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What is the event loop in JavaScript runtimes?

What is the difference between call stack and task queue?
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JAVASCRIPT
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TL;DR

The event loop is concept within the browser runtime environment regarding how asynchronous operations are executed within JavaScript engines. It works as such:

  1. The JavaScript engine starts executing scripts, placing synchronous operations on the call stack.
  2. When an asynchronous operation is encountered (e.g., setTimeout(), HTTP request), it is offloaded to the respective Web API or Node.js API to handle the operation in the background.
  3. Once the asynchronous operation completes, its callback function is placed in the respective queues – task queues (also known as macrotask queues / callback queues) or microtask queues. We will refer to "task queue" as "macrotask queue" from here on to better differentiate from the microtask queue.
  4. The event loop continuously monitors the call stack and executes items on the call stack. If/when the call stack is empty:
    1. Microtask queue is processed. Microtasks include promise callbacks (then, catch, finally), MutationObserver callbacks, and calls to queueMicrotask(). The event loop takes the first callback from the microtask queue and pushes it to the call stack for execution. This repeats until the microtask queue is empty.
    2. Macrotask queue is processed. Macrotasks include web APIs like setTimeout(), HTTP requests, user interface event handlers like clicks, scrolls, etc. The event loop dequeues the first callback from the macrotask queue and pushes it onto the call stack for execution. However, after a macrotask queue callback is processed, the event loop does not proceed with the next macrotask yet! The event loop first checks the microtask queue. Checking the microtask queue is necessary as microtasks have higher priority than macrotask queue callbacks. The macrotask queue callback that was just executed could have added more microtasks!
      1. If the microtask queue is non-empty, process them as per the previous step.
      2. If the microtask queue is empty, the next macrotask queue callback is processed. This repeats until the macrotask queue is empty.
  5. This process continues indefinitely, allowing the JavaScript engine to handle both synchronous and asynchronous operations efficiently without blocking the call stack.

The unfortunate truth is that it is extremely hard to explain the event loop well using only text. We recommend checking out one of the following excellent videos explaining the event loop:

We recommend watching Lydia's video as it is the most modern and concise explanation standing at only 13 minutes long whereas the other videos are at least 30 minutes long. Her video is sufficient for the purpose of interviews.

Event loop in JavaScript

The event loop is the heart of JavaScript's asynchronous operation. It is a mechanism in browsers that handles the execution of code, allowing for asynchronous operations and ensuring that the single-threaded nature of JavaScript engines does not block the execution of the program.

Parts of the event loop

To understand it better we need to understand about all the parts of the system. These components are part of the event loop:

Call stack

Call stack keeps track of the functions being executed in a program. When a function is called, it is added to the top of the call stack. When the function completes, it is removed from the call stack. This allows the program to keep track of where it is in the execution of a function and return to the correct location when the function completes. As the name suggests it is a Stack data structure which follows last-in-first-out.

Web APIs/Node.js APIs

Asynchronous operations like setTimeout(), HTTP requests, file I/O, etc., are handled by Web APIs (in the browser) or C++ APIs (in Node.js). These APIs are not part of the JavaScript engine and run on separate threads, allowing them to execute concurrently without blocking the call stack.

Task queue / Macrotask queue / Callback queue

The task queue, also known as the macrotask queue / callback queue / event queue, is a queue that holds tasks that need to be executed. These tasks are typically asynchronous operations, such as callbacks passed to web APIs (setTimeout(), setInterval(), HTTP requests, etc.), and user interface event handlers like clicks, scrolls, etc.

Microtasks queue

Microtasks are tasks that have a higher priority than macrotasks and are executed immediately after the currently executing script is completed and before the next macrotask is executed. Microtasks are usually used for more immediate, lightweight operations that should be executed as soon as possible after the current operation completes. There is a dedicated microtask queue for microtasks. Microtasks include promises callbacks (then(), catch(), and finally()), await statements, queueMicrotask(), and MutationObserver callbacks.

Event loop order

  1. The JavaScript engine starts executing scripts, placing synchronous operations on the call stack.
  2. When an asynchronous operation is encountered (e.g., setTimeout(), HTTP request), it is offloaded to the respective Web API or Node.js API to handle the operation in the background.
  3. Once the asynchronous operation completes, its callback function is placed in the respective queues – task queues (also known as macrotask queues / callback queues) or microtask queues. We will refer to "task queue" as "macrotask queue" from here on to better differentiate from the microtask queue.
  4. The event loop continuously monitors the call stack and executes items on the call stack. If/when the call stack is empty:
    1. Microtask queue is processed. The event loop takes the first callback from the microtask queue and pushes it to the call stack for execution. This repeats until the microtask queue is empty.
    2. Macrotask queue is processed. The event loop dequeues the first callback from the macrotask queue and pushes it onto the call stack for execution. However, after a macrotask queue callback is processed, the event loop does not proceed with the next macrotask yet! The event loop first checks the microtask queue. Checking the microtask queue is necessary as microtasks have higher priority than macrotask queue callbacks. The macrotask queue callback that was just executed could have added more microtasks!
      1. If the microtask queue is non-empty, process them as per the previous step.
      2. If the microtask queue is empty, the next macrotask queue callback is processed. This repeats until the macrotask queue is empty.
  5. This process continues indefinitely, allowing the JavaScript engine to handle both synchronous and asynchronous operations efficiently without blocking the call stack.

Example

The following code logs some statements using a combination of normal execution, macrotasks, and microtasks.

console.log('Start');


setTimeout(() => {
  console.log('Timeout 1');
}, 0);


Promise.resolve().then(() => {
  console.log('Promise 1');
});


setTimeout(() => {
  console.log('Timeout 2');
}, 0);


console.log('End');


// Console output:
// Start
// End
// Promise 1
// Timeout 1
// Timeout 2

Explanation of the output:

  1. Start and End are logged first because they are part of the initial script.
  2. Promise 1 is logged next because promises are microtasks and microtasks are executed immediately after the items on the call stack.
  3. Timeout 1 and Timeout 2 are logged last because they are macrotasks and are processed after the microtasks.

Further reading and resources

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