How to fix Global augmentation is not allowed outside of external module in TypeScript

// ❌ Wrong - no exports, treated as global script
declare global {
  interface Window {
    customProperty: string;
  }
}

// ✅ Correct - export makes it a module
declare global {
  interface Window {
    customProperty: string;
  }
}

export {}; // Makes this file a module

// ✅ Correct - import makes it a module
import 'some-module'; // Import anything, even if unused

declare global {
  interface Document {
    customMethod(): void;
  }
}

// ✅ Good structure for types/global-augmentation.d.ts
declare global {
  interface Window {
    myCustomAPI: {
      doSomething(): void;
    };
  }

  interface Navigator {
    userAgentData?: {
      brands: Array<{ brand: string; version: string }>;
      mobile: boolean;
      platform: string;
    };
  }
}

export {}; // Required for module context

{
  "include": [
    "src/**/*",
    "types/**/*.d.ts"
  ]
}

// ❌ Script context (no imports/exports) - declare global won't work
// types/custom.d.ts
declare global {
  interface Process {
    customEnv: Record<string, string>;
  }
}

// ✅ Module context (has export) - declare global works
// types/custom.d.ts
declare global {
  interface Process {
    customEnv: Record<string, string>;
  }
}

export {}; // This makes it a module!

// ✅ Creating new global types in script context
// types/globals.d.ts (no imports/exports = script file)
interface MyGlobalType {
  id: string;
  name: string;
}

// This works because file is a script, declarations are global
// No declare global needed

{
  "compilerOptions": {
    "declaration": true,
    "declarationDir": "./types"
  },
  "include": [
    "src/**/*",
    "types/**/*.d.ts",  // Include your .d.ts files
    "global.d.ts"       // Include root-level .d.ts files
  ]
}

# Good structure
types/
├── global-augmentations.d.ts  # Global type extensions
├── module-augmentations.d.ts  # Module-specific extensions
└── custom-types.d.ts          # Custom type definitions

# In tsconfig.json
{
  "include": ["src/**/*", "types/**/*.d.ts"]
}