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Understanding Concurrency and the Event Loop

What is Concurrency?

Concurrency, in the context of programming, refers to the ability of a system to perform multiple tasks or operations simultaneously, making efficient use of resources, especially in applications that require a high degree of responsiveness. In traditional multi-threaded environments, concurrency is often achieved by running tasks on separate threads, allowing them to execute in parallel.

However, JavaScript’s approach to concurrency is distinct. Given its single-threaded nature, JavaScript executes code in a sequence, one task at a time. This might seem like a limitation, but JavaScript overcomes this through asynchronous programming, callbacks, promises, and, most importantly, the event loop.

The Role of the Event Loop

The event loop is the mechanism that allows JavaScript to perform non-blocking, asynchronous operations despite being single-threaded. It plays a pivotal role in JavaScript’s runtime environment, ensuring that tasks are executed in an efficient and orderly manner.

How the Event Loop Works

  1. Call Stack: JavaScript maintains a call stack where it keeps track of all the functions that need to be executed. Since JavaScript is single-threaded, only one function can execute at a time.
  2. Callback Queue: Asynchronous callbacks (e.g., from events, timers, or AJAX requests) are placed in a queue. Once the call stack is empty, meaning all currently running tasks have completed, the event loop transfers tasks from the callback queue to the call stack to be executed.
  3. Web APIs/Runtime Environment: Browser or Node.js APIs handle asynchronous operations like setTimeout, XMLHttpRequest, and event listeners. Once these operations complete, their callbacks are moved to the callback queue, waiting for the event loop to pick them up.
  4. Microtask Queue: A special queue for promises and other microtasks. Tasks in this queue have higher priority and are executed immediately after the current task completes, even before the event loop continues to the next task in the callback queue.

The event loop continuously checks the call stack and determines if there are any tasks waiting in the callback queue. If the call stack is empty, it moves the first task from the queue to the stack, allowing it to execute. This cycle repeats, enabling JavaScript to handle a vast number of operations asynchronously, without blocking the main thread.

Components of JavaScript’s Concurrency Model

Call Stack

The call stack is a fundamental component of JavaScript’s concurrency model, acting as a record of where the program’s execution is at any given moment. When a script calls a function, the interpreter adds it to the call stack and then starts carrying out the function. Any functions that are called by that function are added to the call stack further up, and run where their calls are reached. When the current function is finished, the interpreter takes it off the stack and resumes execution where it left off in the last code listing.

Example of Call Stack Operation

function firstFunction() {
  console.log('First function started');
  console.log('First function ended');

function secondFunction() {
  console.log('Second function started');


In this example:

  1. firstFunction() is called and added to the call stack.
  2. Inside firstFunction(), secondFunction() is called and placed on top of the call stack.
  3. secondFunction() starts and completes, then is removed from the call stack.
  4. Execution resumes in firstFunction(), which then completes and is also removed from the call stack.

Web APIs and the Callback Queue

Web APIs are not part of the JavaScript language itself; instead, they are built into the browser to provide functionality like DOM manipulation, fetching data, setting timers, etc. These APIs can handle operations asynchronously and, upon completion, push the callback function associated with the asynchronous operation to the callback queue.

The callback queue holds callback functions that are associated with asynchronous operations until the call stack is clear. Once the call stack has no more functions to execute, the event loop transfers the first callback in the queue to the call stack, allowing it to execute.

Example of Web APIs and Callback Queue

console.log('Script start');

setTimeout(function() {
  console.log('Timeout ran at last');
}, 0);

console.log('Script end');

Even though setTimeout is set with a delay of 0 milliseconds, the callback function doesn’t execute immediately after “Script end”. It’s because the setTimeout function, handled by Web APIs, places the callback in the callback queue, which will only be executed after the call stack is empty.

Microtask Queue

The microtask queue is similar to the callback queue but with a higher priority. Operations in the microtask queue are processed at the end of the current run of the JavaScript event loop before the event loop checks the callback queue. Promises and other microtasks (like MutationObserver) use the microtask queue to ensure that their callbacks are processed as soon as possible, but still asynchronously.

Differentiating Microtask Queue from Callback Queue

  • Callback Queue: Used for most asynchronous operations like setTimeout, setInterval, and asynchronous Web APIs.
  • Microtask Queue: Used for promises, process.nextTick (in Node.js), and other microtasks. Tasks here take precedence over the callback queue.

Example of Microtask Queue Operation

console.log('Script start');

setTimeout(function() {
}, 0);

Promise.resolve().then(function() {
  console.log('Promise resolved');

console.log('Script end');

In this example, “Promise resolved” logs before “setTimeout” because promise callbacks are placed in the microtask queue, which is processed before the next event loop cycle, whereas setTimeout goes to the callback queue, processed after all microtasks have completed.

The Event Loop in Action

Understanding the event loop in JavaScript is crucial for writing efficient, non-blocking code. Let’s walk through a step-by-step example to illustrate how the event loop processes both synchronous and asynchronous tasks.

Step-by-Step Example

Consider the following code snippet, which includes both synchronous and asynchronous operations:


setTimeout(() => {
  console.log('Timeout Callback');
}, 0);

Promise.resolve().then(() => {
  console.log('Promise Callback');


How the Event Loop Handles This:

  1. Start Execution: The script starts executing from top to bottom. The first console.log('Start') is a synchronous operation and is immediately executed, printing “Start”.
  2. SetTimeout Callback: The setTimeout function is called next. Since it’s an asynchronous operation handled by Web APIs, its callback is set aside to be called after the specified delay (0ms in this case). However, it won’t execute immediately after 0ms but will be moved to the callback queue waiting for the call stack to be empty.
  3. Promise Callback: The Promise.resolve() is encountered next. Promises are also asynchronous but use the microtask queue, which has a higher priority over the callback queue. The callback .then(() => console.log('Promise Callback')) is placed in the microtask queue.
  4. End Execution: The last console.log('End') is executed, printing “End”.
  5. Microtask Queue Processing: Before moving to the next loop cycle, the event loop checks the microtask queue. The promise callback is executed, printing “Promise Callback”.
  6. Callback Queue Processing: Now, the event loop moves to the callback queue. The setTimeout callback is executed, printing “Timeout Callback”.


The process can be visualized as follows:

  1. Call Stack: console.log('Start') -> setTimeout(...) -> Promise.resolve().then(...) -> console.log('End')
  2. Web APIs: Handle setTimeout
  3. Microtask Queue: Promise Callback
  4. Callback Queue: Timeout Callback

After clearing the call stack, the event loop processes the microtask queue first, then the callback queue.

Real-Life Implications and Best Practices

Writing Non-Blocking Code

In a single-threaded environment like JavaScript, writing non-blocking code is essential to maintain application responsiveness. Here are some tips:

  • Use Asynchronous Operations: For I/O operations (e.g., network requests, file reading in Node.js), use asynchronous APIs to prevent blocking the main thread.
  • Leverage Promises and Async/Await: These constructs make handling asynchronous operations more manageable and readable.
  • Break Down Long-Running Tasks: If a task cannot be made asynchronous, consider breaking it down into smaller chunks that can be executed in sequence with timeouts or setImmediate (Node.js), allowing other tasks to be processed in between.

Optimizing for Performance

To ensure your JavaScript code works efficiently with the event loop:

  • Minimize Use of Synchronous Operations: Especially in Node.js, where blocking the main thread can affect the entire application.
  • Smart Use of Promises: While promises are powerful, excessive chaining or incorrect usage can lead to performance issues. Use them judiciously.
  • Event Loop Monitoring: In Node.js, tools like the event-loop-lag module can help monitor the event loop’s health and detect potential bottlenecks.

Real-Life Examples in React

Example 1: Data Fetching on Component Mount

Using useEffect to fetch data asynchronously ensures that the data fetching does not block the initial rendering of the component.

useEffect(() => {
  async function fetchData() {
    const response = await fetch('<>');
    const data = await response.json();
}, []); 

Example 2: Debouncing Search Input

For a search input that triggers a query to the server, debouncing the input can prevent excessive requests, ensuring the event loop is not overwhelmed by rapid state updates.

const [query, setQuery] = useState('');

useEffect(() => {
  const timerId = setTimeout(() => {
  }, 500);

  return () => clearTimeout(timerId);
}, [query]);


Standard JavaScript Exercises

Exercise 1: Asynchronous Output Order

Predict the output order of the following code snippet and explain why:


setTimeout(() => {
}, 0);

Promise.resolve().then(() => console.log('3'));


Exercise 2: Implement a Simple Debounce Function

Write a debounce function that takes a function and a timeout as arguments and returns a debounced version of the original function. The debounced function should only execute after the specified timeout has elapsed without any further calls.

function debounce(func, timeout) {
  // Your implementation here

Exercise 3: Simulate Promise.all

Implement a function that simulates the behavior of Promise.all. The function should accept an array of promises and return a single Promise that resolves to an array of the results of the input promises.

function promiseAll(promises) {
  // Your implementation here

React Exercises

Exercise 1: Fetching Data with useEffect

Create a React component that fetches and displays a list of items from an API endpoint using useEffect. Ensure that the data fetching does not block the rendering of the component.

Exercise 2: State Management with Async Operations

Implement a counter component in React that increments the count after a delay. Use useState to manage the count state and useEffect to handle the asynchronous increment operation.

Exercise 3: Optimizing Component Renders

Create a React component that receives a list of items as props and renders them. Use React.memo or useMemo to optimize the component so that it only re-renders when the list of items changes, not on every parent component re-render.

LeetCode Algorithms

Algorithm 1: Flood Fill (LeetCode #733)

Algorithm 2: Number of Islands (LeetCode #200)

Algorithm 3: Implement Stack using Queues (LeetCode #225)

References and Further Reading

  1. MDN Web Docs – Concurrency model and Event Loop: A comprehensive guide that explains the JavaScript event loop, call stack, and task queues. Visit MDN
  2. “You Don’t Know JS” (book series) by Kyle Simpson – Async & Performance: This book provides an in-depth look at asynchronous JavaScript, covering callbacks, promises, and the event loop. Read on GitHub
  3. JavaScript Info – The Modern JavaScript Tutorial: Offers clear, concise explanations of the event loop, microtasks, and event handling in JavaScript. Visit
  4. “What the heck is the event loop anyway?” by Philip Roberts: A highly recommended talk that explains the JavaScript event loop, call stack, and asynchronous programming in an accessible and engaging way. Watch on YouTube
  5. Node.js Documentation – Event Loop, Timers, and process.nextTick(): Provides insights into how the event loop works in a Node.js environment, which is slightly different from the browser environment. Visit Node.js Docs


Q: What is the event loop in JavaScript, and why is it important?
A: The event loop is a fundamental mechanism in JavaScript that enables non-blocking, asynchronous operations in a single-threaded environment. It’s crucial for managing the execution of tasks, ensuring efficient processing of events, callbacks, and promises, and maintaining application responsiveness.

Q: How does JavaScript handle concurrency given its single-threaded nature?
A: Despite being single-threaded, JavaScript handles concurrency through the event loop, along with the call stack, callback queue, and microtask queue. This model allows JavaScript to perform asynchronous operations, executing tasks in the background without blocking the main thread.

Q: Can you explain the difference between the callback queue and the microtask queue?
A: The callback queue is used for most asynchronous operations like timers and HTTP requests, processed after the current execution stack clears. The microtask queue, with a higher priority, is for promises and process.nextTick (in Node.js), executed immediately after the current task completes and before the event loop continues to the next task.

Q: What are some best practices for writing non-blocking code in JavaScript?
A: Best practices include using asynchronous APIs for I/O tasks, leveraging promises and async/await for clearer asynchronous code, breaking down long-running tasks, and optimizing performance by minimizing synchronous operations that can block the main thread.

Q: How can understanding the event loop improve my coding in React?
A: Knowing how the event loop works helps optimize React applications by efficiently managing asynchronous operations, such as data fetching with useEffect or debouncing user inputs. It ensures that your React components render smoothly, without unnecessary delays or blocking behavior.