--- /dev/null
+<div align="center">
+ <br>
+ <br>
+ <img src="media/logo.svg" alt="type-fest" height="300">
+ <br>
+ <br>
+ <b>A collection of essential TypeScript types</b>
+ <br>
+ <hr>
+</div>
+<br>
+<br>
+
+[](https://travis-ci.com/sindresorhus/type-fest)
+[](https://www.youtube.com/watch?v=9auOCbH5Ns4)
+<!-- Commented out until they actually show anything
+[](https://www.npmjs.com/package/type-fest?activeTab=dependents) [](https://www.npmjs.com/package/type-fest)
+-->
+
+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 >=3.2*
+
+
+## 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/basic.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).
+- [`Mutable`](source/mutable.d.ts) - Convert an object with `readonly` keys into a mutable object. The inverse of `Readonly<T>`.
+- [`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-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.
+
+### Miscellaneous
+
+- [`PackageJson`](source/package-json.d.ts) - Type for [npm's `package.json` file](https://docs.npmjs.com/creating-a-package-json-file).
+
+
+## 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.
+
+
+## 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.
+ <details>
+ <summary>
+ Example
+ </summary>
+
+ [Playground](https://typescript-play.js.org/?target=6#code/KYOwrgtgBAMg9gcxsAbsANlA3gKClAeQDMiAaPKAEWACMwFz8BRAJxbhcagDEBDAF17ocAXxw4AliH7AWRXgGNgUAHJwAJsADCcEEQkJsFXgAcTK3hGAAuKAGd+LKQgDcFEx363wEGrLf46IjIaOi28EioGG5iOArovHZ2qhrAAIJmAEJgEuiaLEb4Jk4oAsoKuvoIYCwCErq2apo6egZQALyF+FCm5pY2UABETelmg1xFnrYAzAAM8xNQQZGh4cFR6AB0xEQUIm4UFa0IABRHVbYACrws-BJCADwjLVUAfACUXfhEHFBnug4oABrYAATygcCIhBoACtgAp+JsQaC7P9ju9Prhut0joCwCZ1GUAGpCMDKTrnAwAbWRPWSyMhKWalQMAF0Dtj8BIoSd8YSZCT0GSOu1OmAQJp9CBgOpPkc7uBgBzOfwABYSOybSnVWp3XQ0sF04FgxnPFkIVkdKB84mkpUUfCxbEsYD8GogKBqjUBKBiWIAen9UGut3u6CeqReBlePXQQQA7skwMl+HAoMU4CgJJoISB0ODeOmbvwIVC1cAcIGmdpzVApDI5IpgJscNL49WMiZsrl8id3lrzScsD0zBYrLZBgAVOCUOCdwa+95uIA)
+
+ ```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<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.
+ <details>
+ <summary>
+ Example
+ </summary>
+
+ [Playground](https://typescript-play.js.org/?target=6#code/AQ4SwOwFwUwJwGYEMDGNgGED21VQGJZwC2wA3gFCjXAzFJgA2A-AFzADOUckA5gNxUaIYjA4ckvGG07c+g6gF8KQkAgCuEFFDA5O6gEbEwUbLm2ESwABQIixACJIoSdgCUYAR3Vg4MACYAPGYuFvYAfACU5Ko0APRxwADKMBD+wFAAFuh2Vv7OSBlYGdmc8ABu8LHKsRyGxqY4oQT21pTCIHQMjOwA5DAAHgACxAAOjDAAdChYxL0ANLHUouKSMH0AEmAAhJhY6ozpAJ77GTCMjMCiV0ToSAb7UJPPC9WRgrEJwAAqR6MwSRQPFGUFocDgRHYxnEfGAowh-zgUCOwF6KwkUl6tXqJhCeEsxDaS1AXSYfUGI3GUxmc0WSneQA)
+
+ ```ts
+ 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',
+ });
+ ```
+ </details>
+
+- [`Readonly<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1415-L1420) - Make all properties in `T` readonly.
+ <details>
+ <summary>
+ Example
+ </summary>
+
+ [Playground](https://typescript-play.js.org/?target=6#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)
+
+ ```ts
+ 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;
+ ```
+ </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`.
+ <details>
+ <summary>
+ Example
+ </summary>
+
+ [Playground](https://typescript-play.js.org/?target=6#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)
+
+ ```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'
+ }
+ ]);
+ ```
+ </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`.
+ <details>
+ <summary>
+ Example
+ </summary>
+
+ [Playground](https://typescript-play.js.org/?target=6#code/AQ4ejYAUHsGcCWAXBMB2dgwGbAKYC2ADgDYwCeeemCaWArgE7ADGMxAhmuQHQBQoYEnJE8wALKEARnkaxEKdMAC8wAOS0kstGuAAfdQBM8ANzxlRjXQbVaWACwC0JPB0NqA3HwGgIwAJJoWozYHCxixnAsjAhStADmwESMMJYo1Fi4HMCIaPEu+MRklHj8gpqyoeHAAKJFFFTAAN4+giDYCIxwSAByHAR4AFw5SDF5Xm2gJBzdfQPD3WPxE5PAlBxdAPLYNQAelgh4aOHDaPQEMowrIAC+3oJ+AMKMrlrAXFhSAFZ4LEhC9g4-0BmA4JBISXgiCkBQABpILrJ5MhUGhYcATGD6Bk4Hh-jNgABrPDkOBlXyQAAq9ngYmJpOAAHcEOCRjAXqwYODfoo6DhakUSph+Uh7GI4P0xER4Cj0OSQGwMP8tP1hgAlX7swwAHgRl2RvIANALSA08ABtAC6AD4VM1Wm0Kow0MMrYaHYJjGYLLJXZb3at1HYnC43Go-QHQDcvA6-JsmEJXARgCDgMYWAhjIYhDAU+YiMAAFIwex0ZmilMITCGF79TLAGRsAgJYAAZRwSEZGzEABFTOZUrJ5Yn+jwnWgeER6HB7AAKJrADpdXqS4ZqYultTG6azVfqHswPBbtauLY7fayQ7HIbAAAMwBuAEoYw9IBq2Ixs9h2eFMOQYPQObALQKJgggABeYhghCIpikkKRpOQRIknAsZUiIeCttECBEP8NSMCkjDDAARMGziuIYxHwYOjDCMBmDNnAuTxA6irdCOBB1Lh5Dqpqn66tISIykawBnOCtqqC0gbjqc9DgpGkxegOliyfJDrRkAA)
+
+ ```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,
+ };
+ ```
+ </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`.
+ <details>
+ <summary>
+ Example
+ </summary>
+
+ [Playground](https://typescript-play.js.org/?target=6#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)
+
+ ```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);
+ }
+ ```
+ </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`.
+ <details>
+ <summary>
+ Example
+ </summary>
+
+ [Playground](https://typescript-play.js.org/?target=6#code/CYUwxgNghgTiAEAzArgOzAFwJYHtXzSwEdkQBJYACgEoAueVZAWwCMQYBuAKDDwGcM8MgBF4AXngBlAJ6scESgHIRi6ty5ZUGdoihgEABXZ888AN5d48ANoiAuvUat23K6ihMQ9ATE0BzV3goPy8GZjZOLgBfLi4Aejj4AEEICBwAdz54MAALKFQQ+BxEeAAHY1NgKAwoIKy0grr4DByEUpgccpgMaXgAaxBerCzi+B9-ZulygDouFHRsU1z8kKMYE1RhaqgAHkt4AHkWACt4EAAPbVRgLLWNgBp9gGlBs8uQa6yAUUuYPQwdgNpKM7nh7mMML4CgA+R5WABqUAgpDeVxuhxO1he0jsXGh8EoOBO9COx3BQPo2PBADckaR6IjkSA6PBqTgsMBzPsicdrEC7OJWXSQNwYvFEgAVTS9JLXODpeDpKBZFg4GCoWa8VACIJykAKiQWKy2YQOAioYikCg0OEMDyhRSy4DyxS24KhAAMjyi6gS8AAwjh5OD0iBFHAkJoEOksC1mnkMJq8gUQKDNttKPlnfrwYp3J5XfBHXqoKpfYkAOI4ansTxaeDADmoRSCCBYAbxhC6TDx6rwYHIRX5bScjA4bLJwoDmDwDkfbA9JMrVMVdM1TN69LgkTgwgkchUahqIA)
+
+ ```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());
+ ```
+ </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 <a href="https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-0.html">here</a>)
+
+ [Playground](https://typescript-play.js.org/?target=6#code/C4TwDgpgBACg9gJ2AOQK4FsBGEFQLxQDOwCAlgHYDmUAPlORtrnQwDasDcAUFwPQBU-WAEMkUOADMowqAGNWwwoSgATCBIqlgpOOSjAAFsOBRSy1IQgr9cKJlSlW1mZYQA3HFH68u8xcoBlHA8EACEHJ08Aby4oKDBUTFZSWXjEFEYcAEIALihkXTR2YSSIAB54JDQsHAA+blj4xOTUsHSACkMzPKD3HHDHNQQAGjSkPMqMmoQASh7g-oihqBi4uNIpdraxPAI2VhmVxrX9AzMAOm2ppnwoAA4ABifuE4BfKAhWSyOTuK7CS7pao3AhXF5rV48E4ICDAVAIPT-cGQyG+XTEIgLMJLTx7CAAdygvRCA0iCHaMwarhJOIQjUBSHaACJHk8mYdeLwxtdcVAAOSsh58+lXdr7Dlcq7A3n3J4PEUdADMcspUE53OluAIUGVTx46oAKuAIAFZGQwCYAKIIBCILjUxaDHAMnla+iodjcIA)
+
+ ```ts
+ 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;
+ ```
+ </details>
+
+- [`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.
+ <details>
+ <summary>
+ Example
+ </summary>
+
+ [Playground](https://typescript-play.js.org/?target=6#code/GYVwdgxgLglg9mABAZwBYmMANgUwBQxgAOIUAXIgIZgCeA2gLoCUFAbnDACaIDeAUIkQB6IYgCypSlBxUATrMo1ECsJzgBbLEoipqAc0J7EMKMgDkiHLnU4wp46pwAPHMgB0fAL58+oSLARECEosLAA5ABUYG2QAHgAxJGdpVWREPDdMylk9ZApqemZEAF4APipacrw-CApEgBogkKwAYThwckQwEHUAIxxZJl4BYVEImiIZKF0oZRwiWVdbeygJmThgOYgcGFYcbhqApCJsyhtpWXcR1cnEePBoeDAABVPzgbTixFeFd8uEsClADcIxGiygIFkSEOT3SmTc2VydQeRx+ZxwF2QQ34gkEwDgsnSuFmMBKiAADEDjIhYk1Qm0OlSYABqZnYka4xA1DJZHJYkGc7yCbyeRA+CAIZCzNAYbA4CIAdxg2zJwVCkWirjwMswuEaACYmCCgA)
+
+ ```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<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.
+ <details>
+ <summary>
+ Example
+ </summary>
+
+ [Playground](https://typescript-play.js.org/?target=6#code/MYGwhgzhAECCBOAXAlqApgWQPYBM0mgG8AoaaFRENALmgkXmQDsBzAblOmCycTV4D8teo1YdO3JiICuwRFngAKClWENmLAJRFOZRAAtkEAHQq00ALzlklNBzIBfYk+KhIMAJJTEYJsDQAwmDA+mgAPAAq0GgAHnxMODCKTGgA7tCKxllg8CwQtL4AngDaALraFgB80EWa1SRkAA6MAG5gfNAB4FABPDJyCrQR9tDNyG0dwMGhtBhgjWEiGgA00F70vv4RhY3hEZXVVinpc42KmuJkkv3y8Bly8EPaDWTkhiZd7r3e8LK3llwGCMXGQWGhEOsfH5zJlsrl8p0+gw-goAAo5MAAW3BaHgEEilU0tEhmzQ212BJ0ry4SOg+kg+gBBiMximIGA0nAfAQLGk2N4EAAEgzYcYcnkLsRdDTvNEYkYUKwSdCme9WdM0MYwYhFPSIPpJdTkAAzDKxBUaZX+aAAQgsVmkCTQxuYaBw2ng4Ok8CYcotSu8pMur09iG9vuObxZnx6SN+AyUWTF8MN0CcZE4Ywm5jZHK5aB5fP4iCFIqT4oRRTKRLo6lYVNeAHpG50wOzOe1zHr9NLQ+HoABybsD4HOKXXRA1JCoKhBELmI5pNaB6Fz0KKBAodDYPAgSUTmqYsAALx4m5nC6nW9nGq14KtaEUA9gR9PvuNCjQ9BgACNvcwNBtAcLiAA)
+
+ ```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<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!');
+ ```
+ </details>
+
+- [`ReturnType<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1461-L1464) – Obtain the return type of a function type.
+ <details>
+ <summary>
+ Example
+ </summary>
+
+ [Playground](https://typescript-play.js.org/?target=6#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-YhAA+qegAAYLKQLQj3ZsEsdccmnGcLor2Dn8xGedHGpEIBzEzspfsfMHDNAANTQACMVaIljV5GQkRA5DYmIpVKQAgAJARO9le33BDXIyi0YuLW2nJFGLqkOvxFB0YPdBSaLZ0IwNzyPkO8-xkGgsLh8Al427a3hWAhXwwHA8EHT5PmgAB1bAQBAANJ24adKWpft72RaBUTgRBUCAj89HAM8xCTaBjggABRQx0DuHJv25P9dCkWRZVIAAiBjoFImpmjlFBgA0NpsjadByDacgIDAEAIAAQmYpjoGYgAZSBsmGPw6DtZiiFA8CoJguDmAQmoZ2QvtUKQLdoAYmBTwgdEiCAA)
+
+ ```ts
+ /** 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[]
+ ```
+ </details>
+
+- [`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.
+ <details>
+ <summary>
+ Example
+ </summary>
+
+ [Playground](https://typescript-play.js.org/?target=6#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-YhAA+qegAAYLKQLQj3ZsEsdccmnGcLor2Dn8xGedHGpEIBzEzspfsfMHDNAANTQACMVaIljV5GQkRA5DYmIpVKQAgAJARO9le33BDXIyi0YuLW2nJFGLqkOvxFB0YPdBSaLZ0IwNzyPkO8-xkGgsLh8Al427a3hWAhXwwHA8EHT5PmgAB1bAQBAANJ24adKWpft72RaBUTgRBUCAj89HAM8xCTaBjggABRQx0DuHJv25P9dCkWRZVIAAiBjoFImpmjlFBgA0NpsjadByDacgIDAEAIAAQmYpjoGYgAZSBsmGPw6DtZiiFA8CoJguDmAQmoZ2QvtUKQLdoAYmBTwgdEiCAA)
+
+ ```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<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`
+ ```
+ </details>
+
+- [`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.
+ <details>
+ <summary>
+ Example
+ </summary>
+
+ [Playground](https://typescript-play.js.org/?target=6#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-SgfXlHqVUE4xiAA)
+
+ ```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;
+ }
+ ```
+ </details>
+
+You can find some examples in the [TypeScript docs](https://www.typescriptlang.org/docs/handbook/advanced-types.html#predefined-conditional-types).
+
+
+## Maintainers
+
+- [Sindre Sorhus](https://github.com/sindresorhus)
+- [Jarek Radosz](https://github.com/CvX)
+- [Dimitri Benin](https://github.com/BendingBender)
+
+
+## License
+
+(MIT OR CC0-1.0)
+
+
+---
+
+<div align="center">
+ <b>
+ <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>
+ </b>
+ <br>
+ <sub>
+ Tidelift helps make open source sustainable for maintainers while giving companies<br>assurances about security, maintenance, and licensing for their dependencies.
+ </sub>
+</div>
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