env_types.ts

// There are two ways you can do this, the first being using a globals.d.ts file.
// Here, you would specify types like this..

// .d.ts
namespace NodeJS {
  interface ProcessEnv {
    SUPER_SECRET_TOKEN: string;
  }
}

// You may have to specify this in the tsconfig.json file.
// tsconfig.json
{
  "typeRoots": ["./path/to/global.d.ts", "./node_modules/@types"],
}

// Option two, in a .ts file.
// .ts
declare global {
  namespace NodeJS {
    export interface ProcessEnv {
      SUPER_SECRET_TOKEN: string;
    }
  }
}

// https://dev.to/isthatcentered/typing-process-env-and-dealing-with-nodeenv-3ilm
// https://dev.to/asjadanis/parsing-env-with-typescript-3jjm