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1<div align="center">
2 <br>
3 <br>
4 <img src="media/logo.svg" alt="type-fest" height="300">
5 <br>
6 <br>
7 <b>A collection of essential TypeScript types</b>
8 <br>
9 <br>
10 <br>
11 <br>
12 <div align="center">
13 <p>
14 <p>
15 <sup>
16 <a href="https://github.com/sponsors/sindresorhus">Sindre Sorhus' open source work is supported by the community</a>
17 </sup>
18 </p>
19 <sup>Special thanks to:</sup>
20 <br>
21 <br>
22 <a href="https://standardresume.co/tech">
23 <img src="https://sindresorhus.com/assets/thanks/standard-resume-logo.svg" width="180"/>
24 </a>
25 </p>
26 </div>
27 <br>
28 <hr>
29</div>
30<br>
31<br>
32
33[![](https://img.shields.io/badge/unicorn-approved-ff69b4.svg)](https://giphy.com/gifs/illustration-rainbow-unicorn-26AHG5KGFxSkUWw1i)
34[![npm dependents](https://badgen.net/npm/dependents/type-fest)](https://www.npmjs.com/package/type-fest?activeTab=dependents) [![npm downloads](https://badgen.net/npm/dt/type-fest)](https://www.npmjs.com/package/type-fest)
35
36Many 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).
37
38Either add this package as a dependency or copy-paste the needed types. No credit required. 👌
39
40PR welcome for additional commonly needed types and docs improvements. Read the [contributing guidelines](.github/contributing.md) first.
41
42## Install
43
44```
45$ npm install type-fest
46```
47
48*Requires TypeScript >=3.4*
49
50## Usage
51
52```ts
53import {Except} from 'type-fest';
54
55type Foo = {
56 unicorn: string;
57 rainbow: boolean;
58};
59
60type FooWithoutRainbow = Except<Foo, 'rainbow'>;
61//=> {unicorn: string}
62```
63
64## API
65
66Click the type names for complete docs.
67
68### Basic
69
70- [`Primitive`](source/basic.d.ts) - Matches any [primitive value](https://developer.mozilla.org/en-US/docs/Glossary/Primitive).
71- [`Class`](source/basic.d.ts) - Matches a [`class` constructor](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes).
72- [`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`.
73- [`JsonObject`](source/basic.d.ts) - Matches a JSON object.
74- [`JsonArray`](source/basic.d.ts) - Matches a JSON array.
75- [`JsonValue`](source/basic.d.ts) - Matches any valid JSON value.
76- [`ObservableLike`](source/basic.d.ts) - Matches a value that is like an [Observable](https://github.com/tc39/proposal-observable).
77
78### Utilities
79
80- [`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).
81- [`Mutable`](source/mutable.d.ts) - Create a type that strips `readonly` from all or some of an object's keys. The inverse of `Readonly<T>`.
82- [`Merge`](source/merge.d.ts) - Merge two types into a new type. Keys of the second type overrides keys of the first type.
83- [`MergeExclusive`](source/merge-exclusive.d.ts) - Create a type that has mutually exclusive keys.
84- [`RequireAtLeastOne`](source/require-at-least-one.d.ts) - Create a type that requires at least one of the given keys.
85- [`RequireExactlyOne`](source/require-exactly-one.d.ts) - Create a type that requires exactly a single key of the given keys and disallows more.
86- [`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.
87- [`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.
88- [`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).
89- [`Promisable`](source/promisable.d.ts) - Create a type that represents either the value or the value wrapped in `PromiseLike`.
90- [`Opaque`](source/opaque.d.ts) - Create an [opaque type](https://codemix.com/opaque-types-in-javascript/).
91- [`SetOptional`](source/set-optional.d.ts) - Create a type that makes the given keys optional.
92- [`SetRequired`](source/set-required.d.ts) - Create a type that makes the given keys required.
93- [`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.
94- [`PromiseValue`](source/promise-value.d.ts) - Returns the type that is wrapped inside a `Promise`.
95- [`AsyncReturnType`](source/async-return-type.d.ts) - Unwrap the return type of a function that returns a `Promise`.
96- [`ConditionalKeys`](source/conditional-keys.d.ts) - Extract keys from a shape where values extend the given `Condition` type.
97- [`ConditionalPick`](source/conditional-pick.d.ts) - Like `Pick` except it selects properties from a shape where the values extend the given `Condition` type.
98- [`ConditionalExcept`](source/conditional-except.d.ts) - Like `Omit` except it removes properties from a shape where the values extend the given `Condition` type.
99- [`UnionToIntersection`](source/union-to-intersection.d.ts) - Convert a union type to an intersection type.
100- [`Stringified`](source/stringified.d.ts) - Create a type with the keys of the given type changed to `string` type.
101- [`FixedLengthArray`](source/fixed-length-array.d.ts) - Create a type that represents an array of the given type and length.
102- [`IterableElement`](source/iterable-element.d.ts) - Get the element type of an `Iterable`/`AsyncIterable`. For example, an array or a generator.
103- [`Entry`](source/entry.d.ts) - Create a type that represents the type of an entry of a collection.
104- [`Entries`](source/entries.d.ts) - Create a type that represents the type of the entries of a collection.
105- [`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.
106- [`Asyncify`](source/asyncify.d.ts) - Create an async version of the given function type.
107
108### Template literal types
109
110*Note:* These require [TypeScript 4.1 or newer](https://devblogs.microsoft.com/typescript/announcing-typescript-4-1/#template-literal-types).
111
112- [`CamelCase`](ts41/camel-case.d.ts) – Convert a string literal to camel-case (`fooBar`).
113- [`KebabCase`](ts41/kebab-case.d.ts) – Convert a string literal to kebab-case (`foo-bar`).
114- [`PascalCase`](ts41/pascal-case.d.ts) – Converts a string literal to pascal-case (`FooBar`)
115- [`SnakeCase`](ts41/snake-case.d.ts) – Convert a string literal to snake-case (`foo_bar`).
116- [`DelimiterCase`](ts41/delimiter-case.d.ts) – Convert a string literal to a custom string delimiter casing.
117- [`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.
118
119### Miscellaneous
120
121- [`PackageJson`](source/package-json.d.ts) - Type for [npm's `package.json` file](https://docs.npmjs.com/creating-a-package-json-file).
122- [`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).
123
124## Declined types
125
126*If we decline a type addition, we will make sure to document the better solution here.*
127
128- [`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.
129- [`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.
130- [`SubType`](https://github.com/sindresorhus/type-fest/issues/22) - The type is powerful, but lacks good use-cases and is prone to misuse.
131- [`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.
132
133## Tips
134
135### Built-in types
136
137There are many advanced types most users don't know about.
138
139- [`Partial<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1401-L1406) - Make all properties in `T` optional.
140 <details>
141 <summary>
142 Example
143 </summary>
144
145 [Playground](https://www.typescriptlang.org/play/#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)
146
147 ```ts
148 interface NodeConfig {
149 appName: string;
150 port: number;
151 }
152
153 class NodeAppBuilder {
154 private configuration: NodeConfig = {
155 appName: 'NodeApp',
156 port: 3000
157 };
158
159 private updateConfig<Key extends keyof NodeConfig>(key: Key, value: NodeConfig[Key]) {
160 this.configuration[key] = value;
161 }
162
163 config(config: Partial<NodeConfig>) {
164 type NodeConfigKey = keyof NodeConfig;
165
166 for (const key of Object.keys(config) as NodeConfigKey[]) {
167 const updateValue = config[key];
168
169 if (updateValue === undefined) {
170 continue;
171 }
172
173 this.updateConfig(key, updateValue);
174 }
175
176 return this;
177 }
178 }
179
180 // `Partial<NodeConfig>`` allows us to provide only a part of the
181 // NodeConfig interface.
182 new NodeAppBuilder().config({appName: 'ToDoApp'});
183 ```
184 </details>
185
186- [`Required<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1408-L1413) - Make all properties in `T` required.
187 <details>
188 <summary>
189 Example
190 </summary>
191
192 [Playground](https://typescript-play.js.org/?target=6#code/AQ4SwOwFwUwJwGYEMDGNgGED21VQGJZwC2wA3gFCjXAzFJgA2A-AFzADOUckA5gNxUaIYjA4ckvGG07c+g6gF8KQkAgCuEFFDA5O6gEbEwUbLm2ESwABQIixACJIoSdgCUYAR3Vg4MACYAPGYuFvYAfACU5Ko0APRxwADKMBD+wFAAFuh2Vv7OSBlYGdmc8ABu8LHKsRyGxqY4oQT21pTCIHQMjOwA5DAAHgACxAAOjDAAdChYxL0ANLHUouKSMH0AEmAAhJhY6ozpAJ77GTCMjMCiV0ToSAb7UJPPC9WRgrEJwAAqR6MwSRQPFGUFocDgRHYxnEfGAowh-zgUCOwF6KwkUl6tXqJhCeEsxDaS1AXSYfUGI3GUxmc0WSneQA)
193
194 ```ts
195 interface ContactForm {
196 email?: string;
197 message?: string;
198 }
199
200 function submitContactForm(formData: Required<ContactForm>) {
201 // Send the form data to the server.
202 }
203
204 submitContactForm({
205 email: 'ex@mple.com',
206 message: 'Hi! Could you tell me more about…',
207 });
208
209 // TypeScript error: missing property 'message'
210 submitContactForm({
211 email: 'ex@mple.com',
212 });
213 ```
214 </details>
215
216- [`Readonly<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1415-L1420) - Make all properties in `T` readonly.
217 <details>
218 <summary>
219 Example
220 </summary>
221
222 [Playground](https://typescript-play.js.org/?target=6#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)
223
224 ```ts
225 enum LogLevel {
226 Off,
227 Debug,
228 Error,
229 Fatal
230 };
231
232 interface LoggerConfig {
233 name: string;
234 level: LogLevel;
235 }
236
237 class Logger {
238 config: Readonly<LoggerConfig>;
239
240 constructor({name, level}: LoggerConfig) {
241 this.config = {name, level};
242 Object.freeze(this.config);
243 }
244 }
245
246 const config: LoggerConfig = {
247 name: 'MyApp',
248 level: LogLevel.Debug
249 };
250
251 const logger = new Logger(config);
252
253 // TypeScript Error: cannot assign to read-only property.
254 logger.config.level = LogLevel.Error;
255
256 // We are able to edit config variable as we please.
257 config.level = LogLevel.Error;
258 ```
259 </details>
260
261- [`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`.
262 <details>
263 <summary>
264 Example
265 </summary>
266
267 [Playground](https://typescript-play.js.org/?target=6#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)
268
269 ```ts
270 interface Article {
271 title: string;
272 thumbnail: string;
273 content: string;
274 }
275
276 // Creates new type out of the `Article` interface composed
277 // from the Articles' two properties: `title` and `thumbnail`.
278 // `ArticlePreview = {title: string; thumbnail: string}`
279 type ArticlePreview = Pick<Article, 'title' | 'thumbnail'>;
280
281 // Render a list of articles using only title and description.
282 function renderArticlePreviews(previews: ArticlePreview[]): HTMLElement {
283 const articles = document.createElement('div');
284
285 for (const preview of previews) {
286 // Append preview to the articles.
287 }
288
289 return articles;
290 }
291
292 const articles = renderArticlePreviews([
293 {
294 title: 'TypeScript tutorial!',
295 thumbnail: '/assets/ts.jpg'
296 }
297 ]);
298 ```
299 </details>
300
301- [`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`.
302 <details>
303 <summary>
304 Example
305 </summary>
306
307 [Playground](https://typescript-play.js.org/?target=6#code/AQ4ejYAUHsGcCWAXBMB2dgwGbAKYC2ADgDYwCeeemCaWArgE7ADGMxAhmuQHQBQoYEnJE8wALKEARnkaxEKdMAC8wAOS0kstGuAAfdQBM8ANzxlRjXQbVaWACwC0JPB0NqA3HwGgIwAJJoWozYHCxixnAsjAhStADmwESMMJYo1Fi4HMCIaPEu+MRklHj8gpqyoeHAAKJFFFTAAN4+giDYCIxwSAByHAR4AFw5SDF5Xm2gJBzdfQPD3WPxE5PAlBxdAPLYNQAelgh4aOHDaPQEMowrIAC+3oJ+AMKMrlrAXFhSAFZ4LEhC9g4-0BmA4JBISXgiCkBQABpILrJ5MhUGhYcATGD6Bk4Hh-jNgABrPDkOBlXyQAAq9ngYmJpOAAHcEOCRjAXqwYODfoo6DhakUSph+Uh7GI4P0xER4Cj0OSQGwMP8tP1hgAlX7swwAHgRl2RvIANALSA08ABtAC6AD4VM1Wm0Kow0MMrYaHYJjGYLLJXZb3at1HYnC43Go-QHQDcvA6-JsmEJXARgCDgMYWAhjIYhDAU+YiMAAFIwex0ZmilMITCGF79TLAGRsAgJYAAZRwSEZGzEABFTOZUrJ5Yn+jwnWgeER6HB7AAKJrADpdXqS4ZqYultTG6azVfqHswPBbtauLY7fayQ7HIbAAAMwBuAEoYw9IBq2Ixs9h2eFMOQYPQObALQKJgggABeYhghCIpikkKRpOQRIknAsZUiIeCttECBEP8NSMCkjDDAARMGziuIYxHwYOjDCMBmDNnAuTxA6irdCOBB1Lh5Dqpqn66tISIykawBnOCtqqC0gbjqc9DgpGkxegOliyfJDrRkAA)
308
309 ```ts
310 // Positions of employees in our company.
311 type MemberPosition = 'intern' | 'developer' | 'tech-lead';
312
313 // Interface describing properties of a single employee.
314 interface Employee {
315 firstName: string;
316 lastName: string;
317 yearsOfExperience: number;
318 }
319
320 // Create an object that has all possible `MemberPosition` values set as keys.
321 // Those keys will store a collection of Employees of the same position.
322 const team: Record<MemberPosition, Employee[]> = {
323 intern: [],
324 developer: [],
325 'tech-lead': [],
326 };
327
328 // Our team has decided to help John with his dream of becoming Software Developer.
329 team.intern.push({
330 firstName: 'John',
331 lastName: 'Doe',
332 yearsOfExperience: 0
333 });
334
335 // `Record` forces you to initialize all of the property keys.
336 // TypeScript Error: "tech-lead" property is missing
337 const teamEmpty: Record<MemberPosition, null> = {
338 intern: null,
339 developer: null,
340 };
341 ```
342 </details>
343
344- [`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`.
345 <details>
346 <summary>
347 Example
348 </summary>
349
350 [Playground](https://typescript-play.js.org/?target=6#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)
351
352 ```ts
353 interface ServerConfig {
354 port: null | string | number;
355 }
356
357 type RequestHandler = (request: Request, response: Response) => void;
358
359 // Exclude `null` type from `null | string | number`.
360 // In case the port is equal to `null`, we will use default value.
361 function getPortValue(port: Exclude<ServerConfig['port'], null>): number {
362 if (typeof port === 'string') {
363 return parseInt(port, 10);
364 }
365
366 return port;
367 }
368
369 function startServer(handler: RequestHandler, config: ServerConfig): void {
370 const server = require('http').createServer(handler);
371
372 const port = config.port === null ? 3000 : getPortValue(config.port);
373 server.listen(port);
374 }
375 ```
376 </details>
377
378- [`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`.
379 <details>
380 <summary>
381 Example
382 </summary>
383
384 [Playground](https://typescript-play.js.org/?target=6#code/CYUwxgNghgTiAEAzArgOzAFwJYHtXzSwEdkQBJYACgEoAueVZAWwCMQYBuAKDDwGcM8MgBF4AXngBlAJ6scESgHIRi6ty5ZUGdoihgEABXZ888AN5d48ANoiAuvUat23K6ihMQ9ATE0BzV3goPy8GZjZOLgBfLi4Aejj4AEEICBwAdz54MAALKFQQ+BxEeAAHY1NgKAwoIKy0grr4DByEUpgccpgMaXgAaxBerCzi+B9-ZulygDouFHRsU1z8kKMYE1RhaqgAHkt4AHkWACt4EAAPbVRgLLWNgBp9gGlBs8uQa6yAUUuYPQwdgNpKM7nh7mMML4CgA+R5WABqUAgpDeVxuhxO1he0jsXGh8EoOBO9COx3BQPo2PBADckaR6IjkSA6PBqTgsMBzPsicdrEC7OJWXSQNwYvFEgAVTS9JLXODpeDpKBZFg4GCoWa8VACIJykAKiQWKy2YQOAioYikCg0OEMDyhRSy4DyxS24KhAAMjyi6gS8AAwjh5OD0iBFHAkJoEOksC1mnkMJq8gUQKDNttKPlnfrwYp3J5XfBHXqoKpfYkAOI4ansTxaeDADmoRSCCBYAbxhC6TDx6rwYHIRX5bScjA4bLJwoDmDwDkfbA9JMrVMVdM1TN69LgkTgwgkchUahqIA)
385
386 ```ts
387 declare function uniqueId(): number;
388
389 const ID = Symbol('ID');
390
391 interface Person {
392 [ID]: number;
393 name: string;
394 age: number;
395 }
396
397 // Allows changing the person data as long as the property key is of string type.
398 function changePersonData<
399 Obj extends Person,
400 Key extends Extract<keyof Person, string>,
401 Value extends Obj[Key]
402 > (obj: Obj, key: Key, value: Value): void {
403 obj[key] = value;
404 }
405
406 // Tiny Andrew was born.
407 const andrew = {
408 [ID]: uniqueId(),
409 name: 'Andrew',
410 age: 0,
411 };
412
413 // Cool, we're fine with that.
414 changePersonData(andrew, 'name', 'Pony');
415
416 // Goverment didn't like the fact that you wanted to change your identity.
417 changePersonData(andrew, ID, uniqueId());
418 ```
419 </details>
420
421- [`NonNullable<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1446-L1449) - Exclude `null` and `undefined` from `T`.
422 <details>
423 <summary>
424 Example
425 </summary>
426 Works with <code>strictNullChecks</code> set to <code>true</code>. (Read more <a href="https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-0.html">here</a>)
427
428 [Playground](https://typescript-play.js.org/?target=6#code/C4TwDgpgBACg9gJ2AOQK4FsBGEFQLxQDOwCAlgHYDmUAPlORtrnQwDasDcAUFwPQBU-WAEMkUOADMowqAGNWwwoSgATCBIqlgpOOSjAAFsOBRSy1IQgr9cKJlSlW1mZYQA3HFH68u8xcoBlHA8EACEHJ08Aby4oKDBUTFZSWXjEFEYcAEIALihkXTR2YSSIAB54JDQsHAA+blj4xOTUsHSACkMzPKD3HHDHNQQAGjSkPMqMmoQASh7g-oihqBi4uNIpdraxPAI2VhmVxrX9AzMAOm2ppnwoAA4ABifuE4BfKAhWSyOTuK7CS7pao3AhXF5rV48E4ICDAVAIPT-cGQyG+XTEIgLMJLTx7CAAdygvRCA0iCHaMwarhJOIQjUBSHaACJHk8mYdeLwxtdcVAAOSsh58+lXdr7Dlcq7A3n3J4PEUdADMcspUE53OluAIUGVTx46oAKuAIAFZGQwCYAKIIBCILjUxaDHAMnla+iodjcIA)
429
430 ```ts
431 type PortNumber = string | number | null;
432
433 /** Part of a class definition that is used to build a server */
434 class ServerBuilder {
435 portNumber!: NonNullable<PortNumber>;
436
437 port(this: ServerBuilder, port: PortNumber): ServerBuilder {
438 if (port == null) {
439 this.portNumber = 8000;
440 } else {
441 this.portNumber = port;
442 }
443
444 return this;
445 }
446 }
447
448 const serverBuilder = new ServerBuilder();
449
450 serverBuilder
451 .port('8000') // portNumber = '8000'
452 .port(null) // portNumber = 8000
453 .port(3000); // portNumber = 3000
454
455 // TypeScript error
456 serverBuilder.portNumber = null;
457 ```
458 </details>
459
460- [`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.
461 <details>
462 <summary>
463 Example
464 </summary>
465
466 [Playground](https://typescript-play.js.org/?target=6#code/GYVwdgxgLglg9mABAZwBYmMANgUwBQxgAOIUAXIgIZgCeA2gLoCUFAbnDACaIDeAUIkQB6IYgCypSlBxUATrMo1ECsJzgBbLEoipqAc0J7EMKMgDkiHLnU4wp46pwAPHMgB0fAL58+oSLARECEosLAA5ABUYG2QAHgAxJGdpVWREPDdMylk9ZApqemZEAF4APipacrw-CApEgBogkKwAYThwckQwEHUAIxxZJl4BYVEImiIZKF0oZRwiWVdbeygJmThgOYgcGFYcbhqApCJsyhtpWXcR1cnEePBoeDAABVPzgbTixFeFd8uEsClADcIxGiygIFkSEOT3SmTc2VydQeRx+ZxwF2QQ34gkEwDgsnSuFmMBKiAADEDjIhYk1Qm0OlSYABqZnYka4xA1DJZHJYkGc7yCbyeRA+CAIZCzNAYbA4CIAdxg2zJwVCkWirjwMswuEaACYmCCgA)
467
468 ```ts
469 function shuffle(input: any[]): void {
470 // Mutate array randomly changing its' elements indexes.
471 }
472
473 function callNTimes<Fn extends (...args: any[]) => any> (func: Fn, callCount: number) {
474 // Type that represents the type of the received function parameters.
475 type FunctionParameters = Parameters<Fn>;
476
477 return function (...args: FunctionParameters) {
478 for (let i = 0; i < callCount; i++) {
479 func(...args);
480 }
481 }
482 }
483
484 const shuffleTwice = callNTimes(shuffle, 2);
485 ```
486 </details>
487
488- [`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.
489 <details>
490 <summary>
491 Example
492 </summary>
493
494 [Playground](https://typescript-play.js.org/?target=6#code/MYGwhgzhAECCBOAXAlqApgWQPYBM0mgG8AoaaFRENALmgkXmQDsBzAblOmCycTV4D8teo1YdO3JiICuwRFngAKClWENmLAJRFOZRAAtkEAHQq00ALzlklNBzIBfYk+KhIMAJJTEYJsDQAwmDA+mgAPAAq0GgAHnxMODCKTGgA7tCKxllg8CwQtL4AngDaALraFgB80EWa1SRkAA6MAG5gfNAB4FABPDJyCrQR9tDNyG0dwMGhtBhgjWEiGgA00F70vv4RhY3hEZXVVinpc42KmuJkkv3y8Bly8EPaDWTkhiZd7r3e8LK3llwGCMXGQWGhEOsfH5zJlsrl8p0+gw-goAAo5MAAW3BaHgEEilU0tEhmzQ212BJ0ry4SOg+kg+gBBiMximIGA0nAfAQLGk2N4EAAEgzYcYcnkLsRdDTvNEYkYUKwSdCme9WdM0MYwYhFPSIPpJdTkAAzDKxBUaZX+aAAQgsVmkCTQxuYaBw2ng4Ok8CYcotSu8pMur09iG9vuObxZnx6SN+AyUWTF8MN0CcZE4Ywm5jZHK5aB5fP4iCFIqT4oRRTKRLo6lYVNeAHpG50wOzOe1zHr9NLQ+HoABybsD4HOKXXRA1JCoKhBELmI5pNaB6Fz0KKBAodDYPAgSUTmqYsAALx4m5nC6nW9nGq14KtaEUA9gR9PvuNCjQ9BgACNvcwNBtAcLiAA)
495
496 ```ts
497 class ArticleModel {
498 title: string;
499 content?: string;
500
501 constructor(title: string) {
502 this.title = title;
503 }
504 }
505
506 class InstanceCache<T extends (new (...args: any[]) => any)> {
507 private ClassConstructor: T;
508 private cache: Map<string, InstanceType<T>> = new Map();
509
510 constructor (ctr: T) {
511 this.ClassConstructor = ctr;
512 }
513
514 getInstance (...args: ConstructorParameters<T>): InstanceType<T> {
515 const hash = this.calculateArgumentsHash(...args);
516
517 const existingInstance = this.cache.get(hash);
518 if (existingInstance !== undefined) {
519 return existingInstance;
520 }
521
522 return new this.ClassConstructor(...args);
523 }
524
525 private calculateArgumentsHash(...args: any[]): string {
526 // Calculate hash.
527 return 'hash';
528 }
529 }
530
531 const articleCache = new InstanceCache(ArticleModel);
532 const amazonArticle = articleCache.getInstance('Amazon forests burining!');
533 ```
534 </details>
535
536- [`ReturnType<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1461-L1464) – Obtain the return type of a function type.
537 <details>
538 <summary>
539 Example
540 </summary>
541
542 [Playground](https://typescript-play.js.org/?target=6#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-YhAA+qegAAYLKQLQj3ZsEsdccmnGcLor2Dn8xGedHGpEIBzEzspfsfMHDNAANTQACMVaIljV5GQkRA5DYmIpVKQAgAJARO9le33BDXIyi0YuLW2nJFGLqkOvxFB0YPdBSaLZ0IwNzyPkO8-xkGgsLh8Al427a3hWAhXwwHA8EHT5PmgAB1bAQBAANJ24adKWpft72RaBUTgRBUCAj89HAM8xCTaBjggABRQx0DuHJv25P9dCkWRZVIAAiBjoFImpmjlFBgA0NpsjadByDacgIDAEAIAAQmYpjoGYgAZSBsmGPw6DtZiiFA8CoJguDmAQmoZ2QvtUKQLdoAYmBTwgdEiCAA)
543
544 ```ts
545 /** Provides every element of the iterable `iter` into the `callback` function and stores the results in an array. */
546 function mapIter<
547 Elem,
548 Func extends (elem: Elem) => any,
549 Ret extends ReturnType<Func>
550 >(iter: Iterable<Elem>, callback: Func): Ret[] {
551 const mapped: Ret[] = [];
552
553 for (const elem of iter) {
554 mapped.push(callback(elem));
555 }
556
557 return mapped;
558 }
559
560 const setObject: Set<string> = new Set();
561 const mapObject: Map<number, string> = new Map();
562
563 mapIter(setObject, (value: string) => value.indexOf('Foo')); // number[]
564
565 mapIter(mapObject, ([key, value]: [number, string]) => {
566 return key % 2 === 0 ? value : 'Odd';
567 }); // string[]
568 ```
569 </details>
570
571- [`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.
572 <details>
573 <summary>
574 Example
575 </summary>
576
577 [Playground](https://typescript-play.js.org/?target=6#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-YhAA+qegAAYLKQLQj3ZsEsdccmnGcLor2Dn8xGedHGpEIBzEzspfsfMHDNAANTQACMVaIljV5GQkRA5DYmIpVKQAgAJARO9le33BDXIyi0YuLW2nJFGLqkOvxFB0YPdBSaLZ0IwNzyPkO8-xkGgsLh8Al427a3hWAhXwwHA8EHT5PmgAB1bAQBAANJ24adKWpft72RaBUTgRBUCAj89HAM8xCTaBjggABRQx0DuHJv25P9dCkWRZVIAAiBjoFImpmjlFBgA0NpsjadByDacgIDAEAIAAQmYpjoGYgAZSBsmGPw6DtZiiFA8CoJguDmAQmoZ2QvtUKQLdoAYmBTwgdEiCAA)
578
579 ```ts
580 class IdleService {
581 doNothing (): void {}
582 }
583
584 class News {
585 title: string;
586 content: string;
587
588 constructor(title: string, content: string) {
589 this.title = title;
590 this.content = content;
591 }
592 }
593
594 const instanceCounter: Map<Function, number> = new Map();
595
596 interface Constructor {
597 new(...args: any[]): any;
598 }
599
600 // Keep track how many instances of `Constr` constructor have been created.
601 function getInstance<
602 Constr extends Constructor,
603 Args extends ConstructorParameters<Constr>
604 >(constructor: Constr, ...args: Args): InstanceType<Constr> {
605 let count = instanceCounter.get(constructor) || 0;
606
607 const instance = new constructor(...args);
608
609 instanceCounter.set(constructor, count + 1);
610
611 console.log(`Created ${count + 1} instances of ${Constr.name} class`);
612
613 return instance;
614 }
615
616
617 const idleService = getInstance(IdleService);
618 // Will log: `Created 1 instances of IdleService class`
619 const newsEntry = getInstance(News, 'New ECMAScript proposals!', 'Last month...');
620 // Will log: `Created 1 instances of News class`
621 ```
622 </details>
623
624- [`Omit<T, K>`](https://github.com/microsoft/TypeScript/blob/71af02f7459dc812e85ac31365bfe23daf14b4e4/src/lib/es5.d.ts#L1446) – Constructs a type by picking all properties from T and then removing K.
625 <details>
626 <summary>
627 Example
628 </summary>
629
630 [Playground](https://typescript-play.js.org/?target=6#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-SgfXlHqVUE4xiAA)
631
632 ```ts
633 interface Animal {
634 imageUrl: string;
635 species: string;
636 images: string[];
637 paragraphs: string[];
638 }
639
640 // Creates new type with all properties of the `Animal` interface
641 // except 'images' and 'paragraphs' properties. We can use this
642 // type to render small hover tooltip for a wiki entry list.
643 type AnimalShortInfo = Omit<Animal, 'images' | 'paragraphs'>;
644
645 function renderAnimalHoverInfo (animals: AnimalShortInfo[]): HTMLElement {
646 const container = document.createElement('div');
647 // Internal implementation.
648 return container;
649 }
650 ```
651 </details>
652
653- [`Uppercase<S extends string>`](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-1.html#template-literal-types) - Transforms every character in a string into uppercase.
654 <details>
655 <summary>
656 Example
657 </summary>
658
659 ```ts
660 type T = Uppercase<'hello'>; // 'HELLO'
661
662 type T2 = Uppercase<'foo' | 'bar'>; // 'FOO' | 'BAR'
663
664 type T3<S extends string> = Uppercase<`aB${S}`>;
665 type T4 = T30<'xYz'>; // 'ABXYZ'
666
667 type T5 = Uppercase<string>; // string
668 type T6 = Uppercase<any>; // any
669 type T7 = Uppercase<never>; // never
670 type T8 = Uppercase<42>; // Error, type 'number' does not satisfy the constraint 'string'
671 ```
672 </details>
673
674- [`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.
675 <details>
676 <summary>
677 Example
678 </summary>
679
680 ```ts
681 type T = Lowercase<'HELLO'>; // 'hello'
682
683 type T2 = Lowercase<'FOO' | 'BAR'>; // 'foo' | 'bar'
684
685 type T3<S extends string> = Lowercase<`aB${S}`>;
686 type T4 = T32<'xYz'>; // 'abxyz'
687
688 type T5 = Lowercase<string>; // string
689 type T6 = Lowercase<any>; // any
690 type T7 = Lowercase<never>; // never
691 type T8 = Lowercase<42>; // Error, type 'number' does not satisfy the constraint 'string'
692 ```
693 </details>
694
695- [`Capitalize<S extends string>`](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-1.html#template-literal-types) - Transforms the first character in a string into uppercase.
696 <details>
697 <summary>
698 Example
699 </summary>
700
701 ```ts
702 type T = Capitalize<'hello'>; // 'Hello'
703
704 type T2 = Capitalize<'foo' | 'bar'>; // 'Foo' | 'Bar'
705
706 type T3<S extends string> = Capitalize<`aB${S}`>;
707 type T4 = T32<'xYz'>; // 'ABxYz'
708
709 type T5 = Capitalize<string>; // string
710 type T6 = Capitalize<any>; // any
711 type T7 = Capitalize<never>; // never
712 type T8 = Capitalize<42>; // Error, type 'number' does not satisfy the constraint 'string'
713 ```
714 </details>
715
716- [`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.
717 <details>
718 <summary>
719 Example
720 </summary>
721
722 ```ts
723 type T = Uncapitalize<'Hello'>; // 'hello'
724
725 type T2 = Uncapitalize<'Foo' | 'Bar'>; // 'foo' | 'bar'
726
727 type T3<S extends string> = Uncapitalize<`AB${S}`>;
728 type T4 = T30<'xYz'>; // 'aBxYz'
729
730 type T5 = Uncapitalize<string>; // string
731 type T6 = Uncapitalize<any>; // any
732 type T7 = Uncapitalize<never>; // never
733 type T8 = Uncapitalize<42>; // Error, type 'number' does not satisfy the constraint 'string'
734 ```
735 </details>
736
737You can find some examples in the [TypeScript docs](https://www.typescriptlang.org/docs/handbook/advanced-types.html#predefined-conditional-types).
738
739## Maintainers
740
741- [Sindre Sorhus](https://github.com/sindresorhus)
742- [Jarek Radosz](https://github.com/CvX)
743- [Dimitri Benin](https://github.com/BendingBender)
744- [Pelle Wessman](https://github.com/voxpelli)
745
746## License
747
748(MIT OR CC0-1.0)
749
750---
751
752<div align="center">
753 <b>
754 <a href="https://tidelift.com/subscription/pkg/npm-type-fest?utm_source=npm-type-fest&utm_medium=referral&utm_campaign=readme">Get professional support for this package with a Tidelift subscription</a>
755 </b>
756 <br>
757 <sub>
758 Tidelift helps make open source sustainable for maintainers while giving companies<br>assurances about security, maintenance, and licensing for their dependencies.
759 </sub>
760</div>
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