Import TypeScript types in JavaScript projects

Let's assume we want to write a Jest config object. Our IDE cannot infer available Jest options:

With JSDoc comments, it's possible to document our JavaScript objects and function parameters. While JSDoc is mostly a documentation tool, editors like WebStorm and VSCode make use of JSDoc comments for code completion. What's interesting is that you can use TypeScript definitions within @type or @param! You'll get an improved IntelliSense without migrating your project to TypeScript:

Here are some common examples:

• Jest configuration file:

// jest.config.js

/** @type {import('@jest/types').Config.InitialOptions} */
module.exports = { ... }

• ESlint configuration file:

// .eslintrc.js

/** @type {import('@types/eslint').Linter.BaseConfig} */
module.exports = { ... }

• Tailwind configuration file:

// tailwind.config.js

/** @type {import('@types/tailwindcss/tailwind-config').TailwindConfig} */
module.exports = { ... }

• Eleventy configuration file (type):

// .eleventy.js

/** @param { import("./types/eleventy").Config } config */
module.exports = function (config) { ... }

Don't forget to add the corresponding dependencies, e.g.:

$ yarn add -D @jest/types         # Jest
$ yarn add -D @types/eslint       # ESlint
$ yarn add -D @types/tailwindcss  # Tailwind

This technique is also useful in projects that are in the middle of a TypeScript migration, i.e., when you mix JavaScript and TypeScript files.