TS7010

'sum', which lacks return-type annotation, implicitly has an 'any' return type.

Broken Code ❌

namespace MyModule {
  export function sum(a: number, b: number);
}

Fixed Code ✔️

You have to add a return-type annotation and preferably a function implementation:

namespace MyModule {
  export function sum(a: number, b: number): number {
    return a + b;
  }
}