Retry with Backoff

Implement a retry function that retries a failing async function up to n times with exponential backoff.

Requirements

• Call the function. If it succeeds, return the result.
• If it fails and retries remain, wait delay * 2^attempt ms, then try again.
• If all retries are exhausted, throw the last error.

Example

let attempt = 0;
const flaky = () => ++attempt < 3 ? Promise.reject('fail') : Promise.resolve('ok');
await retry(flaky, 3, 10); // 'ok' (succeeds on 3rd try)