typescript-tips

Typescript Tips

A curated list of awesome 🔥 TypeScript Tips 🔥

If you enjoy TypeScript and really want to use Typesafe, you can check awesome-typesafe

Contents

• Tips
  • Matt Pocock
  • Carlos Caballero
  • Minko Gechev
  • Cory House
  • Tomek Sułkowski
  • Wes Bos
  • StackBlitz
  • Extending existing types
  • Built-in types
• Contribute
  • Twitter to markdown file
• Credits

Tips

Matt Pocock

• Normal union, a discriminated union, and a type predicate
• Use Object.freeze to ensure your objects are readonly at the type level AND the runtime level
• Inversion of control
• Ultimate TypeScript Thread
• TypeScript Tips Series

Carlos Caballero

• Use look up tables instead of “if”

Minko Gechev

• Enum vs const enums
• Use labeled tuple elements to get better hints from your text editor
• Use as const after literals

Cory House

• Avoid making a property optional when the property isn’t valid in a certain case
• Alias the type’s name when conflicts with an existing identifier

Tomek Sułkowski

• Extract it from a component using the handy ComponentProps

Wes Bos

• Four ways to define an object type in TypeScript
• The difference between any and unknown
• Use TypeScript’s never for making sure you hit every scenario
• Use TypeScript’s never to enforce “one or the other” properties on a type

StackBlitz

• Infers array’s type as const
• How to use extends
• How to use readonly

Extending existing types

• PackageJson - There are a lot of tools that place extra configurations inside the package.json file. You can extend PackageJson to support these additional configurations.
  Example

  [Playground](https://www.typescriptlang.org/play?#code/JYWwDg9gTgLgBDAnmApnA3gBQIYGMDW2A5igFIDOEAdnNuXAEJ0o4HFmVUC+cAZlBBBwA5ElQBaXinIxhAbgCwAKFCRYCZGnQAZYFRgooPfoJHSANntmKlysWlaESFanAC8jZo-YuaAMgwLKwBhal5gIgB+AC44XX1DADpQqnCiLhsgA) ```ts import type { PackageJson as BasePackageJson } from "type-fest"; import type { Linter } from "eslint"; type PackageJson = BasePackageJson & { eslintConfig?: Linter.Config }; ```

Built-in types

There are many advanced types most users don’t know about.

• Partial<T> - Make all properties in T optional.
  Example

  [Playground](https://www.typescriptlang.org/play/#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) ```ts interface NodeConfig { appName: string; port: number; } class NodeAppBuilder { private configuration: NodeConfig = { appName: "NodeApp", port: 3000, }; private updateConfig( key: Key, value: NodeConfig[Key] ) { this.configuration[key] = value; } config(config: Partial) { type NodeConfigKey = keyof NodeConfig; for (const key of Object.keys(config) as NodeConfigKey[]) { const updateValue = config[key]; if (updateValue === undefined) { continue; } this.updateConfig(key, updateValue); } return this; } } // `Partial`` allows us to provide only a part of the // NodeConfig interface. new NodeAppBuilder().config({ appName: "ToDoApp" }); ``` </details>
• Required<T> - Make all properties in T required.
  Example

  [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) { // 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> - Make all properties in T readonly.
  Example

  [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; 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> - From T, pick a set of properties whose keys are in the union K.
  Example

  [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", }, ]); ```
• Record<K, T> - Construct a type with a set of properties K of type T.
  Example

  [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, }; ```
• Exclude<T, U> - Exclude from T those types that are assignable to U.
  Example

  [Playground](https://typescript-play.js.org/?target=6#code/JYOwLgpgTgZghgYwgAgMrQG7QMIHsQzADmyA3gFDLIAOuUYAXMiAK4A2byAPsgM5hRQJHqwC2AI2gBucgF9y5MAE9qKAEoQAjiwj8AEnBAATNtGQBeZAAooWphu26wAGmS3e93bRC8IASgsAPmRDJRlyAHoI5ABRAA8ENhYjFFYOZGVVZBgoXFFkAAM0zh5+QRBhZhYJaAKAOkjogEkQZAQ4X2QAdwALCFbaemRgXmQtFjhOMFwq9K6ULuB0lk6U+HYwZAxJnQaYFhAEMGB8ZCIIMAAFOjAANR2IK0HGWISklIAedCgsKDwCYgAbQA5M9gQBdVzFQJ+JhiSRQMiUYYwayZCC4VHPCzmSzAspCYEBWxgFhQAZwKC+FpgJ43VwARgADH4ZFQSWSBjcZPJyPtDsdTvxKWBvr8rD1DCZoJ5HPopaYoK4EPhCEQmGKcKriLCtrhgEYkVQVT5Nr4fmZLLZtMBbFZgT0wGBqES6ghbHBIJqoBKFdBWQpjfh+DQbhY2tqiHVsbjLMVkAB+ZAAZiZaeQTHOVxu9ySjxNaujNwDVHNvzqbBGkBAdPoAfkQA) ```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); } ```
• Extract<T, U> - Extract from T those types that are assignable to U.
  Example

  [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()); ```
• NonNullable<T> - Exclude null and undefined from T.
  Example

  Works with strictNullChecks set to true. [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; 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> - Obtain the parameters of a function type in a tuple.
  Example

  [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; return function (...args: FunctionParameters) { for (let i = 0; i < callCount; i++) { func(...args); } }; } const shuffleTwice = callNTimes(shuffle, 2); ``` </details>
• ConstructorParameters<T> - Obtain the parameters of a constructor function type in a tuple.
  Example

  [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> = new Map(); constructor(ctr: T) { this.ClassConstructor = ctr; } getInstance(...args: ConstructorParameters): InstanceType { 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> - Obtain the return type of a function type.
  Example

  [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 >(iter: Iterable, callback: Func): Ret[] { const mapped: Ret[] = []; for (const elem of iter) { mapped.push(callback(elem)); } return mapped; } const setObject: Set = 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> - Obtain the instance type of a constructor function type.
  Example

  [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 >(constructor: Constr, ...args: Args): InstanceType { 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> - Constructs a type by picking all properties from T and then removing K.
  Example

  [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; } ```
• Uppercase<S extends string> - Transforms every character in a string into uppercase.
  Example

  ```ts type T = Uppercase<"hello">; // 'HELLO' type T2 = Uppercase<"foo" | "bar">; // 'FOO' | 'BAR' type T3 = Uppercase<`aB${S}`>; type T4 = T3<"xYz">; // 'ABXYZ' type T5 = Uppercase; // string type T6 = Uppercase; // any type T7 = Uppercase; // never type T8 = Uppercase<42>; // Error, type 'number' does not satisfy the constraint 'string' ``` </details>
• Lowercase<S extends string> - Transforms every character in a string into lowercase.
  Example

  ```ts type T = Lowercase<"HELLO">; // 'hello' type T2 = Lowercase<"FOO" | "BAR">; // 'foo' | 'bar' type T3 = Lowercase<`aB${S}`>; type T4 = T3<"xYz">; // 'abxyz' type T5 = Lowercase; // string type T6 = Lowercase; // any type T7 = Lowercase; // never type T8 = Lowercase<42>; // Error, type 'number' does not satisfy the constraint 'string' ``` </details>
• Capitalize<S extends string> - Transforms the first character in a string into uppercase.
  Example

  ```ts type T = Capitalize<"hello">; // 'Hello' type T2 = Capitalize<"foo" | "bar">; // 'Foo' | 'Bar' type T3 = Capitalize<`aB${S}`>; type T4 = T3<"xYz">; // 'ABxYz' type T5 = Capitalize; // string type T6 = Capitalize; // any type T7 = Capitalize; // never type T8 = Capitalize<42>; // Error, type 'number' does not satisfy the constraint 'string' ``` </details>
• Uncapitalize<S extends string> - Transforms the first character in a string into lowercase.
  Example

  ```ts type T = Uncapitalize<"Hello">; // 'hello' type T2 = Uncapitalize<"Foo" | "Bar">; // 'foo' | 'bar' type T3 = Uncapitalize<`AB${S}`>; type T4 = T3<"xYz">; // 'aBxYz' type T5 = Uncapitalize; // string type T6 = Uncapitalize; // any type T7 = Uncapitalize; // never type T8 = Uncapitalize<42>; // Error, type 'number' does not satisfy the constraint 'string' ``` </details>

You can find some examples in the TypeScript docs.

Contribute

Contributions welcome! Read the contribution guidelines first.

Twitter to markdown file

Create .envrc and fill the value then Use tweet-to-markdown

# .envrc
export TTM_API_KEY=YOUR_API_KEY

Then run the direnv command

 direnv allow .

Finally, the generate markdown from a twitter url

npx tweet-to-markdown https://twitter.com/mattpocockuk/status/1509964736275927042\?s\=20\&t\=sA-g5MNM5TPjN6Ozs1qxgA

Credits

This project is made by community and especially the wonderful people and projects listed in this document

Open Source

• type-fest: for 2 sections (Extending existing type, Built-in types)

Tech Twitter

• Matt Pocock ✈️ Modern Frontends
• Carlos Caballero
• Minko Gechev (@mgechev@mstdn.social)
• Cory House
• Tomek Sułkowski
• Wes Bos
• StackBlitz

Author

👤 Huynh Duc Dung

• Website: https://productsway.com/
• Twitter: @jellydn
• Github: @jellydn

Stargazers

Show your support

Give a ⭐️ if this project helped you!