Async operations return Promises

Callback-based async APIs are incompatible with async/await, require consumers to wrap calls in new Promise() to compose them, and produce inverted error-handling patterns compared to modern JavaScript. ISO 25010 interoperability degrades when your API cannot be composed with Promise.all, Promise.race, or async middleware chains. Callback-only APIs also make it harder to surface errors correctly — thrown errors inside callbacks are swallowed by the event loop if the callback doesn't explicitly handle them.

Convert callback-based methods to async functions returning typed Promises.

// Before — callback pattern:
export function fetchData(
  url: string,
  callback: (err: Error | null, data?: ResponseData) => void
) { /* ... */ }

// After — Promise pattern:
export async function fetchData(url: string): Promise<ResponseData> {
  const response = await fetch(url)
  if (!response.ok) throw new ApiError('Request failed', response.status)
  return response.json() as Promise<ResponseData>
}

If you need to maintain backward compatibility during a migration, add the Promise API alongside the callback API and deprecate the callback variant with a JSDoc @deprecated tag.