frontend-async-best-practices

---
name: frontend-async-best-practices
description: Async/await and Promise optimization guidelines. Use when writing, reviewing, or refactoring asynchronous code to eliminate waterfalls and maximize parallelism. Triggers on tasks involving data fetching, loaders, actions, or Promise handling.
license: MIT
---

# Async Best Practices

Performance optimization patterns for asynchronous JavaScript code. Contains 5 rules focused on eliminating request waterfalls and maximizing parallelism.

**Impact: CRITICAL** - Waterfalls are the #1 performance killer. Each sequential await adds full network latency.

## When to Apply

Reference these guidelines when:

- Writing Remix loaders or actions
- Implementing data fetching logic
- Working with multiple async operations
- Reviewing code for waterfall patterns
- Optimizing response times

## Rules Summary

### parallel (CRITICAL) — @rules/parallel.md

Use `Promise.all()` for independent operations.

```typescript
// Bad: 3 sequential round trips
const user = await fetchUser();
const posts = await fetchPosts();
const comments = await fetchComments();

// Good: 1 parallel round trip
const [user, posts, comments] = await Promise.all([
  fetchUser(),
  fetchPosts(),
  fetchComments(),
]);
```

### defer-await (HIGH) — @rules/defer-await.md

Move await into branches where actually used.

```typescript
// Bad: always waits even when skipping
async function handle(skip: boolean) {
  let data = await fetchData();
  if (skip) return { skipped: true };
  return process(data);
}

// Good: only waits when needed
async function handle(skip: boolean) {
  if (skip) return { skipped: true };
  let data = await fetchData();
  return process(data);
}
```

### dependencies (CRITICAL) — @rules/dependencies.md

Chain dependent operations, parallelize independent ones.

```typescript
// Bad: profile waits for config unnecessarily
const [user, config] = await Promise.all([fetchUser(), fetchConfig()]);
const profile = await fetchProfile(user.id);

// Good: profile starts as soon as user resolves
const userPromise = fetchUser();
const profilePromise = userPromise.then((user) => fetchProfile(user.id));

const [user, config, profile] = await Promise.all([
  userPromise,
  fetchConfig(),
  profilePromise,
]);
```

### api-routes (CRITICAL) — @rules/api-routes.md

Start promises early, await late in loaders.

```typescript
// Bad: sequential execution
export async function loader() {
  let session = await auth();
  let config = await fetchConfig();
  return { session, config };
}

// Good: parallel execution
export async function loader() {
  let sessionPromise = auth();
  let configPromise = fetchConfig();
  const [session, config] = await Promise.all([sessionPromise, configPromise]);
  return { session, config };
}
```

### suspense-boundaries (HIGH) — @rules/suspense-boundaries.md

Use Suspense to show UI immediately while data loads.

```tsx
// Bad: entire page blocked by data
async function Page() {
  let data = await fetchData();
  return (
    <Layout>
      <Content data={data} />
    </Layout>
  );
}

// Good: layout shows immediately, content streams in
function Page() {
  return (
    <Layout>
      <Suspense fallback={<Skeleton />}>
        <Content />
      </Suspense>
    </Layout>
  );
}
```