Type declarations with extension .d.ts in Typescript

A type declaration is a file with the extension .d.ts. It’s a way to attach TypeScript types to JavaScript code that would otherwise be untyped.

• Type declarations can only define types, no values.
• Type declarations can only declare that there exists a value defined somewhere in JS file using declare keyword

A type declaration has to live in a script-mode .ts or .d.ts file.

See [[DefinitelyTyped maintains module type declaration for open source projects]].