namespace and module are both thought of as outdated keywords to organize the code. Instead, it is generally preferable to use ES2015 module syntax (e.g. import/export).

However, this rule still allows the use of these keywords in the following two cases:

• they are used for defining "ambient" namespaces along with declare keywords
• they are written in TypeScript's type definition files: .d.ts

Invalid:

// foo.ts
module mod {}
namespace ns {}

// bar.d.ts
// all usage of `module` and `namespace` keywords are allowed in `.d.ts`

Valid:

// foo.ts
declare global {}
declare module mod1 {}
declare module "mod2" {}
declare namespace ns {}

// bar.d.ts
module mod1 {}
namespace ns1 {}
declare global {}
declare module mod2 {}
declare module "mod3" {}
declare namespace ns2 {}