Understanding the Fascinating Event Loop in JavaScript

Understanding the Fascinating Event Loop in JavaScript


The event loop in JavaScript is a critical concept to understand for writing optimal code. In this comprehensive guide, we will demystify the JavaScript event loop by walking through it step-by-step.

What is the Event Loop in JavaScript?

The event loop handles executing multiple tasks in the JavaScript engine:

  • It processes the program code
  • Executes asynchronous callbacks
  • Handles external events
  • Interacts with other APIs

It allows JavaScript to perform non-blocking I/O operations even with its single threaded nature.

Event Loop in Action

Let’s see the event loop in action with an example:

console.log('Hi'); // (A) 

setTimeout(() => {
}, 1000); // (B)

console.log('Bye'); // (C)

The output will be:


Let’s walk through how this works:

  1. Hi is logged by the initial code (A)
  2. setTimeout registers the callback (B)
  3. Bye is logged (C)
  4. After 1 second, the callback fires (B)

The magic is that Bye didn’t have to wait for the blocking timeout to complete before being logged. The event loop enables this.

Call Stack and Callback Queue

The event loop works using a call stack and callback queue:

  • The call stack handles execution contexts pushed on top to run code
  • The callback queue queues callbacks that are ready to be executed

When the stack is empty, the event loop passes callbacks from the queue to the call stack.

call stack

Event Loop Phases

The event loop runs across multiple phases to handle execution:

  • Timers – Executes pending timers
  • Pending callbacks – Executes I/O callbacks
  • Idle/Prepare – Used internally
  • Poll – Retrieve new I/O events
  • Check – Execute setImmediate callbacks
  • Close callbacks – Fire close event callbacks

This orderly processing allows coordinating events.

event loop
Source: https://nodejs.org/en/docs/guides/event-loop-timers-and-nexttick

Have a look below video for more detail in eventloop

Non-blocking I/O

JavaScript uses non-blocking I/O operations so code runs without waiting:

const data = fetch(url); // Doesn't block  

// More code runs here while fetch completes

const result = await data; // Resumes after fetching

The event loop enables coordinating these async operations.

Concurrency and Parallelism

JavaScript provides:

  • Concurrency – Switch between tasks efficiently via the event loop
  • Parallelism – Web workers run code in parallel on separate threads

So the main thread handles high-priority tasks while parallelizing less urgent ones with workers.

Use Cases

Some common cases to leverage the event loop:

  • Network requests
  • Background processing
  • Async task coordination
  • Database access
  • Listening for user input events

Properly structuring code to avoid blocking the loop is key.

Frequently Asked Questions

Q: What schedules and handles code execution in JavaScript?

A: The event loop handles coordinating code execution across multiple phases like handling timers, I/O callbacks, immediate callbacks etc.

Q: How does JavaScript provide non-blocking asynchronous execution?

A: The event loop enables asynchronous actions by using callbacks, promises, await etc. to execute tasks out of normal sequential order without blocking.

Q: What is the difference between the call stack and callback queue?

A: The call stack handles currently running code while the callback queue lines up callbacks ready to be invoked next.

Q: What are the different phases of the event loop?

A: The main phases are timers, I/O callbacks, prepare, poll, check, and close callbacks. Each handles a different aspect of coordinating code execution.

Q: How can long-running tasks avoid blocking the event loop?

A: Move long tasks like heavy computation to web workers so they run in a separate thread and don’t block the main event loop.

Q: What role do Web Workers play in relation to the event loop?

A: Web workers allow long-running code to execute in a separate thread in parallel so that it doesn’t block the main event loop thread.

Q: What are some common use cases that use the event loop?

A: Async I/O operations like network requests, incoming connections, user input events are handled efficiently via the event loop.

Q: What causes a “blocked event loop” and how can it be prevented?

A: Blocking the main thread with long synchronous operations can cause event loop blocking. This can be avoided by proper async patterns and offloading heavy work using web workers.

Q: Are setTimeout callbacks guaranteed to fire after the specified delay?

A: No, setTimeout schedules a minimum delay but not guaranteed. Callbacks are queued in the order received and must wait for any pending main thread activity to complete.

Q: How does await/async syntax interplay with the event loop?

A: Async/await are built on promises. They allow asynchronous code to be written sequentially while still leveraging the event loop asynchronously under the hood.


The event loop is a fundamental JavaScript concept that enables coordinating asynchronous actions like serving requests and handling user input. This guide provided a beginner-friendly introduction to the event loop with visual diagrams, step-by-step examples, and common use cases. We explored key concepts like the call stack, callback queue, concurrency, and factors that affect event loop performance. I hope this overview helps demystify the JavaScript event loop and how to harness its capabilities in your code.

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