Promise Combinators: Real-World Examples in JavaScript

Promise Combinators: Real-World Examples in JavaScript

Promise combinators are used in real applications. Here are practical examples.

1. Loading Multiple Resources (Promise.all)

async function loadPage(userId) {
  const [user, posts, friends] = await Promise.all([
    fetch(`/api/users/${userId}`).then((r) => r.json()),
    fetch(`/api/users/${userId}/posts`).then((r) => r.json()),
    fetch(`/api/users/${userId}/friends`).then((r) => r.json()),
  ]);
  return { user, posts, friends };
}
``
### 2. Best-Effort Data Fetching (Promise.allSettled)

```js
async function loadDashboard(userId) {
  const results = await Promise.allSettled([
    fetchNotifications(userId),
    fetchRecommendations(userId),
    fetchTrending(),
  ]);
  return {
    notifications: results[0].status === "fulfilled" ? results[0].value : [],
    recommendations: results[1].status === "fulfilled" ? results[1].value : [],
    trending: results[2].status === "fulfilled" ? results[2].value : [],
  };
}
``
Some widgets may fail, but the dashboard still loads with available data.

### 3. Timeout for API Calls (Promise.race)

```js
async function fetchWithTimeout(url, ms) {
  return Promise.race([
    fetch(url),
    new Promise((_, reject) => setTimeout(() => reject(new Error("timeout")), ms)),
  ]);
}
``
### 4. First Successful Mirror (Promise.any)

```js
async function fetchFromMirrors(path) {
  return Promise.any([
    fetch(`https://mirror1.example.com${path}`),
    fetch(`https://mirror2.example.com${path}`),
    fetch(`https://mirror3.example.com${path}`),
  ]);
}
``
Try multiple mirrors; use whichever responds first.

### 5. Batch Processing with Limit

```js
async function processBatch(items, batchSize, processFn) {
  for (let i = 0; i < items.length; i += batchSize) {
    await Promise.all(
      items.slice(i, i + batchSize).map(processFn)
    );
  }
}
``
Process `batchSize` items at a time to avoid overwhelming the server.

### 6. Polling Until Success (Promise.any pattern)

```js
async function pollUntilReady(url, maxAttempts, delay) {
  for (let i = 0; i < maxAttempts; i++) {
    try {
      const res = await fetch(url);
      if (res.ok) return await res.json();
    } catch {}
    await new Promise((resolve) => setTimeout(resolve, delay));
  }
  throw new Error("Polling failed");
}
``
### The Takeaway

Real-world examples: `Promise.all` for loading multiple resources, `allSettled` for best-effort dashboards, `race` for timeouts, `any` for first successful mirror, batching with `Promise.all` and limits, and polling. Each combinator has a specific real-world use case.