Creating a TypeSpec Library

A TypeSpec library is a package that includes TypeSpec types, decorators, emitters or linters. These libraries are npm packages with some additional TypeSpec-specific metadata and conventions. This guide will walk you through the process of creating a new TypeSpec library, adding types to it, and distributing it on the public npm registry. Further sections will delve into the specifics of creating decorators, emitters and linters.

While this guide assumes that you’ll be using TypeScript to develop your library, you can skip the TypeScript-related steps if you prefer to use plain JavaScript.

Prerequisites

You’ll need to have both Node and npm installed. If you’re planning to develop multiple libraries simultaneously, it’s recommended to set up a monorepo to simplify the development process. TypeSpec itself uses pnpm.

Setting up with templates

You can use the following templates:

# Create a TypeSpec library (Decorators & Linters) with TypeScript enabled.
tsp init --template library-ts

# Create a TypeSpec emitter with TypeScript enabled.
tsp init --template emitter-ts

Standard package structure

Here’s a high-level overview of what a TypeSpec package typically contains. Each of these files will be explained in more detail in the following sections.

• dist/index.js - The main file for your Node library
• lib/main.tsp - The main file for your TypeSpec types (optional)
• src/index.ts - The main file for your Node library in TypeScript
• src/lib.ts - The file that defines your TypeSpec library
• package.json - Metadata about your TypeSpec package

Step 1: Initial setup

You can skip this step if you’ve used one of the templates above.

a. Initialize your package directory & package.json

Run the following commands:

mkdir myLibrary
cd myLibrary
npm init

After completing the wizard, you’ll have a package.json file that defines your TypeSpec library.

Unlike Node libraries which support CommonJS (cjs), TypeSpec libraries must be ECMAScript Modules. To specify this, open your package.json and add the following top-level configuration key:

  "type": "module"

b. Install TypeSpec dependencies

Run the following command:

npm install --save-peer @typespec/compiler

You might need to install other dependencies from the TypeSpec standard library. For example, if you want to use the metadata found in @typespec/openapi, you’ll need to install that as well.

Refer to the dependency section for more information on defining your dependencies.

c. Define your main files

Your package.json needs to refer to two main files: your Node module main file, and your TypeSpec main. The Node module main file is specified by the "main" key in your package.json file, and it defines the entry point for your library when it’s used as a Node library. This must reference a JS file. The TypeSpec main defines the entry point for your library when it’s used from a TypeSpec program, and it can reference either a JS file (when your library doesn’t contain any TypeSpec types) or a TypeSpec file.

  "main": "dist/src/index.js",
  "exports": {
    ".": {
      "typespec": "./lib/main.tsp"
    },
    // Additional named export are possible
    "./experimental": {
      "typespec": "./lib/experimental.tsp"
    },
    // Wildcard export as well
    "./lib/*": {
      "typespec": "./lib/*.tsp"
    }
  }

d. Install and initialize TypeScript

Run the following commands:

npm install -D typescript
npx tsc --init --strict

This will create a tsconfig.json file. You’ll need to make a few changes to this file. Open tsconfig.json and set the following settings:

"module": "Node16",           // This and next setting tells TypeScript to use the new ESM import system to resolve types.
"moduleResolution": "Node16",
"target": "es2019",
"rootDir": ".",
"outDir": "./dist",
"sourceMap": true,

e. Create lib.ts

Open ./src/lib.ts and create your library definition that registers your library with the TypeSpec compiler and defines any diagnostics your library will emit. Make sure to export the library definition as $lib.

Caution

If $lib is not accessible from your library package (for example, import {$lib} from "my-library";), some features such as linting and emitter option validation will not be available.

For example:

import { createTypeSpecLibrary } from "@typespec/compiler";

export const $lib = createTypeSpecLibrary({
  name: "myLibrary",
  diagnostics: {},
} as const);

// Optional but convenient, these are meant to be used locally in your library.
export const { reportDiagnostic, createDiagnostic } = $lib;

Diagnostics are used for linters and decorators, which are covered in subsequent topics.

f. Create index.ts

Open ./src/index.ts and import your library definition:

// Re-export $lib so the compiler can access it and register your library correctly.
export { $lib } from "./lib.js";

g. Build TypeScript

TypeSpec can only import JavaScript files, so any changes made to TypeScript sources need to be compiled before they are visible to TypeSpec. To do this, run npx tsc -p . in your library’s root directory. If you want to re-run the TypeScript compiler whenever files are changed, you can run npx tsc -p . --watch.

Alternatively, you can add these as scripts in your package.json to make them easier to invoke. Consider adding the following:

  "scripts": {
    "clean": "rimraf ./dist ./temp",
    "build": "tsc -p .",
    "watch": "tsc -p . --watch",
    "test": "node --test ./dist/test/**/*.test.js", // Node 22+
    "test": "node --test ./dist/test/"              // Node 18, 20
  }

You can then run npm run build or npm run watch to build or watch your library.

h. Add your main TypeSpec file

Open ./lib/main.tsp and import your JS entrypoint. This ensures that when TypeSpec imports your library, the code to define the library is run. When we add decorators in later topics, this import will ensure those get exposed as well.

import "../dist/index.js";

Step 2: Adding TypeSpec types to your library

Open ./lib/main.tsp and add any types you want to be available when users import this library. It’s strongly recommended to put these types in a namespace that corresponds with the library name. For example, your ./lib/main.tsp file might look like:

import "../dist/index.js";

namespace MyLibrary;
model Person {
  name: string;
  age: uint8;
}

Controlling your library’s API surface

When authoring a library, not every type you define is meant for consumers to use directly. Some types are implementation details that support your library internally. You can use the internal modifier to prevent consumers from accessing these types.

Caution

The internal modifier is an experimental feature. See Access Modifiers for full details.

import "../dist/index.js";

namespace MyLibrary;

/** This model is part of the public API. */
model Person {
  name: string;
  age: uint8;
}

/** This model is an implementation detail and cannot be accessed by consumers. */
internal model PersonValidator {
  rules: string[];
}

Consumers who try to reference PersonValidator will receive a compiler error. This helps you evolve your library’s internals without worrying about breaking consumers who might have depended on them.

Step 3: Defining dependencies

When defining dependencies in a TypeSpec library, follow these rules:

• Use peerDependencies for all TypeSpec libraries (and the compiler) that you use in your own library or emitter.
• Use devDependencies for other TypeSpec libraries that are only used in tests.
• Use dependencies or devDependencies for any other packages, depending on whether they’re used in library code or in test/dev scripts.

TypeSpec libraries are defined using peerDependencies to avoid having multiple versions of the compiler or library running at the same time.

Example

{
  "dependencies": {
    "yaml": "~2.3.1", // This is a regular package this library/emitter will use
  },
  "peerDependencies": {
    // These are all TypeSpec libraries this library/emitter depends on
    "@typespec/compiler": "~0.43.0",
    "@typespec/http": "~0.43.1",
    "@typespec/openapi": "~0.43.0",
  },
  "devDependencies": {
    // This TypeSpec library is only used in the tests but is not required to use this library.
    "@typespec/versioning": "~0.43.0",
    // TypeScript is only used during development
    "typescript": "~5.0.2",
  },
}

Step 4: Testing your TypeSpec library

Testing see documentation for adding tests to your library.

Step 5: Publishing your TypeSpec library

To publish your library to the public npm registry, follow the instructions in the npm documentation.

Step 6: Importing your TypeSpec library

Once your TypeSpec library is published, users can install and use it just like any of the standard TypeSpec libraries. First, they need to install it:

npm install $packageName

Next, they can import it into their TypeSpec program and use the namespace (if desired):

import "MyLibrary";
using MyLibrary;

model Employee extends Person {
  job: string;
}

Step 7: Next steps

TypeSpec libraries can contain more than just types. For more details on how to write decorators, emitters and linters, refer to the subsequent topics.