Configuring TypeScript in Deno

TypeScript comes with a load of different options that can be configured, but Deno strives to make it easy to use TypeScript with Deno. Lots of different options frustrates that goal. To make things easier, Deno configures TypeScript to “just work” and shouldn’t require additional configuration.

That being said, Deno does support using a TypeScript configuration file, though like the rest of Deno, the detection and use of use of a configuration file is not automatic. To use a TypeScript configuration file with Deno, you have to provide a path on the command line. For example:

> deno run --config ./tsconfig.json main.ts

⚠️ Do consider though that if you are creating libraries that require a configuration file, all of the consumers of your modules will require that configuration file too if you distribute your modules as TypeScript. In addition, there could be settings you do in the configuration file that make other TypeScript modules incompatible. Honestly it is best to use the Deno defaults and to think long and hard about using a configuration file.

How Deno uses a configuration file

Deno does not process a TypeScript configuration file like tsc does, as there are lots of parts of a TypeScript configuration file that are meaningless in a Deno context or would cause Deno to not function properly if they were applied.

Deno only looks at the compilerOptions section of a configuration file, and even then it only considers certain compiler options, with the rest being ignored.

Here is a table of compiler options that can be changed, their default in Deno and any other notes about that option:

For a full list of compiler options and how they affect TypeScript, please refer to the TypeScript Handbook

What an implied tsconfig.json looks like

It is impossible to get tsc to behave like Deno. It is also difficult to get the TypeScript language service to behave like Deno. This is why we have built a language service directly into Deno. That being said, it can be useful to understand what is implied.

If you were to write a tsconfig.json for Deno, it would look something like this:

{
  "compilerOptions": {
    "allowJs": true,
    "esModuleInterop": true,
    "experimentalDecorators": true,
    "inlineSourceMap": true,
    "isolatedModules": true,
    "jsx": "react",
    "lib": ["deno.window"],
    "module": "esnext",
    "strict": true,
    "target": "esnext",
    "useDefineForClassFields": true
  }
}

You can’t copy paste this into a tsconfig.json and get it to work, specifically because of the built in type libraries that are custom to Deno which are provided to the TypeScript compiler. This can somewhat be mocked by running deno types on the command line and piping the output to a file and including that in the files as part of the program, removing the "lib" option, and setting the "noLib" option to true.

If you use the --unstable flag, Deno will change the "lib" option to [ "deno.window", "deno.unstable" ]. If you are trying to load a worker, that is type checked with "deno.worker" instead of "deno.window". See Type Checking Web Workers for more information on this.

Using the “lib” property

Deno has several libraries built into it that are not present in other platforms, like tsc. This is what enables Deno to properly check code written for Deno. In some situations though, this automatic behavior can cause challenges, for example like writing code that is intended to also run in a browser. In these situations the "lib" property of a tsconfig.json can be used to modify the behavior of Deno when type checking code.

The built-in libraries that are of interest to users:

• "deno.ns" - This includes all the custom Deno global namespace APIs plus the Deno additions to import.meta. This should generally not conflict with other libraries or global types.
• "deno.unstable" - This includes the addition unstable Deno global namespace APIs.
• "deno.window" - This is the “default” library used when checking Deno main runtime scripts. It includes the "deno.ns" as well as other type libraries for the extensions that are built into Deno. This library will conflict with libraries like "dom" and "dom.iterable" that are standard TypeScript libraries.
• "deno.worker" - This is the library used when checking a Deno web worker script. For more information about web workers, check out Type Checking Web Workers.
• "dom.asynciterable" - TypeScript currently does not include the DOM async iterables that Deno implements (plus several browsers), so we have implemented it ourselves until it becomes available in TypeScript.

These are common libraries that Deno doesn’t use, but are useful when writing code that is intended to also work in another runtime:

• "dom" - The main browser global library that ships with TypeScript. The type definitions conflict in many ways with "deno.window" and so if "dom" is used, then consider using just "deno.ns" to expose the Deno specific APIs.
• "dom.iterable" - The iterable extensions to the browser global library.
• "scripthost" - The library for the Microsoft Windows Script Host.
• "webworker" - The main library for web workers in the browser. Like "dom" this will conflict with "deno.window" or "deno.worker", so consider using just "deno.ns" to expose the Deno specific APIs.
• "webworker.importscripts" - The library that exposes the importScripts() API in the web worker.
• "webworker.iterable" - The library that adds iterables to objects within a web worker. Modern browsers support this.

Targeting Deno and the Browser

A common use case is writing code that works in Deno and the browser, and have the code “sniff” to determine if it is running in the browser or in Deno. If that is the case a common configuration of a tsconfig.json would look like this:

{
  "compilerOptions": {
    "target": "esnext",
    "lib": ["dom", "dom.iterable", "dom.asynciterable", "deno.ns"]
  }
}

This should allow most code to be type checked properly by Deno.

If you expect to run the code in Deno with the --unstable flag, then you will want to add that library to the mix as well:

{
  "compilerOptions": {
    "target": "esnext",
    "lib": [
      "dom",
      "dom.iterable",
      "dom.asynciterable",
      "deno.ns",
      "deno.unstable"
    ]
  }
}

Typically when you use the "lib" option in TypeScript, you need to include an “es” library as well. In the case of "deno.ns" and "deno.unstable", they automatically include "esnext" when you bring them in.

The biggest “danger” when doing something like this, is that the type checking is significantly looser, and there is no way to validate that you are doing sufficient and effective feature detection in your code, which may lead to what could be trivial errors becoming runtime errors.