How to fix Ambient declaration 'X' cannot be declared in a non-ambient context in TypeScript

// ❌ WRONG - in a regular .ts file
declare namespace MyNamespace {
  interface MyInterface {
    name: string;
  }
}

export function doSomething() {
  // implementation
}

// ✅ CORRECT - in a .d.ts file
declare namespace MyNamespace {
  interface MyInterface {
    name: string;
  }

  function doSomething(): void;
}

src/
├── types/
│   └── my-namespace.d.ts      # Type definitions only
├── lib/
│   └── my-module.ts            # Implementation code
└── index.ts

declare namespace MyNamespace {
  interface User {
    id: string;
    name: string;
  }

  function getUserById(id: string): User | null;
}

namespace MyNamespace {
  export interface User {
    id: string;
    name: string;
  }

  export function getUserById(id: string): User | null {
    // implementation
    return null;
  }
}

export = MyNamespace;

// ✅ This is allowed - declare is inside a namespace block
declare namespace GlobalTypes {
  interface Window {
    customProperty: string;
  }
}

// ✅ This is allowed - augmenting module types
declare module 'express' {
  interface Request {
    userId?: string;
  }
}

// Implementation code can follow
export function setupMiddleware() {
  // ...
}

// ❌ WRONG
declare interface User {
  name: string;
}

// ✅ CORRECT - no declare keyword
interface User {
  name: string;
}

class UserClass implements User {
  name: string = '';
}

export { User, UserClass };

{
  "compilerOptions": {
    "declaration": true,
    "declarationDir": "./dist/types",
    "outDir": "./dist",
    "rootDir": "./src",
    "typeRoots": ["./node_modules/@types", "./src/types"],
    "skipLibCheck": true
  },
  "include": ["src/**/*"],
  "exclude": ["node_modules", "dist"]
}