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.

• ID: async-promises

• Severity: medium

• What to look for: List all async operations in the public API. For each, examine exported functions and methods that perform asynchronous operations (network requests, file I/O, database queries). Check whether they:

  1. Return Promise<T> or use async/await
  2. Use callbacks instead of Promises
  3. Return void but perform async work (fire-and-forget)
  4. Have consistent async patterns (all async functions return Promises, not a mix of callbacks and Promises)

• Pass criteria: All asynchronous operations in the public API return Promises (either explicitly or via async functions). The package does not require consumers to pass callback functions for async results — 100% of async operations must return Promises (no callback-only APIs). Report: "X async operations found, all Y return Promises."

• Fail criteria: The public API uses callbacks for asynchronous operations (e.g., client.fetch(url, callback)), or async methods return void and report results through event emitters or side channels without also providing a Promise-based API.

• Skip (N/A) when: The package is entirely synchronous — no async operations, no I/O, no network calls. Pure computation libraries, parsers, and formatters typically fall into this category. Also skip for Go (uses channels/goroutines) and Rust (uses different async patterns).

• Detail on fail: "The createClient().fetch() method takes a callback parameter instead of returning a Promise. Consumers cannot use async/await: client.fetch(url, (err, data) => { ... }). This is a Node.js callback pattern from before Promises were standard."

• Remediation: Promises are the standard async pattern in modern JavaScript. They compose with async/await, error handling is consistent, and they work with every modern framework.

  // Before — callback pattern:
  export function fetchData(
    url: string,
    callback: (err: Error | null, data?: any) => void
  ) {
    // ...
  }
  
  // After — Promise pattern:
  export async function fetchData(url: string): Promise<ResponseData> {
    const response = await fetch(url)
    if (!response.ok) throw new ApiError(response.status)
    return response.json()
  }