Type Declaration Files

Declaration files in TypeScript play a crucial role in providing type information for external JavaScript libraries that lack native TypeScript support. These files, typically with a .d.ts extension, enable TypeScript to understand the structure, types, and interfaces of the associated JavaScript code.

Creating a Basic Declaration File

Suppose you have a JavaScript library, myLibrary.js, with the following code:

// myLibrary.js function greet(name) { console.log(`Hello, ${name}!`); }

To create a declaration file for this library, you can make a myLibrary.d.ts file:

// myLibrary.d.ts declare function greet(name: string): void;

Now, when you import myLibrary into a TypeScript file, TypeScript understands the types:

// app.ts import { greet } from './myLibrary'; greet('Alice'); // TypeScript recognizes the 'name' parameter as a string

Using DefinitelyTyped for Popular Libraries

For widely-used JavaScript libraries without official TypeScript support, the DefinitelyTyped repository provides a vast collection of community-contributed declaration files. Installing these declaration files with npm allows TypeScript to seamlessly integrate with popular libraries. For example, to use the jQuery library:

npm install --save-dev @types/jquery

Now, TypeScript recognizes jQuery methods and their types:

// app.ts import * as $ from 'jquery'; const element = $('#myElement'); element.hide(); // TypeScript recognizes the 'hide' method

Extending and Augmenting Declarations

Declaration files can be extended or augmented to provide additional type information or overwrite existing definitions. For instance, if you want to extend the declaration for the greet function in myLibrary.d.ts to accept an optional second parameter:

// myLibrary.d.ts declare function greet(name: string, greeting?: string): void;

Now, TypeScript understands both the original and extended versions of the greet function.

Global Declarations

For libraries that add global variables or functions, global declaration files can be used. Suppose you have a JavaScript file, globals.js, that declares a global variable:

// globals.js window.myGlobal = 'Hello, global!';

Create a global declaration file, globals.d.ts

// globals.d.ts declare var myGlobal: string;

Now, TypeScript recognizes myGlobal as a global variable in your code.

Using Third-Party Libraries

Obtain type definitions from DefinitelyTyped or create them manually.

// Using Lodash with type definitions: import * as _ from 'lodash'; const filteredArray = _.filter([1, 2, 3], (num) => num > 1);

Creating Ambient Modules

Represent modules not following standard module systems.

// Declaring an ambient module: declare module 'legacy-module' { export function doSomething(): void; }


Declaration files in TypeScript, denoted by the .d.ts extension, serve as crucial tools for providing type information to external JavaScript libraries lacking native TypeScript support. Whether creating custom declarations for specific code or utilizing community-contributed declarations through DefinitelyTyped, these files empower TypeScript to understand and enforce type safety, enabling seamless integration with a wide range of external libraries and enhancing the overall development experience.