minimal adjustments

[dotfiles/.git] / .config / coc / extensions / node_modules / coc-prettier / node_modules / @eslint / eslintrc / node_modules / type-fest / readme.md

<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>

[![Build Status](https://travis-ci.com/sindresorhus/type-fest.svg?branch=master)](https://travis-ci.com/sindresorhus/type-fest)
[![](https://img.shields.io/badge/unicorn-approved-ff69b4.svg)](https://www.youtube.com/watch?v=9auOCbH5Ns4)
<!-- Commented out until they actually show anything
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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)


---

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        <sub>
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