Zod is a Typescript package that lets you declare your data types and do runtime type checking.

Lots of possible utilities for this library. Validation and type guards come to mind first, but Zod also includes a lot of helpful methods such as default and transform.

Quick example

import { z } from 'zod'

const Post = z.object({
  id: z.string().uuid(),
  title: z.string().min(1),
  body: z.string()
})

// Declare the type
type Post = z.infer<typeof Post>

const myPost = {
  id: 'c2cffa40-8169-4e9b-bce4-af21bc17b033',
  title: 'Validate schemas and declare types with Zod',
  body: 'Lorem ipsum dolor sit amet, consectetur adipiscing elit.'
}

Post.safeParse(myPost)

// {
//   success: true,
//   data: {
//     id: 'c2cffa40-8169-4e9b-bce4-af21bc17b033',
//     title: 'Validate schemas and declare types with Zod',
//     body: 'Lorem ipsum dolor sit amet, consectetur adipiscing elit.'
//   }
// }

Useful libraries for coloring programming language syntax.

Lowlight and Refractor are two interesting libraries that wrap highlight.js and Prism respectively.

From Titus Wormer:

Virtual syntax highlighting for virtual DOMs and non-HTML things.

Instead of returning a HTML string with <span>s and class names, it returns an array of objects that you can use to create HTML or anything else really. Super useful for creating React components without using something like dangerouslySetInnerHTML.