Event Loop & Asynchronous Programming Interview Questions

The Event Loop is the mechanism that allows Node.js to perform non-blocking I/O operations despite JavaScript being single-threaded. It works in phases: 1) Timers (setTimeout, setInterval), 2) Pending callbacks (I/O callbacks), 3) Idle, prepare (internal use), 4) Poll (new I/O events), 5) Check (setImmediate), 6) Close callbacks. Each phase has a FIFO queue of callbacks to execute. Example: console.log('1'); setTimeout(() => console.log('2'), 0); Promise.resolve().then(() => console.log('3')); console.log('4'); // Output: 1, 4, 3, 2

Different asynchronous patterns in Node.js serve different purposes: 1) Callbacks: Traditional pattern with error-first callback style. Example: fs.readFile('file.txt', (err, data) => {}). 2) Promises: Chain-able operations with .then() and .catch(). Example: fetch('url').then(res => res.json()). 3) Async/Await: Syntactic sugar over promises for cleaner code. Example: async function getData() { const response = await fetch('url'); return response.json(); }. Async/await provides better error handling and code readability compared to callbacks and raw promises.

The microtask queue handles high-priority tasks like Promise callbacks and process.nextTick. Characteristics: 1) Processes before the next event loop phase, 2) Higher priority than macrotasks (setTimeout, setInterval), 3) Includes Promise callbacks and process.nextTick callbacks. Example: process.nextTick(() => console.log('1')); Promise.resolve().then(() => console.log('2')); setTimeout(() => console.log('3'), 0); // Output: 1, 2, 3

Error handling in async operations can be done through multiple patterns: 1) Callbacks: Use error-first pattern. Example: fs.readFile('file.txt', (err, data) => { if (err) handle(err); }). 2) Promises: Use .catch() or try/catch with async/await. Example: async function readFile() { try { const data = await fs.promises.readFile('file.txt'); } catch (err) { handle(err); } }. 3) Event emitters: Use error event handlers. Example: stream.on('error', (err) => handle(err));

EventEmitter implements the Observer pattern for event-driven programming. Features: 1) Register event listeners, 2) Emit events, 3) Handle events asynchronously, 4) Multiple listeners per event. Example: const EventEmitter = require('events'); class MyEmitter extends EventEmitter {} const myEmitter = new MyEmitter(); myEmitter.on('event', (data) => console.log(data)); myEmitter.emit('event', 'Hello World');

Node.js provides several strategies for CPU-intensive tasks: 1) Worker Threads: Parallel execution of CPU tasks. Example: const { Worker } = require('worker_threads'); const worker = new Worker('./worker.js'); 2) Child Processes: Separate Node.js processes. Example: const { fork } = require('child_process'); const child = fork('cpu_task.js'); 3) Chunking: Breaking large tasks into smaller ones. Example: function processChunk(data, callback) { setImmediate(() => { /* process chunk */ callback(); }); }

Best practices for Promise chains include: 1) Always return values in .then(), 2) Use single catch at the end, 3) Avoid nesting promises, 4) Use Promise.all() for parallel operations, 5) Handle rejections properly. Example: fetchUser(id).then(user => { return fetchPosts(user.id); }).then(posts => { return processPosts(posts); }).catch(error => { handleError(error); });

Custom async iterators can be implemented using Symbol.asyncIterator: Example: class AsyncRange { constructor(start, end) { this.start = start; this.end = end; } async *[Symbol.asyncIterator]() { for(let i = this.start; i <= this.end; i++) { await new Promise(resolve => setTimeout(resolve, 1000)); yield i; } } } async function iterate() { for await (const num of new AsyncRange(1, 3)) { console.log(num); } }

Key differences: 1) Parallel: Multiple tasks execute simultaneously (Worker Threads, Child Processes), 2) Concurrent: Tasks progress simultaneously but execute on single thread (async/await, Promises). Example of parallel: const { Worker } = require('worker_threads'); const workers = [new Worker('./worker.js'), new Worker('./worker.js')]; Example of concurrent: async function concurrent() { const [result1, result2] = await Promise.all([task1(), task2()]); }

Async error handling patterns include: 1) Try-catch with async/await. Example: async function handle() { try { await riskyOperation(); } catch (err) { handleError(err); } } 2) Error boundaries: class ErrorBoundary extends EventEmitter { async execute(fn) { try { return await fn(); } catch (err) { this.emit('error', err); } } } 3) Error-first callbacks: function operation(callback) { doAsync((err, result) => callback(err, result)); }

Node.js timer functions work through: 1) Timer phase in event loop, 2) Min heap data structure for efficient timer management, 3) Internal scheduling using libuv. Example: const start = Date.now(); setTimeout(() => console.log(Date.now() - start), 1000); // Actual delay might be longer due to event loop phases and CPU load. setTimeout and setInterval are not exact timing mechanisms.

Async hooks track lifecycle of async operations: 1) init: Resource creation, 2) before: Callback execution start, 3) after: Callback completion, 4) destroy: Resource cleanup. Example: const async_hooks = require('async_hooks'); const hook = async_hooks.createHook({ init(asyncId, type) { console.log(`${type}: ${asyncId}`); }, destroy(asyncId) { console.log(`Destroyed: ${asyncId}`); } }).enable();

Custom promises can be implemented using the Promise constructor: Example: function customPromise(value) { return new Promise((resolve, reject) => { if (value) { setTimeout(() => resolve(value), 1000); } else { reject(new Error('Invalid value')); } }); } class AsyncOperation { static async execute() { return new Promise((resolve, reject) => { // Async operation logic }); } }

Promise execution phases include: 1) Pending: Initial state, 2) Fulfilled: Successful completion, 3) Rejected: Error state. Methods: .then() for fulfillment, .catch() for rejection, .finally() for cleanup. Example: new Promise((resolve, reject) => { doAsync() }).then(result => { // Fulfilled }).catch(error => { // Rejected }).finally(() => { // Cleanup });

Throttling and debouncing control function execution frequency: 1) Throttle: Limit execution rate. Example: function throttle(fn, delay) { let last = 0; return function(...args) { const now = Date.now(); if (now - last >= delay) { last = now; return fn.apply(this, args); } }; } 2) Debounce: Delay execution until pause. Example: function debounce(fn, delay) { let timeoutId; return function(...args) { clearTimeout(timeoutId); timeoutId = setTimeout(() => fn.apply(this, args), delay); }; }

Race conditions can be handled through: 1) Promise.race() for first completion, 2) Mutex implementation for synchronization, 3) Atomic operations, 4) Proper state management. Example: class Mutex { constructor() { this.locked = false; this.queue = []; } async acquire() { return new Promise(resolve => { if (!this.locked) { this.locked = true; resolve(); } else { this.queue.push(resolve); } }); } release() { if (this.queue.length > 0) { const next = this.queue.shift(); next(); } else { this.locked = false; } } }

Event loop starvation occurs when CPU-intensive tasks block the event loop from processing other events. Prevention methods include: 1) Breaking long tasks into smaller chunks using setImmediate, 2) Using Worker Threads for CPU-intensive work, 3) Implementing proper concurrency controls. Example: function processArray(arr) { let index = 0; function nextChunk() { if (index < arr.length) { let chunk = arr.slice(index, index + 1000); // Process chunk setImmediate(() => nextChunk()); // Schedule next chunk index += 1000; } } nextChunk(); }

Cancellable promises can be implemented using: 1) AbortController for fetch operations, 2) Custom cancel tokens, 3) Wrapper classes with cancel functionality. Example: class CancellablePromise { constructor(executor) { this.abortController = new AbortController(); this.promise = new Promise((resolve, reject) => { executor(resolve, reject, this.abortController.signal); }); } cancel() { this.abortController.abort(); } } const operation = new CancellablePromise((resolve, reject, signal) => { signal.addEventListener('abort', () => reject(new Error('Cancelled'))); });

Patterns for concurrent database operations include: 1) Connection pooling, 2) Transaction management, 3) Batch operations, 4) Rate limiting. Example: class DBOperations { constructor(pool) { this.pool = pool; } async batchInsert(records, batchSize = 1000) { for (let i = 0; i < records.length; i += batchSize) { const batch = records.slice(i, i + batchSize); await Promise.all(batch.map(record => this.insert(record))); } } async transaction(operations) { const client = await this.pool.connect(); try { await client.query('BEGIN'); const results = await Promise.all(operations.map(op => op(client))); await client.query('COMMIT'); return results; } catch (e) { await client.query('ROLLBACK'); throw e; } finally { client.release(); } } }

Retry mechanisms can be implemented using: 1) Exponential backoff, 2) Maximum retry limits, 3) Retry delay calculation, 4) Error type checking. Example: async function retry(operation, maxRetries = 3, delay = 1000) { for (let i = 0; i < maxRetries; i++) { try { return await operation(); } catch (error) { if (i === maxRetries - 1) throw error; const waitTime = delay * Math.pow(2, i); await new Promise(resolve => setTimeout(resolve, waitTime)); } } }

Memory leak prevention strategies include: 1) Proper event listener cleanup, 2) Closing resources (files, connections), 3) Monitoring reference counts, 4) Using WeakMap/WeakSet. Example: class ResourceManager { constructor() { this.resources = new WeakMap(); } async useResource(key, resource) { try { this.resources.set(key, resource); await resource.process(); } finally { resource.close(); // Cleanup this.resources.delete(key); } } }

Async iterators can be implemented using Symbol.asyncIterator: Example: class DataSource { constructor(data) { this.data = data; } async *[Symbol.asyncIterator]() { for (const item of this.data) { await new Promise(resolve => setTimeout(resolve, 100)); yield item; } } } async function processData() { const source = new DataSource([1, 2, 3]); for await (const item of source) { console.log(item); } }

Rate limiting patterns include: 1) Token bucket algorithm, 2) Sliding window, 3) Fixed window counting, 4) Leaky bucket algorithm. Example: class RateLimiter { constructor(limit, interval) { this.limit = limit; this.interval = interval; this.tokens = limit; this.lastRefill = Date.now(); } async acquire() { const now = Date.now(); const timePassed = now - this.lastRefill; this.tokens += Math.floor(timePassed / this.interval) * this.limit; this.tokens = Math.min(this.tokens, this.limit); this.lastRefill = now; if (this.tokens <= 0) { throw new Error('Rate limit exceeded'); } this.tokens--; return true; } }

Timeout handling includes: 1) Promise race with timeout promise, 2) AbortController integration, 3) Cleanup on timeout. Example: function withTimeout(promise, timeout) { const timeoutPromise = new Promise((_, reject) => { setTimeout(() => reject(new Error('Operation timed out')), timeout); }); return Promise.race([promise, timeoutPromise]); } async function fetchWithTimeout(url, timeout = 5000) { const controller = new AbortController(); const id = setTimeout(() => controller.abort(), timeout); try { const response = await fetch(url, { signal: controller.signal }); clearTimeout(id); return response; } catch (err) { clearTimeout(id); throw err; } }

Async dependency injection patterns include: 1) Factory functions, 2) Async constructors, 3) Dependency containers. Example: class Container { constructor() { this.dependencies = new Map(); } async register(key, factory) { this.dependencies.set(key, factory); } async resolve(key) { const factory = this.dependencies.get(key); if (!factory) throw new Error(`No dependency found for ${key}`); return await factory(); } } class Service { static async create(container) { const dependency = await container.resolve('dependency'); return new Service(dependency); } }

Distributed event patterns include: 1) Message queues, 2) Pub/sub systems, 3) Event buses, 4) Webhook implementations. Example: class DistributedEventEmitter { constructor(redis) { this.redis = redis; this.handlers = new Map(); this.redis.subscribe('events'); this.redis.on('message', (channel, message) => { const event = JSON.parse(message); const handlers = this.handlers.get(event.type) || []; handlers.forEach(handler => handler(event.data)); }); } on(eventType, handler) { const handlers = this.handlers.get(eventType) || []; handlers.push(handler); this.handlers.set(eventType, handlers); } emit(eventType, data) { this.redis.publish('events', JSON.stringify({ type: eventType, data })); } }

Async middleware patterns include: 1) Pipeline pattern, 2) Chain of responsibility, 3) Composition pattern. Example: class MiddlewareChain { constructor() { this.middlewares = []; } use(middleware) { this.middlewares.push(middleware); return this; } async execute(context) { return this.middlewares.reduce((promise, middleware) => { return promise.then(() => middleware(context)); }, Promise.resolve()); } } const chain = new MiddlewareChain(); chain.use(async (ctx) => { ctx.validated = true; }).use(async (ctx) => { if (ctx.validated) ctx.processed = true; });

Backpressure handling strategies include: 1) Implementing pause/resume mechanisms, 2) Buffer size limits, 3) Throttling, 4) Flow control. Example: class ThrottledStream extends Readable { constructor(source, rate) { super(); this.source = source; this.rate = rate; this.lastRead = Date.now(); } _read(size) { const now = Date.now(); const timeSinceLastRead = now - this.lastRead; if (timeSinceLastRead < this.rate) { setTimeout(() => this._read(size), this.rate - timeSinceLastRead); return; } this.lastRead = now; const chunk = this.source.read(size); this.push(chunk); } }

Async error boundaries can be implemented using: 1) Higher-order functions, 2) Class wrappers, 3) Decorator patterns. Example: class AsyncBoundary { constructor(errorHandler) { this.errorHandler = errorHandler; } wrap(target) { return new Proxy(target, { get: (obj, prop) => { const value = obj[prop]; if (typeof value === 'function') { return async (...args) => { try { return await value.apply(obj, args); } catch (error) { return this.errorHandler(error); } }; } return value; } }); } } const safeFetch = new AsyncBoundary(error => console.error(error)) .wrap({ async fetch(url) { const response = await fetch(url); return response.json(); } });