Promise.allSettled, Promise.any, and Promise.race Explained

// Rejects immediately if ANY promise rejects
const results = await Promise.all([
  fetch("/api/users"),
  fetch("/api/posts"),
  fetch("/api/comments")
]);
// If /api/posts fails, you get NOTHING

const results = await Promise.allSettled([
  fetch("/api/users"),
  fetch("/api/posts"),    // Even if this fails...
  fetch("/api/comments")
]);
// results = [
//   { status: "fulfilled", value: Response },
//   { status: "rejected", reason: Error },
//   { status: "fulfilled", value: Response }
// ]

// Filter successes
const successful = results
  .filter(r => r.status === "fulfilled")
  .map(r => r.value);

// Returns the first promise to settle (fulfill OR reject)
const result = await Promise.race([
  fetch("/api/data"),
  new Promise((_, reject) =>
    setTimeout(() => reject(new Error("Timeout")), 5000)
  )
]);
// Timeout pattern: either data arrives or 5s timeout

// Returns the first promise to FULFILL (ignores rejections)
const fastest = await Promise.any([
  fetch("https://cdn1.example.com/data"),
  fetch("https://cdn2.example.com/data"),
  fetch("https://cdn3.example.com/data")
]);
// Gets data from whichever CDN responds first
// Only rejects if ALL promises reject (AggregateError)

async function resilientFetch(url, { timeout = 5000, retries = 3 } = {}) {
  for (let i = 0; i < retries; i++) {
    try {
      return await Promise.race([
        fetch(url),
        new Promise((_, reject) =>
          setTimeout(() => reject(new Error("Timeout")), timeout)
        )
      ]);
    } catch (err) {
      if (i === retries - 1) throw err;
      await new Promise(r => setTimeout(r, 1000 * Math.pow(2, i)));
    }
  }
}