I've been using TypeScript for over four years now, and, overall, it's been a great experience. With time, the friction of using it has minimized until it became zero, making me much more productive when writing types or even approaching problems type-first. Although I'm far from being a true type wizard, I dare consider myself proficient with the language, having gone thourgh my share of type gymnastics, conditional types, nested generics, and contemplating the sacred difference between type and interface. Thruthfully, I thought I understood the language rather well.

Until I didn't. See, there's one particular thing about TypeScript that I got totally wrong, and I believe you did too. And it's not some contrived corner case you've never heard of and likely won't ever use. Quite the opposite. It's something you, and any other TypeScript developer interacted directly hundreds of times, something that's been swimming right under our noses all along.

I'm talking about tsconfig.json.

And no, this isn't about how complex it can get (I confess I can't explain target and module without a moment of thought). Instead, it's something rather simple. It's about what tsconfig.json actually does.

"Well, it's a configuration file, it configures TypeScript, duh." Right! It does, but not in a way you would expect. Let me show you.

Libraries, tests, and the truth

There's a great example behind every great discovery. I will do my best for this to be both.

Let's write a simple frontend application. And I mean it, no frameworks, no dependencies. Simple.

1// src/app.ts
2const greetingText = document.createElement('p')
3greetingText.innerText = 'Hello, John!'

Create a paragraph element and greet John. Simple. So far so good.

But where does the document come from? You can say it's a global variable in JavaScript and, by all means, you would be right. There's just one thing. We aren't in JavaScript. Not yet, really. We are looking at some TypeScript code in our IDE. It'd have to be compiled to become JavaScript, land in the browser, and for the browser to expose the document global to it. So how does TypeScript knows of the document, its presence, and its methods?

TypeScript does that by loading a default definition library called lib.dom. Think of it as a .d.ts file containing a bunch of types to describe JavaScript globals, because that's precisely what it is. You can see that for yourself by holding CMD (CTRL on Windows) and clicking on the document object. Mystery solved.

Since our application is, naturally, the best thing since sliced bread, let's add some automated tests for it. For this step, we will betray our notion of simplicity and install a testing framework called Vitest. Next, we write the test itself:

1// src/app.test.ts
2it('greets John', async () => {
3 await import('./app')
4 const greetingText = document.querySelector('p')
5 expect(greetingText).toHaveText('Hello, John!')

Once we try to run this test, TypeScript would interfere with an error:

Cannot find name 'it'. Do you need to install type definitions for a test runner?

It hurts me to admit it but the compiler has the point. Where would it come from? It's not a global like document, it has to come from somewhere. Well, actually, it's quite common for the testing frameworks to extend the global object and expose functions like it and expect globally so you can access them in each test without having to import them explicitly.

We follow a conveniently present section of our testing framework's documentation and enable global it by modifying tsconfig.json:

1// tsconfig.json
3 "compilerOptions": {
4 "types": ["vitest/globals"]
5 },
6 "include": ["src"]

By using the compilerOptions.types, we are asking TypeScript to load additional types, in this case from vitest/globals, that declare the global it function. The compiler grins at our efforts and lets the test pass, making us feel particuarly good about ourselves and this whole strictly typed languages ordeal.

But not. So. Fast.

The issue

We will take a slight step to the side but I promise it will all make sense in the end.

Let me ask you this: What happens if you reference a non-existing code in TypeScript? Yep, a wavy red line and the Cannot find name type error, that's what happens. We've just seen it a moment ago trying to call it() in a test.

Jump back to the app.ts module and add a reference to a non-existing global variable called test:

1// src/app.ts
2// ...application code.

We haven't defined test. It's not a browser global, and it certainly doesn't exist in any of TypeScript default libraries. It's a mistake, a bug, it has to go red.

Only, it doesn't. As the red wavy line doesn't reveal itself beneath the code, power courses through you. Authority. Confusion. To make things worse, not only does TypeScript not produce an error here, it actually tries being helpful, suggesting us to type test, showing us its call signature, saying it comes from some TestApi namespace. But that's a type from Vitest, how can this be. . .

Would this code compile? Sure. Would it work in the browser? Nope. It will throw like a seasoned pitcher on his brightest day. How come? Isn't the entire purpose of using TypeScript to guard against mistakes like this?

The test here is what I refer to as a ghostly definition. It's a valid type definition that describes something that just doesn't exist. Yet another TypeScript shenanigan, say you. Don't hurry blaming the tool, say I. Here's what's happening.

(More than) one config to rule them all

Move the app.test.ts test module from the src directory to a newly created test directory. Open it. Wait, is that a type error on it again? Didn't we fixed that already by adding vitest/globals to our tsconfig.json?

The thing is, TypeScript doesn't know what to do with the test directory. In fact, TypeScript doesn't even know it exists since all we point to in tsconfig.json is src:

1// tsconfig.json
3 "compilerOptions": {
4 "types": ["vitest/globals"]
5 },
6 "include": ["src"]

As I mentioned before, the way TypeScript configuration works is not entirely obvious (at least to me). For a long time I used to think that the include option stands for which modules to include in the compilation, and exclude, respectively, controls which modules to exclude. If we consult TypeScript documentation on the matter, we will read this:

include, specifies an array of filenames or patterns to include in the program.

The way I come to understand what include does is slightly different and more specific than what's stated in the docs.

The include option controls what modules to apply this TypeScript configuration to.

You read it right. If a TypeScript module is located outside of the directories listed in the include option, that tsconfig.json will have no effect on that module at all. Respectively, the exclude option allows to filter out which file patterns must be not be affected by the current configuration.

Okay, so we add test to include and move on with our day, what's the big deal?

1// tsconfig.json
3 "compilerOptions": {
4 "types": ["vitest/globals"]
5 },
6 "include": ["src", "test"]

This is where most developers get it completely wrong. By adding new directories to include, you are expanding this configuration to affect all of them. While this change fixes the testing framework types in test, it will leak them to all src modules! You've just made your entire source code one haunted mansion, unleashing hundreds of ghostly types upon it. Things that don't exist will be typed, thing that are typed may clash with other definitions, and the overall experience with using TypeScript will degrade drastically, especially as your application grows over time.

So, what's the solution then? Should we go and create a bunch of tsconfig.json for every directory?

Well, actually, yeah, you should. Except, not for every directory, but for every environment your code is meant to run.

Runtimes and concerns

Behind-the-scenes of a modern web application is an exquisite salad of modules. The immediate source of your app is meant to be compiled, minified, code-split, bundled, and shipped to your users. Then there are test files, which are TypeScript modules also, never to be compiled or shipped to anyone. There may also be Storybook stories, Playwright tests, maybe a custom *.ts script or two to automate things—all helpful, all having different intentions and meant to run in different environments.

But what we write our modules for matters. It matters for TypeScript too. Why do you think it gives you the Document type by default? Because it knows you're likely developing a web app. Developing a Node.js server instead? Be so kind to state that intention and install @types/node. The compiler cannot guess for you, you need to tell it what you want.

And you communicate that intention through tsconfig.json. But not just the root-level one. TypeScript can handle nested configurations remarkably well. Because it was designed to do that. All you have to do is be explicit about your intentions.

You do so by strategically placing the tsconfig.json files across your project. Here's an example:

1# The root-level configuration to list the common options.
2# All the other configurations will extend from it.
3# We want to set "noEmit": true and "declarations": false,
4# as well as "skipLibCheck": true in this one.
5- tsconfig.json
7# The root-level configuration for the actual application build.
8# This is where we want to emit modules and type declarations.
9- tsconfig.build.json
11- /e2e
12 # A configuration for end-to-end test files
13 # that are meant to either run in or describe the browser
14 # runtime. We'd want the "lib" library here, and perhaps
15 # additional types, like "dom.iterable".
16 - tsconfig.json
17 - Login.test.ts
19- /src
20 # Another config, this time for the source code.
21 # This can be skipped it the default configuration
22 # covers the source, but the source may require additional
23 # types in certain situations.
24 - App.tsx
25 - util.ts
27- /test
28 # TypeScript configuration for test files. This is where
29 # we add things like "vitest/globals", Node.js globals,
30 # and other, test-related types.
31 - tsconfig.json
32 - App.test.tsx

Woah, that's a lot of configs! Well, that's a lot of intentions as well: from the source files to various testing levels to the production build. All meant to be type-safe. And you make them type-safe by introducing directory-scoped configurations that extend the base ./tsconfig.json at the root.

For example, this is what the TypeScript configuration for the src files may look like:

1// src/tsconfig.json
3 // Extend the root-level config to reuse common options.
4 "extends": "./tsconfig.json",
5 "compilerOptions": {
6 // Compile to the code that runs in the browser.
7 "target": "es2015",
8 "module": "esnext",
9 // Support JSX, we're running React here.
10 "jsx": "react"
11 },
12 // Apply this config only to the source files.
13 "include": ["src"],
14 "exclude": ["node_modules"]

In contrast, here's a configuration for the integration tests in the test directory:

1// test/tsconfig.json
3 "extends": "../tsconfig.json",
4 "compilerOptions": {
5 // No transpiling here, let's stay on the edge.
6 "target": "esnext",
7 "module": "esnext",
8 // Integration tests run in Node.js.
9 // Let's also add those test runner's globals.
10 "types": ["@types/node", "vitest/globals"]
11 },
12 // We care only about the test files here.
13 "include": ["**/*.test.ts"]

When writing TypeScript configurations, remember this:

You should have as many TypeScript configurations as there are layers in your project: source code, Node.js testing, in-browser testing, third-party tooling, etc.

TypeScript will automatically pick up the tsconfig.json closest to the module it type-checks, giving you the ability to extend while deviating where necessary.

The practical aspect

For better or worse, we are moving towards the era where developer tooling is abstracted from us. It's fair to expect your framework of choice to handle this configuration jungle for you. In fact, some frameworks already do this. Take Vite as an example. I'm quite confident you can find a multi-configuration setup for TypeScript in about any other project.

But I want you to understand that TypeScript is still your tool, abstracted or not, and you would do good by learning more about it, understanding it better, and using it right.