Type Declaration Files in TypeScript: A Complete Guide

Updated: January 31, 2024 By: Guest Contributor Post a comment


TypeScript enhances JavaScript with types, elevating our ol’ twain of thought into the clarity of strong typing. The keystone in this construct is declaring types, possibly through files succinctly ending with .d.ts. These files are essential for the TypeScript compiler, serving as a bridge between your strongly-typed syntax and the wild west of JavaScript’s dynamic types.

The Basics of .d.ts files

Before we set sail into the mighty river of TypeScript, it’s pivotal to understand the role of type declaration files. Like a river’s map, a .d.ts file outlines the structure of objects, variables, and functions, making sure the code flows without unexpected rapids.

// Example of a simple declaration file

declare module 'example' {
  export function sayHello(name: string): string;

Expanding Your Horizons with Advanced Declarations

As our coding vessel ventures forth, deeper into the great unknown, one must learn to declare namespaces, modules, and even global variables or functions.

// Advanced module and namespace declaration

declare namespace MyLibrary {
  function doThings(): void;

declare module 'MyLib' {
  export = MyLibrary;

Generics and Declaration Merging

The true finesse of TypeScript comes into play when we harness the power of generics and declaration merging. It’s akin to navigating through the cut-throughs of the Mississippi; giving your code the ability to deal with various type ‘currents’.

// Declaration merging example

declare namespace Animals {
  interface Bear { honey: boolean; }

declare namespace Animals {
  interface Bear { fish: boolean; }
  // Combined into one Bear interface

Delving into Module Augmentation

On our coding journey, there comes a time when we must enhance an existing library without altering its original course. This is module augmentation – adding custom type flair to established modules.

// Module augmentation example

import 'moment';

// Augment the 'moment' module with a new function

declare module 'moment' {
  export function twainQuote(): string;

The Artful Craft of Authoring Your Own .d.ts Files

Now, let’s turn to writing our own declaration files – crafting the rules that our TypeScript world abides by. Pay attention, as rushing this delicate art will only lead to chaos on the fluid banks of your application.

// Example of custom declaration file

declare module 'myCustomModule' {
  export function myFunction(a: number, b: number): number;

Defining External JavaScript Libraries

As the inherently curious typical developer, we may need to work together with external libraries that were developed in the era before TypeScript’s structured approach. Despair not! Reach for declaration files that type these external artifacts, maintaining their venerable aura whilst conferring them the gift of types.

// Declaring external JavaScript library types

declare function oldLibraryFunction(argument: number): string;

Tooling and Best Practices

Ever the crafty ones, we TypeScripters rely on tools of fine make to manage our declaration files. The trusty tsc can generate these for us, whispers of @types packages add the wisdom of crowdsourced types, and a linter ensures our declaration syntax is sharper than a pinewood canoe.

// Tooling example: Linting your declaration file

tslint --project my-project/tsconfig.json my-declarations.d.ts


In this grand voyage, we’ve skirmished with the basics of TypeScript’s type declaration files and sailed through the stormy seas of advanced concepts. Authoring and understanding .d.ts files is much like narrating great stories; it starts with embracing the rules, follows with deliberate practice, and ends in the mastery that allows you to guide others through the treacherous but exhilarating twists and turns of TypeScript.