Ambient module

Learn to declare ambient modules.

What is an ambient module?

An ambient module tells TypeScript about a JS module that exists but has no type information. It is declared with declare module and the module name is quoted. Functions and variables exported inside gain types.

declare module "old-module" {
  export function greet(name: string): string;
}

Wildcard module

With the wildcard declare module "*" you can describe all modules matching a pattern. For example, it is used to give type information when importing CSS or image files. The default export is specified with export default.

declare module "*.css" {
  const content: string;
  export default content;
}

Exercise Declare the "selam" ambient module

Declare an ambient module "selam". Export a function merhaba(name: string): string inside it. Then print "modül hazır" with console.log.

Show example solution

declare module "selam" {
  export function merhaba(name: string): string;
}
console.log("modül hazır");

Test yourself

What is the purpose of declare module "x" { ... }?
- Creates a new JavaScript module
- Loads the module at runtime
- Compiles the module's JavaScript code
- Tells TypeScript about an existing JS module
Show answer

Tells TypeScript about an existing JS module — declare module provides type information for an existing JS module; it does not create a new module or generate JavaScript code.

Ambient module keyword

declare ____ "eski" { export function x(): void; }

Show answer

declare module "eski" { export function x(): void; }

Ambient module parts

declare module

Ambient module start
export function

Function inside module
"*"

Wildcard
export default

Default export

Next lesson: Triple-slash directive

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