Event Loop


The event loop is a crucial part of JavaScript’s concurrency model, responsible for executing asynchronous operations in a non-blocking manner. Understanding how it works is key to writing efficient and performant JavaScript code.

JavaScript is single-threaded, meaning it can only execute one piece of code at a time. However, many operations, like network requests or user interactions, are inherently asynchronous. They take time to complete, and blocking the main thread while waiting would lead to a frozen and unresponsive user interface. This is where the event loop comes in.

Here’s a simplified breakdown:

  1. Call Stack: When a function is called, it’s added to the call stack. JavaScript executes functions synchronously, one by one, from the top of the stack.

  2. Web APIs: When an asynchronous operation is encountered (e.g., setTimeout, fetch), it’s handed off to the browser’s Web APIs. These APIs handle the operation in the background, freeing up the call stack.

  3. Callback Queue: Once an asynchronous operation completes, its associated callback function is placed in the callback queue. This queue acts as a holding area for functions waiting to be executed.

  4. Event Loop’s Role: The event loop continuously monitors the call stack and the callback queue. When the call stack is empty, it takes the first callback function from the queue and places it onto the stack for execution. This cycle repeats, ensuring that asynchronous operations are processed as soon as the main thread is free.

Example:

console.log("Start");

setTimeout(() => {
  console.log("Inside setTimeout - this runs asynchronously");
}, 0);

console.log("End");

Output:

Start
End
Inside setTimeout - this runs asynchronously

Even though setTimeout has a delay of 0 milliseconds, “Inside setTimeout” is logged after “End”. This demonstrates that the setTimeout callback is placed in the callback queue and executed only after the call stack becomes empty.

In essence, the event loop enables JavaScript to handle asynchronous operations without blocking the main thread, ensuring a smooth and responsive user experience. It’s the core mechanism that allows JavaScript to be non-blocking despite being single-threaded.