Declaration Merging & Ambient Declarations

Declaration Merging

When you declare the same identifier multiple times, TypeScript merges the definitions.

interface Window { foo: string; }
interface Window { bar: number; }
// Window now has both foo and bar

Merging Functions

function combine(a: string, b: string): string;
function combine(a: number, b: number): number;
function combine(a: any, b: any) {
  return a + b;
}

Ambient Declarations (declare)

Declare the shape of a value provided by external script or library.

// globals.d.ts
declare const $: (selector: string) => any;

Now you can use $ without TypeScript complaining.

Module Augmentation

// myLib.d.ts (external)
declare module "my-lib" { export function foo(): void; }
// augmentation
import "my-lib";
declare module "my-lib" { export function bar(): number; }