async-await-patterns

name	async-await-patterns
description	Use when writing JavaScript or TypeScript code with asynchronous operations, fixing promise-related bugs, or converting callback/promise patterns to async/await. Triggers: 'promise chain', 'unhandled rejection', 'race condition in JS', 'callback hell', 'Promise.all', 'sequential vs parallel async', 'missing await'. Enforces async/await discipline over raw promises.

Senior JavaScript/TypeScript Engineer. Reputation depends on production-grade asynchronous code. Prevents race conditions, memory leaks, and unhandled promise rejections through disciplined async patterns.

<CRITICAL_INSTRUCTION> Use async/await for ALL asynchronous operations instead of raw promises, callbacks, or blocking patterns. This is critical to application stability. NOT optional. NOT negotiable. </CRITICAL_INSTRUCTION>

Invariant Principles

1. Explicit async boundary: Function containing await MUST be marked async. Compiler enforces; no exceptions.
2. Await ALL promises: Every promise-returning call requires await. Missing await = bug.
3. Structured error handling: try-catch wraps async operations. Unhandled rejections crash applications.
4. Pattern consistency: async/await XOR promise chains. Never mix in same function.
5. Parallelism via combinators: Independent operations use Promise.all/allSettled. Sequential only when dependencies exist.

Required Reasoning

Before writing ANY async code, verify:

1. Is this operation asynchronous? (API calls, file I/O, timers, database queries)
2. Did I mark the containing function as async?
3. Did I use await for every promise-returning operation?
4. Did I add proper try-catch error handling?
5. Did I avoid mixing async/await with .then()/.catch()?
6. Can independent operations run in parallel with Promise.all?

Now write asynchronous code following this checklist.

Core Pattern

async function operationName(): Promise<ReturnType> {
  try {
    const result = await asyncOperation();
    return result;
  } catch (error) {
    throw error; // Handle or rethrow with context
  }
}

Forbidden Patterns: Quick Reference

Anti-pattern	Fix
.then()/.catch() chains	async/await with try-catch
const x = asyncFn() (missing await)	const x = await asyncFn()
function with await inside	async function
Await without try-catch	Wrap in try-catch
Mix async/await + .then()	Pure async/await
Callbacks when promises available	async/await
Sequential awaits for independent ops	Promise.all

Forbidden Patterns: Detailed Examples

### Raw Promise Chains Instead of Async/Await

// BAD
function fetchData() {
  return fetch('/api/data')
    .then(response => response.json())
    .then(data => processData(data))
    .catch(error => handleError(error));
}

// CORRECT
async function fetchData() {
  try {
    const response = await fetch('/api/data');
    const data = await response.json();
    return processData(data);
  } catch (error) {
    handleError(error);
    throw error;
  }
}

### Forgetting await Keyword

// BAD - returns Promise instead of value
async function getData() {
  const data = fetchFromDatabase(); // Forgot await!
  return data.id; // Error: data is a Promise
}

// CORRECT
async function getData() {
  const data = await fetchFromDatabase();
  return data.id;
}

### Missing async Keyword on Function

// BAD - SyntaxError
function loadUser() {
  const user = await database.getUser();
  return user;
}

// CORRECT
async function loadUser() {
  const user = await database.getUser();
  return user;
}

### Missing Error Handling

// BAD - unhandled promise rejection if save fails
async function saveData(data) {
  const result = await database.save(data);
  return result;
}

// CORRECT
async function saveData(data) {
  try {
    const result = await database.save(data);
    return result;
  } catch (error) {
    console.error('Save failed:', error);
    throw new Error('Failed to save data');
  }
}

### Mixing Async/Await with Promise Chains

// BAD
async function processUser() {
  const user = await getUser();
  return updateUser(user)
    .then(result => result.data)
    .catch(error => console.error(error));
}

// CORRECT
async function processUser() {
  try {
    const user = await getUser();
    const result = await updateUser(user);
    return result.data;
  } catch (error) {
    console.error(error);
    throw error;
  }
}

Parallel vs Sequential

// PARALLEL: independent operations
const [a, b, c] = await Promise.all([fetchA(), fetchB(), fetchC()]);

// SEQUENTIAL: each depends on previous
const inventory = await checkInventory();
const payment = await processPayment(inventory);
const order = await createOrder(payment);

// FAULT-TOLERANT: continue despite failures
const results = await Promise.allSettled([op1(), op2(), op3()]);
// Each result: { status: 'fulfilled', value } or { status: 'rejected', reason }

Complete Real-World Example

async function updateUserProfile(userId: string, updates: ProfileUpdates): Promise<User> {
  try {
    const user = await database.users.findById(userId);

    if (!user) {
      throw new Error(`User ${userId} not found`);
    }

    const validatedUpdates = await validateProfileData(updates);
    const updatedUser = await database.users.update(userId, validatedUpdates);

    await Promise.all([
      notificationService.send(userId, 'Profile updated'),
      auditLog.record('profile_update', { userId, updates: validatedUpdates })
    ]);

    return updatedUser;

  } catch (error) {
    if (error instanceof ValidationError) {
      throw new BadRequestError('Invalid profile data', error);
    }
    if (error instanceof DatabaseError) {
      throw new ServiceError('Database operation failed', error);
    }
    throw new Error(`Failed to update profile: ${error.message}`);
  }
}

Recommended: Python Asyncio Pipeline Architecture

For Python scripts with multiple network requests or I/O operations, consider using asyncio with pipeline queues. This is not mandatory, but works well when a script naturally decomposes into producer/consumer stages:

• Network I/O: 3-5 parallel workers
• File I/O: 2-3 parallel workers
• Rate-limited APIs: Single worker with delays
• Shared state: Single worker or locks

This pattern shines when you have distinct stages (fetch, transform, write) that can overlap. For simple scripts with a handful of sequential requests, asyncio.gather or Promise.all is sufficient.

Self-Check

Before submitting ANY asynchronous code, verify:

• Did I mark the function as async?
• Did I use await for EVERY promise-returning operation?
• Did I wrap await operations in try-catch blocks?
• Did I avoid using .then()/.catch() chains?
• Did I avoid mixing async/await with promise chains?
• Did I avoid using callbacks when async/await is available?
• Did I consider whether operations can run in parallel with Promise.all()?
• Did I provide meaningful error messages in catch blocks?
• Does error handling preserve error context?

If NO to ANY item above: STOP. Rewrite using proper async/await before proceeding.

<FINAL_EMPHASIS> Use async/await for ALL asynchronous operations. NEVER use raw promise chains when async/await is clearer. NEVER forget the await keyword. NEVER omit error handling. This is critical to code quality and application stability. Non-negotiable. </FINAL_EMPHASIS>