TypeScript

TypeScript allows programmers to define type guards so that programmers have more control over the dynamic nature of the typing of JavaScript. Since type guards are completely defined by the programmer, they also evade type checking from the TypeScript compiler: Type checking won’t catch any error if the type guard doesn’t correctly determine the type. This can be caused by typos, updating the type without updating the type guard, etc. For example, consider the following interface and its well-written type guard: ...

Array.includes is a commonly used function in JavaScript. However, in TypeScript, its typing is quite strict: The element being searched for must have the same type as that of the array element. This causes type errors if the array is of a literal type. For example, with the following code snippet: const okNumbers = [1, 3, 5, 7] as const; console.log(`2 is OK? ${okNumbers.includes(2)}`); The TypeScript compiler complains: Argument of type ‘2’ is not assignable to parameter of type ‘1 | 3 | 5 | 7’. ...

React allows specifying the style of an HTML element from within the app, which is a powerful tool when the style depends on the data or logic of the app. On the other hand, CSS variables, also known as CSS custom properties, provide great flexibility for CSS. For example, the following code specifies a CSS variable of the h1 element: export default function App() { return <h1 style={{ "--component-style": "classical" }}>Hello World!</h1>; } However, the TypeScript compiler will complain: src/App.tsx:2:23 - error TS2353: Object literal may only specify known properties, and '"--component-style"' does not exist in type 'Properties<string | number, string & {}>'. 2 return <h1 style={{ "--component-style": "classical" }}>Hello World!</h1> ~~~~~~~~~~~~~~~~~~~ node_modules/@types/react/index.d.ts:2908:9 2908 style?: CSSProperties | undefined; ~~~~~ The expected type comes from property 'style' which is declared here on type 'DetailedHTMLProps<HTMLAttributes<HTMLHeadingElement>, HTMLHeadingElement>' In this post, we discuss how to properly type CSS variables in React. Here, we avoid using type assertions and strive for type safety by typing as narrowly as possible. ...

Motivation TypeDoc is a document generator that, among other things, converts comments in TypeScript source code into rendered HTML documentation. The generated HTML documentation can be hosted on static website hosting services, such as GitHub Pages, GitLab Pages, Netlify, and CloudFlare Pages. All of them by default use a 404.html file as the content of the 404 page of the site. However, TypeDoc does not generate a 404.html by default. How can we make TypeDoc generate such a 404.html file? ...

Motivation Very often in TypeScript, we would like to wrap an existing function into a wrapper function, with some additional preprocessing or postprocessing. For example, function existingFunc(arg1: number, arg2: string): number { return arg1 + arg2.length; } function wrapperFunc(arg1: number, arg2: string): number { // Preprocess arg1 and arg2... return existingFunc(arg1, arg2); } In this case, the wrapper function often shares the same parameter types and return type. If the existing function is already typed, we can reuse those typing information. This would reduce redundancy and is also able to reflect any changes that would happen in the typing of the existing function in the future. ...