Parallel Execution

Introduction

By default, await pauses execution, making operations sequential. To run independent operations in parallel, use Promise.all().

Sequential vs Parallel

// ❌ Sequential (slow) — each waits for the previous
async function slow() {
  const users = await fetchUsers();
  const posts = await fetchPosts();     // waits for users
  const comments = await fetchComments(); // waits for posts
}

// ✅ Parallel (fast)
async function fast() {
  const [users, posts, comments] = await Promise.all([
    fetchUsers(),
    fetchPosts(),
    fetchComments()
  ]);
}

Visual Explanation

sequenceDiagram
    participant App as Code
    participant API as API Server

    Note over App,API: Sequential (3 seconds total)
    App->>API: fetchUsers() — 1s
    App->>API: fetchPosts() — 1s after users
    App->>API: fetchComments() — 1s after posts

    Note over App,API: Parallel (1 second total)
    App->>API: fetchUsers(), fetchPosts(), fetchComments() — all at once
    API-->>App: All return within ~1s

When to Use Parallel

async function loadDashboard() {
  const start = performance.now();

  // Independent data — parallel
  const [user, notifications, preferences] = await Promise.all([
    fetchUser(),
    fetchNotifications(),
    fetchPreferences()
  ]);

  // Dependent data — sequential
  const posts = await fetchPosts(user.id);
  const comments = await Promise.all(
    posts.map(post => fetchComments(post.id)) // Parallel within sequential
  );

  console.log(`Loaded in ${performance.now() - start}ms`);
  return { user, notifications, preferences, posts, comments };
}

Summary

Sequential await = slow for independent operations
Use Promise.all() with await for parallel execution
Only parallelize truly independent operations
Combine sequential and parallel for complex data flows

Parallel Execution

Parallel Execution

Introduction

Sequential vs Parallel

Visual Explanation

When to Use Parallel

Summary

Further Reading