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 <hr> 10</div> 11<br> 12<br> 13 14[![Build Status](https://travis-ci.com/sindresorhus/type-fest.svg?branch=master)](https://travis-ci.com/sindresorhus/type-fest) 15[![](https://img.shields.io/badge/unicorn-approved-ff69b4.svg)](https://www.youtube.com/watch?v=9auOCbH5Ns4) 16<!-- Commented out until they actually show anything 17[![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) 18--> 19 20Many 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). 21 22Either add this package as a dependency or copy-paste the needed types. No credit required. 23 24PR welcome for additional commonly needed types and docs improvements. Read the [contributing guidelines](.github/contributing.md) first. 25 26 27## Install 28 29``` 30$ npm install type-fest 31``` 32 33*Requires TypeScript >=3.2* 34 35 36## Usage 37 38```ts 39import {Except} from 'type-fest'; 40 41type Foo = { 42 unicorn: string; 43 rainbow: boolean; 44}; 45 46type FooWithoutRainbow = Except<Foo, 'rainbow'>; 47//=> {unicorn: string} 48``` 49 50 51## API 52 53Click the type names for complete docs. 54 55### Basic 56 57- [`Primitive`](source/basic.d.ts) - Matches any [primitive value](https://developer.mozilla.org/en-US/docs/Glossary/Primitive). 58- [`Class`](source/basic.d.ts) - Matches a [`class` constructor](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes). 59- [`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`. 60- [`JsonObject`](source/basic.d.ts) - Matches a JSON object. 61- [`JsonArray`](source/basic.d.ts) - Matches a JSON array. 62- [`JsonValue`](source/basic.d.ts) - Matches any valid JSON value. 63- [`ObservableLike`](source/basic.d.ts) - Matches a value that is like an [Observable](https://github.com/tc39/proposal-observable). 64 65### Utilities 66 67- [`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). 68- [`Mutable`](source/mutable.d.ts) - Convert an object with `readonly` keys into a mutable object. The inverse of `Readonly<T>`. 69- [`Merge`](source/merge.d.ts) - Merge two types into a new type. Keys of the second type overrides keys of the first type. 70- [`MergeExclusive`](source/merge-exclusive.d.ts) - Create a type that has mutually exclusive keys. 71- [`RequireAtLeastOne`](source/require-at-least-one.d.ts) - Create a type that requires at least one of the given keys. 72- [`RequireExactlyOne`](source/require-one.d.ts) - Create a type that requires exactly a single key of the given keys and disallows more. 73- [`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. 74- [`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. 75- [`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). 76- [`Promisable`](source/promisable.d.ts) - Create a type that represents either the value or the value wrapped in `PromiseLike`. 77- [`Opaque`](source/opaque.d.ts) - Create an [opaque type](https://codemix.com/opaque-types-in-javascript/). 78- [`SetOptional`](source/set-optional.d.ts) - Create a type that makes the given keys optional. 79- [`SetRequired`](source/set-required.d.ts) - Create a type that makes the given keys required. 80 81### Miscellaneous 82 83- [`PackageJson`](source/package-json.d.ts) - Type for [npm's `package.json` file](https://docs.npmjs.com/creating-a-package-json-file). 84 85 86## Declined types 87 88*If we decline a type addition, we will make sure to document the better solution here.* 89 90- [`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. 91- [`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. 92 93 94## Tips 95 96### Built-in types 97 98There are many advanced types most users don't know about. 99 100- [`Partial<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1401-L1406) - Make all properties in `T` optional. 101 <details> 102 <summary> 103 Example 104 </summary> 105 106 [Playground](https://typescript-play.js.org/?target=6#code/KYOwrgtgBAMg9gcxsAbsANlA3gKClAeQDMiAaPKAEWACMwFz8BRAJxbhcagDEBDAF17ocAXxw4AliH7AWRXgGNgUAHJwAJsADCcEEQkJsFXgAcTK3hGAAuKAGd+LKQgDcFEx363wEGrLf46IjIaOi28EioGG5iOArovHZ2qhrAAIJmAEJgEuiaLEb4Jk4oAsoKuvoIYCwCErq2apo6egZQALyF+FCm5pY2UABETelmg1xFnrYAzAAM8xNQQZGh4cFR6AB0xEQUIm4UFa0IABRHVbYACrws-BJCADwjLVUAfACUXfhEHFBnug4oABrYAATygcCIhBoACtgAp+JsQaC7P9ju9Prhut0joCwCZ1GUAGpCMDKTrnAwAbWRPWSyMhKWalQMAF0Dtj8BIoSd8YSZCT0GSOu1OmAQJp9CBgOpPkc7uBgBzOfwABYSOybSnVWp3XQ0sF04FgxnPFkIVkdKB84mkpUUfCxbEsYD8GogKBqjUBKBiWIAen9UGut3u6CeqReBlePXQQQA7skwMl+HAoMU4CgJJoISB0ODeOmbvwIVC1cAcIGmdpzVApDI5IpgJscNL49WMiZsrl8id3lrzScsD0zBYrLZBgAVOCUOCdwa+95uIA) 107 108 ```ts 109 interface NodeConfig { 110 appName: string; 111 port: number; 112 } 113 114 class NodeAppBuilder { 115 private configuration: NodeConfig = { 116 appName: 'NodeApp', 117 port: 3000 118 }; 119 120 config(config: Partial<NodeConfig>) { 121 type NodeConfigKey = keyof NodeConfig; 122 123 for (const key of Object.keys(config) as NodeConfigKey[]) { 124 const updateValue = config[key]; 125 126 if (updateValue === undefined) { 127 continue; 128 } 129 130 this.configuration[key] = updateValue; 131 } 132 133 return this; 134 } 135 } 136 137 // `Partial<NodeConfig>`` allows us to provide only a part of the 138 // NodeConfig interface. 139 new NodeAppBuilder().config({appName: 'ToDoApp'}); 140 ``` 141 </details> 142 143- [`Required<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1408-L1413) - Make all properties in `T` required. 144 <details> 145 <summary> 146 Example 147 </summary> 148 149 [Playground](https://typescript-play.js.org/?target=6#code/AQ4SwOwFwUwJwGYEMDGNgGED21VQGJZwC2wA3gFCjXAzFJgA2A-AFzADOUckA5gNxUaIYjA4ckvGG07c+g6gF8KQkAgCuEFFDA5O6gEbEwUbLm2ESwABQIixACJIoSdgCUYAR3Vg4MACYAPGYuFvYAfACU5Ko0APRxwADKMBD+wFAAFuh2Vv7OSBlYGdmc8ABu8LHKsRyGxqY4oQT21pTCIHQMjOwA5DAAHgACxAAOjDAAdChYxL0ANLHUouKSMH0AEmAAhJhY6ozpAJ77GTCMjMCiV0ToSAb7UJPPC9WRgrEJwAAqR6MwSRQPFGUFocDgRHYxnEfGAowh-zgUCOwF6KwkUl6tXqJhCeEsxDaS1AXSYfUGI3GUxmc0WSneQA) 150 151 ```ts 152 interface ContactForm { 153 email?: string; 154 message?: string; 155 } 156 157 function submitContactForm(formData: Required<ContactForm>) { 158 // Send the form data to the server. 159 } 160 161 submitContactForm({ 162 email: 'ex@mple.com', 163 message: 'Hi! Could you tell me more about…', 164 }); 165 166 // TypeScript error: missing property 'message' 167 submitContactForm({ 168 email: 'ex@mple.com', 169 }); 170 ``` 171 </details> 172 173- [`Readonly<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1415-L1420) - Make all properties in `T` readonly. 174 <details> 175 <summary> 176 Example 177 </summary> 178 179 [Playground](https://typescript-play.js.org/?target=6#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) 180 181 ```ts 182 enum LogLevel { 183 Off, 184 Debug, 185 Error, 186 Fatal 187 }; 188 189 interface LoggerConfig { 190 name: string; 191 level: LogLevel; 192 } 193 194 class Logger { 195 config: Readonly<LoggerConfig>; 196 197 constructor({name, level}: LoggerConfig) { 198 this.config = {name, level}; 199 Object.freeze(this.config); 200 } 201 } 202 203 const config: LoggerConfig = { 204 name: 'MyApp', 205 level: LogLevel.Debug 206 }; 207 208 const logger = new Logger(config); 209 210 // TypeScript Error: cannot assign to read-only property. 211 logger.config.level = LogLevel.Error; 212 213 // We are able to edit config variable as we please. 214 config.level = LogLevel.Error; 215 ``` 216 </details> 217 218- [`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`. 219 <details> 220 <summary> 221 Example 222 </summary> 223 224 [Playground](https://typescript-play.js.org/?target=6#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) 225 226 ```ts 227 interface Article { 228 title: string; 229 thumbnail: string; 230 content: string; 231 } 232 233 // Creates new type out of the `Article` interface composed 234 // from the Articles' two properties: `title` and `thumbnail`. 235 // `ArticlePreview = {title: string; thumbnail: string}` 236 type ArticlePreview = Pick<Article, 'title' | 'thumbnail'>; 237 238 // Render a list of articles using only title and description. 239 function renderArticlePreviews(previews: ArticlePreview[]): HTMLElement { 240 const articles = document.createElement('div'); 241 242 for (const preview of previews) { 243 // Append preview to the articles. 244 } 245 246 return articles; 247 } 248 249 const articles = renderArticlePreviews([ 250 { 251 title: 'TypeScript tutorial!', 252 thumbnail: '/assets/ts.jpg' 253 } 254 ]); 255 ``` 256 </details> 257 258- [`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`. 259 <details> 260 <summary> 261 Example 262 </summary> 263 264 [Playground](https://typescript-play.js.org/?target=6#code/AQ4ejYAUHsGcCWAXBMB2dgwGbAKYC2ADgDYwCeeemCaWArgE7ADGMxAhmuQHQBQoYEnJE8wALKEARnkaxEKdMAC8wAOS0kstGuAAfdQBM8ANzxlRjXQbVaWACwC0JPB0NqA3HwGgIwAJJoWozYHCxixnAsjAhStADmwESMMJYo1Fi4HMCIaPEu+MRklHj8gpqyoeHAAKJFFFTAAN4+giDYCIxwSAByHAR4AFw5SDF5Xm2gJBzdfQPD3WPxE5PAlBxdAPLYNQAelgh4aOHDaPQEMowrIAC+3oJ+AMKMrlrAXFhSAFZ4LEhC9g4-0BmA4JBISXgiCkBQABpILrJ5MhUGhYcATGD6Bk4Hh-jNgABrPDkOBlXyQAAq9ngYmJpOAAHcEOCRjAXqwYODfoo6DhakUSph+Uh7GI4P0xER4Cj0OSQGwMP8tP1hgAlX7swwAHgRl2RvIANALSA08ABtAC6AD4VM1Wm0Kow0MMrYaHYJjGYLLJXZb3at1HYnC43Go-QHQDcvA6-JsmEJXARgCDgMYWAhjIYhDAU+YiMAAFIwex0ZmilMITCGF79TLAGRsAgJYAAZRwSEZGzEABFTOZUrJ5Yn+jwnWgeER6HB7AAKJrADpdXqS4ZqYultTG6azVfqHswPBbtauLY7fayQ7HIbAAAMwBuAEoYw9IBq2Ixs9h2eFMOQYPQObALQKJgggABeYhghCIpikkKRpOQRIknAsZUiIeCttECBEP8NSMCkjDDAARMGziuIYxHwYOjDCMBmDNnAuTxA6irdCOBB1Lh5Dqpqn66tISIykawBnOCtqqC0gbjqc9DgpGkxegOliyfJDrRkAA) 265 266 ```ts 267 // Positions of employees in our company. 268 type MemberPosition = 'intern' | 'developer' | 'tech-lead'; 269 270 // Interface describing properties of a single employee. 271 interface Employee { 272 firstName: string; 273 lastName: string; 274 yearsOfExperience: number; 275 } 276 277 // Create an object that has all possible `MemberPosition` values set as keys. 278 // Those keys will store a collection of Employees of the same position. 279 const team: Record<MemberPosition, Employee[]> = { 280 intern: [], 281 developer: [], 282 'tech-lead': [], 283 }; 284 285 // Our team has decided to help John with his dream of becoming Software Developer. 286 team.intern.push({ 287 firstName: 'John', 288 lastName: 'Doe', 289 yearsOfExperience: 0 290 }); 291 292 // `Record` forces you to initialize all of the property keys. 293 // TypeScript Error: "tech-lead" property is missing 294 const teamEmpty: Record<MemberPosition, null> = { 295 intern: null, 296 developer: null, 297 }; 298 ``` 299 </details> 300 301- [`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`. 302 <details> 303 <summary> 304 Example 305 </summary> 306 307 [Playground](https://typescript-play.js.org/?target=6#code/JYOwLgpgTgZghgYwgAgMrQG7QMIHsQzADmyA3gFDLIAOuUYAXMiAK4A2byAPsgM5hRQJHqwC2AI2gBucgF9y5MAE9qKAEoQAjiwj8AEnBAATNtGQBeZAAooWphu26wAGmS3e93bRC8IASgsAPmRDJRlyAHoI5ABRAA8ENhYjFFYOZGVVZBgoXFFkAAM0zh5+QRBhZhYJaAKAOkjogEkQZAQ4X2QAdwALCFbaemRgXmQtFjhOMFwq9K6ULuB0lk6U+HYwZAxJnQaYFhAEMGB8ZCIIMAAFOjAANR2IK0HGWISklIAedCgsKDwCYgAbQA5M9gQBdVzFQJ+JhiSRQMiUYYwayZCC4VHPCzmSzAspCYEBWxgFhQAZwKC+FpgJ43VwARgADH4ZFQSWSBjcZPJyPtDsdTvxKWBvr8rD1DCZoJ5HPopaYoK4EPhCEQmGKcKriLCtrhgEYkVQVT5Nr4fmZLLZtMBbFZgT0wGBqES6ghbHBIJqoBKFdBWQpjfh+DQbhY2tqiHVsbjLMVkAB+ZAAZiZaeQTHOVxu9ySjxNaujNwDVHNvzqbBGkBAdPoAfkQA) 308 309 ```ts 310 interface ServerConfig { 311 port: null | string | number; 312 } 313 314 type RequestHandler = (request: Request, response: Response) => void; 315 316 // Exclude `null` type from `null | string | number`. 317 // In case the port is equal to `null`, we will use default value. 318 function getPortValue(port: Exclude<ServerConfig['port'], null>): number { 319 if (typeof port === 'string') { 320 return parseInt(port, 10); 321 } 322 323 return port; 324 } 325 326 function startServer(handler: RequestHandler, config: ServerConfig): void { 327 const server = require('http').createServer(handler); 328 329 const port = config.port === null ? 3000 : getPortValue(config.port); 330 server.listen(port); 331 } 332 ``` 333 </details> 334 335- [`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`. 336 <details> 337 <summary> 338 Example 339 </summary> 340 341 [Playground](https://typescript-play.js.org/?target=6#code/CYUwxgNghgTiAEAzArgOzAFwJYHtXzSwEdkQBJYACgEoAueVZAWwCMQYBuAKDDwGcM8MgBF4AXngBlAJ6scESgHIRi6ty5ZUGdoihgEABXZ888AN5d48ANoiAuvUat23K6ihMQ9ATE0BzV3goPy8GZjZOLgBfLi4Aejj4AEEICBwAdz54MAALKFQQ+BxEeAAHY1NgKAwoIKy0grr4DByEUpgccpgMaXgAaxBerCzi+B9-ZulygDouFHRsU1z8kKMYE1RhaqgAHkt4AHkWACt4EAAPbVRgLLWNgBp9gGlBs8uQa6yAUUuYPQwdgNpKM7nh7mMML4CgA+R5WABqUAgpDeVxuhxO1he0jsXGh8EoOBO9COx3BQPo2PBADckaR6IjkSA6PBqTgsMBzPsicdrEC7OJWXSQNwYvFEgAVTS9JLXODpeDpKBZFg4GCoWa8VACIJykAKiQWKy2YQOAioYikCg0OEMDyhRSy4DyxS24KhAAMjyi6gS8AAwjh5OD0iBFHAkJoEOksC1mnkMJq8gUQKDNttKPlnfrwYp3J5XfBHXqoKpfYkAOI4ansTxaeDADmoRSCCBYAbxhC6TDx6rwYHIRX5bScjA4bLJwoDmDwDkfbA9JMrVMVdM1TN69LgkTgwgkchUahqIA) 342 343 ```ts 344 declare function uniqueId(): number; 345 346 const ID = Symbol('ID'); 347 348 interface Person { 349 [ID]: number; 350 name: string; 351 age: number; 352 } 353 354 // Allows changing the person data as long as the property key is of string type. 355 function changePersonData< 356 Obj extends Person, 357 Key extends Extract<keyof Person, string>, 358 Value extends Obj[Key] 359 > (obj: Obj, key: Key, value: Value): void { 360 obj[key] = value; 361 } 362 363 // Tiny Andrew was born. 364 const andrew = { 365 [ID]: uniqueId(), 366 name: 'Andrew', 367 age: 0, 368 }; 369 370 // Cool, we're fine with that. 371 changePersonData(andrew, 'name', 'Pony'); 372 373 // Goverment didn't like the fact that you wanted to change your identity. 374 changePersonData(andrew, ID, uniqueId()); 375 ``` 376 </details> 377 378- [`NonNullable<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1446-L1449) - Exclude `null` and `undefined` from `T`. 379 <details> 380 <summary> 381 Example 382 </summary> 383 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>) 384 385 [Playground](https://typescript-play.js.org/?target=6#code/C4TwDgpgBACg9gJ2AOQK4FsBGEFQLxQDOwCAlgHYDmUAPlORtrnQwDasDcAUFwPQBU-WAEMkUOADMowqAGNWwwoSgATCBIqlgpOOSjAAFsOBRSy1IQgr9cKJlSlW1mZYQA3HFH68u8xcoBlHA8EACEHJ08Aby4oKDBUTFZSWXjEFEYcAEIALihkXTR2YSSIAB54JDQsHAA+blj4xOTUsHSACkMzPKD3HHDHNQQAGjSkPMqMmoQASh7g-oihqBi4uNIpdraxPAI2VhmVxrX9AzMAOm2ppnwoAA4ABifuE4BfKAhWSyOTuK7CS7pao3AhXF5rV48E4ICDAVAIPT-cGQyG+XTEIgLMJLTx7CAAdygvRCA0iCHaMwarhJOIQjUBSHaACJHk8mYdeLwxtdcVAAOSsh58+lXdr7Dlcq7A3n3J4PEUdADMcspUE53OluAIUGVTx46oAKuAIAFZGQwCYAKIIBCILjUxaDHAMnla+iodjcIA) 386 387 ```ts 388 type PortNumber = string | number | null; 389 390 /** Part of a class definition that is used to build a server */ 391 class ServerBuilder { 392 portNumber!: NonNullable<PortNumber>; 393 394 port(this: ServerBuilder, port: PortNumber): ServerBuilder { 395 if (port == null) { 396 this.portNumber = 8000; 397 } else { 398 this.portNumber = port; 399 } 400 401 return this; 402 } 403 } 404 405 const serverBuilder = new ServerBuilder(); 406 407 serverBuilder 408 .port('8000') // portNumber = '8000' 409 .port(null) // portNumber = 8000 410 .port(3000); // portNumber = 3000 411 412 // TypeScript error 413 serverBuilder.portNumber = null; 414 ``` 415 </details> 416 417- [`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. 418 <details> 419 <summary> 420 Example 421 </summary> 422 423 [Playground](https://typescript-play.js.org/?target=6#code/GYVwdgxgLglg9mABAZwBYmMANgUwBQxgAOIUAXIgIZgCeA2gLoCUFAbnDACaIDeAUIkQB6IYgCypSlBxUATrMo1ECsJzgBbLEoipqAc0J7EMKMgDkiHLnU4wp46pwAPHMgB0fAL58+oSLARECEosLAA5ABUYG2QAHgAxJGdpVWREPDdMylk9ZApqemZEAF4APipacrw-CApEgBogkKwAYThwckQwEHUAIxxZJl4BYVEImiIZKF0oZRwiWVdbeygJmThgOYgcGFYcbhqApCJsyhtpWXcR1cnEePBoeDAABVPzgbTixFeFd8uEsClADcIxGiygIFkSEOT3SmTc2VydQeRx+ZxwF2QQ34gkEwDgsnSuFmMBKiAADEDjIhYk1Qm0OlSYABqZnYka4xA1DJZHJYkGc7yCbyeRA+CAIZCzNAYbA4CIAdxg2zJwVCkWirjwMswuEaACYmCCgA) 424 425 ```ts 426 function shuffle(input: any[]): void { 427 // Mutate array randomly changing its' elements indexes. 428 } 429 430 function callNTimes<Fn extends (...args: any[]) => any> (func: Fn, callCount: number) { 431 // Type that represents the type of the received function parameters. 432 type FunctionParameters = Parameters<Fn>; 433 434 return function (...args: FunctionParameters) { 435 for (let i = 0; i < callCount; i++) { 436 func(...args); 437 } 438 } 439 } 440 441 const shuffleTwice = callNTimes(shuffle, 2); 442 ``` 443 </details> 444 445- [`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. 446 <details> 447 <summary> 448 Example 449 </summary> 450 451 [Playground](https://typescript-play.js.org/?target=6#code/MYGwhgzhAECCBOAXAlqApgWQPYBM0mgG8AoaaFRENALmgkXmQDsBzAblOmCycTV4D8teo1YdO3JiICuwRFngAKClWENmLAJRFOZRAAtkEAHQq00ALzlklNBzIBfYk+KhIMAJJTEYJsDQAwmDA+mgAPAAq0GgAHnxMODCKTGgA7tCKxllg8CwQtL4AngDaALraFgB80EWa1SRkAA6MAG5gfNAB4FABPDJyCrQR9tDNyG0dwMGhtBhgjWEiGgA00F70vv4RhY3hEZXVVinpc42KmuJkkv3y8Bly8EPaDWTkhiZd7r3e8LK3llwGCMXGQWGhEOsfH5zJlsrl8p0+gw-goAAo5MAAW3BaHgEEilU0tEhmzQ212BJ0ry4SOg+kg+gBBiMximIGA0nAfAQLGk2N4EAAEgzYcYcnkLsRdDTvNEYkYUKwSdCme9WdM0MYwYhFPSIPpJdTkAAzDKxBUaZX+aAAQgsVmkCTQxuYaBw2ng4Ok8CYcotSu8pMur09iG9vuObxZnx6SN+AyUWTF8MN0CcZE4Ywm5jZHK5aB5fP4iCFIqT4oRRTKRLo6lYVNeAHpG50wOzOe1zHr9NLQ+HoABybsD4HOKXXRA1JCoKhBELmI5pNaB6Fz0KKBAodDYPAgSUTmqYsAALx4m5nC6nW9nGq14KtaEUA9gR9PvuNCjQ9BgACNvcwNBtAcLiAA) 452 453 ```ts 454 class ArticleModel { 455 title: string; 456 content?: string; 457 458 constructor(title: string) { 459 this.title = title; 460 } 461 } 462 463 class InstanceCache<T extends (new (...args: any[]) => any)> { 464 private ClassConstructor: T; 465 private cache: Map<string, InstanceType<T>> = new Map(); 466 467 constructor (ctr: T) { 468 this.ClassConstructor = ctr; 469 } 470 471 getInstance (...args: ConstructorParameters<T>): InstanceType<T> { 472 const hash = this.calculateArgumentsHash(...args); 473 474 const existingInstance = this.cache.get(hash); 475 if (existingInstance !== undefined) { 476 return existingInstance; 477 } 478 479 return new this.ClassConstructor(...args); 480 } 481 482 private calculateArgumentsHash(...args: any[]): string { 483 // Calculate hash. 484 return 'hash'; 485 } 486 } 487 488 const articleCache = new InstanceCache(ArticleModel); 489 const amazonArticle = articleCache.getInstance('Amazon forests burining!'); 490 ``` 491 </details> 492 493- [`ReturnType<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1461-L1464) – Obtain the return type of a function type. 494 <details> 495 <summary> 496 Example 497 </summary> 498 499 [Playground](https://typescript-play.js.org/?target=6#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-YhAA+qegAAYLKQLQj3ZsEsdccmnGcLor2Dn8xGedHGpEIBzEzspfsfMHDNAANTQACMVaIljV5GQkRA5DYmIpVKQAgAJARO9le33BDXIyi0YuLW2nJFGLqkOvxFB0YPdBSaLZ0IwNzyPkO8-xkGgsLh8Al427a3hWAhXwwHA8EHT5PmgAB1bAQBAANJ24adKWpft72RaBUTgRBUCAj89HAM8xCTaBjggABRQx0DuHJv25P9dCkWRZVIAAiBjoFImpmjlFBgA0NpsjadByDacgIDAEAIAAQmYpjoGYgAZSBsmGPw6DtZiiFA8CoJguDmAQmoZ2QvtUKQLdoAYmBTwgdEiCAA) 500 501 ```ts 502 /** Provides every element of the iterable `iter` into the `callback` function and stores the results in an array. */ 503 function mapIter< 504 Elem, 505 Func extends (elem: Elem) => any, 506 Ret extends ReturnType<Func> 507 >(iter: Iterable<Elem>, callback: Func): Ret[] { 508 const mapped: Ret[] = []; 509 510 for (const elem of iter) { 511 mapped.push(callback(elem)); 512 } 513 514 return mapped; 515 } 516 517 const setObject: Set<string> = new Set(); 518 const mapObject: Map<number, string> = new Map(); 519 520 mapIter(setObject, (value: string) => value.indexOf('Foo')); // number[] 521 522 mapIter(mapObject, ([key, value]: [number, string]) => { 523 return key % 2 === 0 ? value : 'Odd'; 524 }); // string[] 525 ``` 526 </details> 527 528- [`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. 529 <details> 530 <summary> 531 Example 532 </summary> 533 534 [Playground](https://typescript-play.js.org/?target=6#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-YhAA+qegAAYLKQLQj3ZsEsdccmnGcLor2Dn8xGedHGpEIBzEzspfsfMHDNAANTQACMVaIljV5GQkRA5DYmIpVKQAgAJARO9le33BDXIyi0YuLW2nJFGLqkOvxFB0YPdBSaLZ0IwNzyPkO8-xkGgsLh8Al427a3hWAhXwwHA8EHT5PmgAB1bAQBAANJ24adKWpft72RaBUTgRBUCAj89HAM8xCTaBjggABRQx0DuHJv25P9dCkWRZVIAAiBjoFImpmjlFBgA0NpsjadByDacgIDAEAIAAQmYpjoGYgAZSBsmGPw6DtZiiFA8CoJguDmAQmoZ2QvtUKQLdoAYmBTwgdEiCAA) 535 536 ```ts 537 class IdleService { 538 doNothing (): void {} 539 } 540 541 class News { 542 title: string; 543 content: string; 544 545 constructor(title: string, content: string) { 546 this.title = title; 547 this.content = content; 548 } 549 } 550 551 const instanceCounter: Map<Function, number> = new Map(); 552 553 interface Constructor { 554 new(...args: any[]): any; 555 } 556 557 // Keep track how many instances of `Constr` constructor have been created. 558 function getInstance< 559 Constr extends Constructor, 560 Args extends ConstructorParameters<Constr> 561 >(constructor: Constr, ...args: Args): InstanceType<Constr> { 562 let count = instanceCounter.get(constructor) || 0; 563 564 const instance = new constructor(...args); 565 566 instanceCounter.set(constructor, count + 1); 567 568 console.log(`Created ${count + 1} instances of ${Constr.name} class`); 569 570 return instance; 571 } 572 573 574 const idleService = getInstance(IdleService); 575 // Will log: `Created 1 instances of IdleService class` 576 const newsEntry = getInstance(News, 'New ECMAScript proposals!', 'Last month...'); 577 // Will log: `Created 1 instances of News class` 578 ``` 579 </details> 580 581- [`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. 582 <details> 583 <summary> 584 Example 585 </summary> 586 587 [Playground](https://typescript-play.js.org/?target=6#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-SgfXlHqVUE4xiAA) 588 589 ```ts 590 interface Animal { 591 imageUrl: string; 592 species: string; 593 images: string[]; 594 paragraphs: string[]; 595 } 596 597 // Creates new type with all properties of the `Animal` interface 598 // except 'images' and 'paragraphs' properties. We can use this 599 // type to render small hover tooltip for a wiki entry list. 600 type AnimalShortInfo = Omit<Animal, 'images' | 'paragraphs'>; 601 602 function renderAnimalHoverInfo (animals: AnimalShortInfo[]): HTMLElement { 603 const container = document.createElement('div'); 604 // Internal implementation. 605 return container; 606 } 607 ``` 608 </details> 609 610You can find some examples in the [TypeScript docs](https://www.typescriptlang.org/docs/handbook/advanced-types.html#predefined-conditional-types). 611 612 613## Maintainers 614 615- [Sindre Sorhus](https://github.com/sindresorhus) 616- [Jarek Radosz](https://github.com/CvX) 617- [Dimitri Benin](https://github.com/BendingBender) 618 619 620## License 621 622(MIT OR CC0-1.0) 623 624 625--- 626 627<div align="center"> 628 <b> 629 <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> 630 </b> 631 <br> 632 <sub> 633 Tidelift helps make open source sustainable for maintainers while giving companies<br>assurances about security, maintenance, and licensing for their dependencies. 634 </sub> 635</div> 636