What are Declaration Files (.d.ts)? | TypeScript Interview Questions

What You'll Learn

What .d.ts files are
When you need them
How to create them

What are Declaration Files?

Declaration files (.d.ts) contain type information without implementation. They describe the shape of JavaScript code.

code.tsTypeScript

// math.d.ts
declare function add(a: number, b: number): number;
declare function subtract(a: number, b: number): number;

declare const PI: number;

Why Use Them?

Add types to JavaScript libraries
Share types without implementation
@types packages use them

Finding Type Definitions

$ terminalBash

# Install types for lodash
npm install --save-dev @types/lodash

# Many packages include their own types
npm install axios  # types included

Creating Declaration Files

code.tsTypeScript

// For a module
declare module "my-library" {
  export function doSomething(value: string): number;
  export interface Config {
    debug: boolean;
  }
}

// For global variables
declare global {
  interface Window {
    myGlobal: string;
  }
}

Generate from TypeScript

data.jsonJSON

// tsconfig.json
{
  "compilerOptions": {
    "declaration": true,
    "declarationDir": "./types"
  }
}