Declaration Files

.d.ts Files

Declaration files (.d.ts) describe the types of existing JavaScript code. They let TypeScript understand libraries that weren't written in TypeScript.

DefinitelyTyped

The @types npm packages provide type declarations for thousands of popular JavaScript libraries. Installing @types/lodash gives TypeScript full knowledge of Lodash's API.

Writing Your Own

When a library has no @types package, you may need to write your own declaration file. Use declare module to tell TypeScript about the module.