Readonly members

• Modifiers: readonly
• Allowed: UPPER_CASE, camelCase, snake_case
• Leading underscore: allowed

Why This Rule

Readonly members are immutable properties that cannot be reassigned after initialization. They can represent constants, configuration values, or environment variables:

• UPPER_CASE: For environment variables and true constants (VITE_API_BASE_URL, MAX_RETRIES)
• camelCase: For instance-specific readonly values (apiKey, userId)
• snake_case: For mapping to external data structures (user_id, created_at)

When to use each:

interface ImportMetaEnv {
  readonly VITE_API_BASE_URL: string; // WHY: UPPER_CASE for environment variables
  readonly VITE_DEBUG_MODE: boolean; // WHY: UPPER_CASE for env vars
}

class ApiClient {
  public readonly apiKey: string; // WHY: camelCase for instance-specific readonly
  public readonly user_id: string; // WHY: snake_case to mirror external data
}

Leading underscore: Allowed for flexibility, though not commonly used for readonly members.

Why allow multiple formats:

Readonly members serve different purposes. Environment variables are conventionally UPPER_CASE, while other readonly properties may follow different conventions based on their source or usage.

References:

• TypeScript Handbook - Readonly Properties

✅ Good

interface ImportMetaEnv {
  readonly VITE_API_BASE_URL: string; // WHY: UPPER_CASE for env vars
  readonly VITE_DEBUG: boolean; // WHY: UPPER_CASE
}

class Example {
  public readonly apiKey = 'key'; // WHY: camelCase for instance value
  public readonly user_id = '123'; // WHY: snake_case for external mapping
}

❌ Bad

interface ImportMetaEnv {
  readonly ViteApiBaseUrl: string; // WHY: PascalCase not allowed
}

class Example {
  public readonly ApiKey = 'key'; // WHY: PascalCase not allowed
}