The Problem: Browsers vs. Native Apps

• On the Web: Storing tokens in localStorage is risky because any XSS (Cross-Site Scripting) vulnerability can leak your user's session. HttpOnly Cookies are the gold standard here because JavaScript can't touch them.

• In Mobile: React Native doesn't handle cookies as gracefully as browsers do. Instead, mobile OSs provide "Secure Enclaves" (like Keychain or Keystore) which are incredibly secure places to store raw strings.

The Solution: The Dual-Response Pattern

Your NestJS backend should detect the client type and respond accordingly:

1. For Web: Set a Set-Cookie header with httpOnly: true. This keeps the token out of the browser's reach.

2. For Mobile: Return the accessToken in the JSON response body. The mobile app can then save this to React Native Keychain.

NestJS Implementation (Quick Look)

In your Auth Controller, use a custom header (like x-platform) to decide your strategy:

typescript

@Post('login')
async login(@Req() req: Request, @Res({ passthrough: true }) res: Response) {
  const { accessToken, user } = await this.authService.validateUser(req.body);
  const isMobile = req.headers['x-platform'] === 'mobile';

  if (isMobile) {
    // Return token directly for mobile storage
    return { accessToken, user };
  }

  // Secure cookie for web users
  res.cookie('access_token', accessToken, {
    httpOnly: true,
    secure: true, // Use false for local dev
    sameSite: 'lax',
  });
  return { user };
}

Use code with caution.

Why This Wins

• Max Security: Web users are protected from XSS via cookies; mobile users are protected via native device encryption.

• Developer Experience: Your mobile team doesn't have to fight cookie-persistence libraries, and your web team doesn't have to worry about manual header management.

• One Backend: Your Guards can simply check both the cookies object and the Authorization header to validate the user, keeping your logic DRY.