Configuring TypeScript

Deno’s flexibility shines in its equal treatment of TypeScript and JavaScript. Whether you’re transitioning from JavaScript to TypeScript or vice versa, Deno has features to ease the journey.

Type Checking JavaScript Jump to heading#

You may wish to make your JavaScript more type-sound without adding type annotations everywhere. Deno supports using the TypeScript type checker to type check JavaScript. You can mark individual files by adding the check JavaScript pragma to the file:

// @ts-check

This will cause the type checker to infer type information about the JavaScript code and raise any issues as diagnostic issues.

These can be turned on for all JavaScript files in a program by providing a configuration file with the check JS option set to true, as below. Then use the --config option when running on the command line.

{
  "compilerOptions": {
    "checkJs": true
  }
}

Using JSDoc in JavaScript Jump to heading#

When type-checking JavaScript or importing JavaScript into TypeScript, JSDoc annotations can provide additional type information beyond what can just be inferred from the code itself. Deno supports this seamlessly if you annotate your code inline with the supported TypeScript JSDoc.

For example to set the type of an array use the following JSDoc comment:

/** @type {string[]} */
const a = [];

Skipping type checking Jump to heading#

You might have TypeScript code that you are experimenting with, where the syntax is valid but not fully type safe. You can bypass type checking for a whole program by passing the --no-check flag.

You can also skip whole files being type checked, including JavaScript if you have check JS enabled, by using the nocheck pragma:

// @ts-nocheck

Renaming JS files to TS files Jump to heading#

TypeScript files benefit from the TypeScript compiler being able to do more thorough safety checks of your code. This is often referred to as strict mode. When you rename a .js file to .ts you'll might see new type errors that you TypeScript wasn't able to detect before.

Configuring TypeScript in Deno Jump to heading#

TypeScript offers many configuration options, which can be daunting if you're just starting out with TS. Deno aims to simplify using TypeScript, instead of drowning you in countless settings. Deno configures TypeScript to just work out of the box. No extra configuration headaches required!

However, if you do want to change the TypeScript compiler options, Deno allows you to do so in your deno.json file. Provide a path on the command line, or use the default. For example:

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

TS Compiler Options Jump to heading#

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.

Using the "lib" property Jump to heading#

If you're working on a project that ships code to multiple runtimes, like browsers for example, you can tweak the default types via the "lib" property within the compilerOptions.

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 are not enabled by default, 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 Jump to heading#

You may want to write code that seamlessly runs in both Deno and the browser. In this case you'll need to conditionally check the execution environment before using any APIs exclusive to one or the other. In such cases, a typical compilerOptions configuration might look like this:

{
  "compilerOptions": {
    "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 should add that library to the mix as well:

{
  "compilerOptions": {
    "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.

Types and Type Declarations Jump to heading#

Deno applies a design principle of no non-standard module resolution. When TypeScript checks a file, it focuses solely on its types. In contrast, the tsc compiler employs intricate logic to resolve those types. By default, tsc expects ambiguous module specifiers with extensions (e.g., .ts, .d.ts or .js). Deno, however, deals with explicit specifiers.

Here’s where it gets interesting: Imagine you want to consume a TypeScript file that’s already transpiled to JavaScript, along with its type definition file (mod.js and mod.d.ts). If you import mod.js into Deno, it strictly follows your request and imports the JavaScript file. But here’s the catch: Your code won’t be as thoroughly type-checked as if TypeScript considered the mod.d.ts file alongside the mod.js file.

To address this, Deno offers two solutions, each catering to specific scenarios:

As the Importer: If you know what types should apply to a JavaScript module, you can enhance type checking by explicitly specifying the types.

As the Supplier: If you’re the provider or host of the module, everyone consuming it benefits without worrying about type resolution.

Providing types when importing Jump to heading#

If you are consuming a JavaScript module and you have either created types (a .d.ts file) or have otherwise obtained the types you want to use, you can instruct Deno to use that file when type checking, instead of the JavaScript file, using the @ts-types compiler hint.

For example if you have a JavaScript module, coolLib.js, and a separate coolLib.d.ts file, you would import it like this:

// @ts-types="./coolLib.d.ts"
import * as coolLib from "./coolLib.js";

When you’re performing type checking on coolLib and using it in your file, the TypeScript type definitions from coolLib.d.ts will take precedence over examining the JavaScript file.

Providing types when hosting Jump to heading#

If you have control over the module’s source code or how the file is hosted on a web server, there are two ways to let Deno know about the types for a specific module (which won’t require any special action from the importer).

@ts-self-types Jump to heading#

If you are providing a JavaScript file, and want to provide a declaration file that contains the types for this file, you can specify a @ts-self-types directive in the JS file, pointing to the declaration file.

For example, if you make a coolLib.js library, and write its type definitions in coolLib.d.ts the ts-self-types directive would look like this:

// @ts-self-types="./coolLib.d.ts"

// ... the rest of the JavaScript ...

X-TypeScript-Types Jump to heading#

Deno supports a header for remote modules that instructs Deno where to locate the types for a given module. For example, a response for https://example.com/coolLib.js might look something like this:

HTTP/1.1 200 OK
Content-Type: application/javascript; charset=UTF-8
Content-Length: 648
X-TypeScript-Types: ./coolLib.d.ts

When seeing this header, Deno would attempt to retrieve https://example.com/coolLib.d.ts and use that when type checking the original module.

Using ambient or global types Jump to heading#

Overall it is better to use module/UMD type definitions with Deno, where a module expressly imports the types it depends upon. Modular type definitions can express augmentation of the global scope via the declare global in the type definition. For example:

declare global {
  var AGlobalString: string;
}

This would make AGlobalString available in the global namespace when importing the type definition.

In some cases though, when leveraging other existing type libraries, it may not be possible to leverage modular type definitions. Therefore there are ways to include arbitrary type definitions when type checking programmes.

Triple-slash directive Jump to heading#

This option couples the type definitions to the code itself. By adding a triple-slash types directive in a TS file (not a JS file!), near the type of a module, type checking the file will include the type definition. For example:

/// <reference types="./types.d.ts" />

The specifier provided is resolved just like any other specifier in Deno, which means it requires an extension, and is relative to the module referencing it. It can be a fully qualified URL as well:

/// <reference types="https://deno.land/x/pkg@1.0.0/types.d.ts" />

Suppling "types" in deno.json Jump to heading#

Another option is to provide a "types" value to the "compilerOptions" in your deno.json. For example:

{
  "compilerOptions": {
    "types": [
      "./types.d.ts",
      "https://deno.land/x/pkg@1.0.0/types.d.ts",
      "/Users/me/pkg/types.d.ts"
    ]
  }
}

Like the triple-slash reference above, the specifier supplied in the "types" array will be resolved like other specifiers in Deno. In the case of relative specifiers, it will be resolved relative to the path to the config file. Make sure to tell Deno to use this file by specifying --config=path/to/file flag.

Type Checking Web Workers Jump to heading#

When Deno loads a TypeScript module in a web worker, it will automatically type check the module and its dependencies against the Deno web worker library. This can present a challenge in other contexts like deno check or in editors. There are a couple of ways to instruct Deno to use the worker libraries instead of the standard Deno libraries.

Triple-slash directives Jump to heading#

This option couples the library settings with the code itself. By adding the following triple-slash directives near the top of the entry point file for the worker script, Deno will now type check it as a Deno worker script, irrespective of how the module is analyzed:

/// <reference no-default-lib="true" />
/// <reference lib="deno.worker" />

The first directive ensures that no other default libraries are used. If this is omitted, you will get some conflicting type definitions, because Deno will try to apply the standard Deno library as well. The second instructs Deno to apply the built-in Deno worker type definitions plus dependent libraries (like "esnext").

The one disadvantage of this, is that it makes the code less portable to other non-Deno platforms like tsc, as it is only Deno which has the "deno.worker" library built into it.

Providing "lib" setting in deno.json Jump to heading#

You can provide a "lib" option in your deno.json file to instruct Deno to use library files. For example:

{
  "compilerOptions": {
    "target": "esnext",
    "lib": ["deno.worker"]
  }
}

Then when running deno subcommand, you would need to pass the --config path/to/file argument, or if you are using an IDE which leverages the Deno language server, set the deno.config setting.

If you also have non-worker scripts, you will either need to omit the --config argument, or have one that is configured to meet the needs of your non-worker scripts.

Important points Jump to heading#

Type declaration semantics Jump to heading#

Type declaration files (.d.ts files) follow the same semantics as other files in Deno. This means that declaration files are assumed to be module declarations (UMD declarations) and not ambient/global declarations. It is unpredictable how Deno will handle ambient/global declarations.

In addition, if a type declaration imports something else, like another .d.ts file, its resolution follow the normal import rules of Deno. For a lot of the .d.ts files that are generated and available on the web, they may not be compatible with Deno.

esm.sh is a CDN which provides type declarations by default (via the X-TypeScript-Types header). It can be disabled by appending ?no-dts to the import URL:

import React from "https://esm.sh/react?no-dts";

Behavior of JavaScript when type checking Jump to heading#

When you import JavaScript code into TypeScript within Deno, even if you’ve set checkJs to false (which is the default behavior for Deno), the TypeScript compiler will still analyze the JavaScript module. It tries to infer the shape of the exports from that module to validate the import in your TypeScript file.

Usually, this isn’t an issue when importing a standard ES module. However, there are cases where TypeScript’s analysis might fail, for example, with modules that have special packaging or are global UMD (Universal Module Definition) modules. When faced with such situations, the best approach is to provide some form of type information using one of the methods mentioned earlier.

Internals Jump to heading#

While it isn't required to understand how Deno works internally to be able to leverage TypeScript with Deno well, it can help to understand how it works.

Before any code is executed or compiled, Deno generates a module graph by parsing the root module, and then detecting all of its dependencies, and then retrieving and parsing those modules, recursively, until all the dependencies are retrieved.

For each dependency, there are two potential "slots" that are used. There is the code slot and the type slot. As the module graph is filled out, if the module is something that is or can be emitted to JavaScript, it fills the code slot, and type only dependencies, like .d.ts files fill the type slot.

When the module graph is built, and there is a need to type check the graph, Deno starts up the TypeScript compiler and feeds it the names of the modules that need to be potentially emitted as JavaScript. During that process, the TypeScript compiler will request additional modules, and Deno will look at the slots for the dependency, offering it the type slot if it is filled before offering it the code slot.

This means when you import a .d.ts module, or you use one of the solutions above to provide alternative type modules for JavaScript code, that is what is provided to TypeScript instead when resolving the module.