapitalize<'Hello'>; // 'hello'
type T2 = Uncapitalize<'Foo' | 'Bar'>; // 'foo' | 'bar'
type T3<Sextendsstring> = Uncapitalize<`AB${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'
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