Installing types from DefinitelyTyped

Types from any library would be a dev dependency and all you have to do is install it from npm. The following command installs express types:

npm install --save-dev @types/express

This command downloads the express type to the @types folder, and TypeScript looks in the @types folder to resolve mappings for that type. As we only need it during development time, we have added the --save-dev option.