JSDoc · TypeScript · jsconfig.json

Type-checked JavaScript. No build step.

Write your types as JSDoc comments and let TypeScript's checker read them through one jsconfig.json file. Your .js files run exactly as written, and your editor and CI catch type errors before your users do.

Start in 5 minutes Browse the reference

Set up checking in three steps

TypeScript 5.9.3 TypeScript 5.9.3, run 21 October 2026: No errors. 6.0.3 TypeScript 6.0.3, run 21 October 2026: the same as 5.9.3. No errors. 7.0.2 TypeScript 7.0.2, run 21 October 2026: the same as 5.9.3. No errors. checked 21 October 2026

Start a project

Terminal

npm init -y
npm pkg set type=module
npm install --save-dev typescript

Create jsconfig.json

jsconfig.json

{
  "compilerOptions": {
    "checkJs": true,
    "noEmit": true,
    "strict": true,
    "module": "nodenext",
    "target": "es2022"
  }
}

Check your code
Terminal
```
npm pkg set scripts.lint="tsc -p jsconfig.json"
npm run lint
```
This config checks index.js in the quick start on every build of this site.

0 build steps
1 config file
5 minutes

The full quick start

Why bother

What you get

The checking TypeScript users rely on, in the JavaScript you already write.

Editor hints

Completions, hovers and inline errors in every .js file, from comments you would write anyway.
Errors before runtime

Typos, wrong arguments and missing null checks fail in your editor and in CI, not in production.
Docs from the same comments

The JSDoc that types a function also documents it, for the checker, the editor and the next reader.
Publishable types

TypeScript can turn the same comments into .d.ts files, so people who install your package get types too.

The reference

Look it up

Tag and type pages pair the JSDoc with its TypeScript spelling, and every example in the reference passed (or failed exactly as shown) in tsc.

JSDoc tags

Every JSDoc tag as TypeScript reads it: syntax, support level, checked examples.
Type expressions

What goes inside the braces: unions, generics, object shapes, each next to its TypeScript spelling.
Config options

The jsconfig.json options that matter for JavaScript, and what each one changes.
Errors

The errors JavaScript projects meet first, what causes each one, and how to fix it.
Editors

Making your editor show the same errors as tsc, one editor at a time.
TypeScript releases

What each TypeScript release from 4.0 to 7.0 changed for JavaScript checked with JSDoc.

Recipes

How do I type this?

Answers to specific questions about real code, each one checked end to end. Six places to start, and 58 recipes in all.

- @param
- Optional parameters
- Default values
How do I mark a parameter as optional in JSDoc?

Some of my function's parameters can be left out, and some have default values. How do I write that in JSDoc so callers may skip them and tsc still checks what they pass?
Read the recipe
- @template
- Generics
- keyof
How do I write a generic function in JavaScript with @template?

I have small helpers like last(), indexById() and pluck() that work on any array. With @param {any[]} they lose the element type. How do I make them generic in JSDoc?
Read the recipe
- Enums
- @typedef
- Object.freeze
How do I write an enum in JavaScript with JSDoc?

In TypeScript I would write enum OrderStatus. My code is plain JavaScript. How do I get a fixed set of named values that tsc checks, and what is wrong with the @enum tag?
Read the recipe
- @import
- @typedef
- import() types
How do I import and re-export types between JavaScript files?

I define Order and LineItem with @typedef in one file. How do I use them in other files, and how do I collect the types of several modules in one place for the rest of the app to import?
Read the recipe
- DOM
- querySelector
- instanceof
How do I type document.querySelector results in JavaScript?

document.querySelector('#email').value fails with 'possibly null' and 'Property value does not exist on type Element'. The element is always on the page. What is the right way to get an input element in JSDoc-checked code?
Read the recipe
- React
- JSX
- @typedef
How do I type React component props in a .jsx file?

My React app is written in .jsx files. How do I declare a component's props in JSDoc so a missing or wrong prop is caught where the component is used, without switching to .tsx?
Read the recipe

All 58 recipes, by topic

Guides

Read further

The longer reads: migrating a codebase, the quirks of JSDoc, shipping types, CI. Dated, so you know how current they are.

21 October 2026 · Style

A JSDoc style guide for type-checked JavaScript

House rules for JSDoc that TypeScript checks: one comment style, where @typedefs live, naming, @import, @satisfies over casts, and Prettier settings.
21 October 2026 · Adoption

Checking JavaScript in CI

Run tsc on every pull request: a GitHub Actions workflow, caching, monorepos, a pre-commit hook, and failing only on new errors with a baseline file.
21 October 2026 · Adoption

Choosing a strictness level for checked JavaScript

Relaxed, recommended or strict: what each level of jsconfig.json strictness catches, what it costs in an existing codebase, and an order to turn options on.
21 October 2026 · Concepts

How TypeScript reads a JavaScript file

How tsc reads a .js file: types from initializers, this and module.exports assignments, which JSDoc comments count, and what .js allows that .ts does not.
21 October 2026 · Adoption

Migrating a JavaScript codebase to checked JavaScript, step by step

Move an existing JavaScript project to type-checked JavaScript one step at a time: per file, per folder, then noImplicitAny and strict, measuring as you go.

All 10 guides

Compare

Weigh it up

Checked JavaScript next to the alternatives: TypeScript files, Node's type stripping, Flow, other runtimes and TypeScript's own versions.

JSDoc-typed JavaScript vs TypeScript files

The same checker reads both. What differs is syntax, the build step, and what each can express: a dated comparison with examples checked by tsc.
JSDoc-typed JavaScript vs Node's type stripping

Node.js now runs .ts files by erasing their types. What that changes about the no-build-step argument, what it does not do, and which Node.js versions do it.
jsconfig.json vs tsconfig.json

The same file format with different defaults: what a jsconfig.json turns on by itself, how tsc and editors find each file, and when to use which.
TypeScript 5.9 vs 6.0 vs 7.0 for JavaScript projects

What changes for checked JavaScript across TypeScript 5.9, 6.0 and the 7.0 native port: defaults, removed options, JSDoc 7.0 drops, speed, and an upgrade path.
JSDoc with TypeScript vs Flow

Two ways to type JavaScript in comments: TypeScript's checker reading JSDoc, and Flow's comment types. Where each stands today, and how to move from Flow.
Checking JavaScript in Node.js, Deno and Bun

What each runtime checks before and while it runs your JavaScript, which config each reads, and the commands, with one sample file checked by tsc.

All comparisons

Where the name comes from

A talk, a tool, and now a reference

The name comes from a 2022 conference talk by AJ ONeal that made the case for typed JavaScript without transpiling, and from the setup tool he published with it. This site is an independent reference for the same idea.

The history

Check your first file today

One config file and one npm script. Your code runs exactly as it does now; it just gets checked first.

Start in 5 minutes

Type-checked JavaScript. No build step.

Type-checked JavaScript. No build step.

Set up checking in three steps

Editor hints

Errors before runtime

Docs from the same comments

Publishable types

A talk, a tool, and now a reference

Check your first file today