Jasmin Virdi

Posted on Mar 28, 2022 • Edited on Oct 8, 2022

Difference between the Event Loop in Browser and Node Js?

#javascript #node #programming #webdev

Every JS developer must have heard of the term Event Loop. Both JS and Node Js is based on the principle of event loop which has similarities and dissimilarities to some extent. Let's discuss the event loop in brief and find the difference between them. 📖

Event Loop in Browser

Let's cover each section in brief here:

Heap - It stores all the object reference and variables that we define in our function.
Call Stack - All the function that we use in our code is stacked here in LIFO manner such that the last function is at the top and first function is at the bottom.
Web API's - These API's are provided by browser which provides additional functionality over V8 engine. The functions which uses these API's are pushed to this container which on completion of the response of Web API is popped out of this container.
Queues - The queues are used to compute the asynchronous code response such that it doesn't block engine to execute further.
- Macro Task Queue - This queue executes async functions like DOM events, Ajax calls and setTimeout and has lower priority than Job queue.
- Micro Task Queue - This queue executes async functions which uses promises and has higher precedence over Message Queue.

The event loop checks the Call Stack, if the stack is empty it pushes the functions in the Queues to Call Stack and runs it. Functions already present are given higher priority and runs first in comparison to functions in message queue.

Event Loop in Node Js

The Node Server consist of following parts:

Event Queue - On completion of the Thread Pool a callback function is issued and sent to the event queue. When call stack is empty the event goes through the event queue and sends callback to the call stack.
Thread Pool - The thread pool is composed of 4 threads which delegates operations that are too heavy for the event loop. I/O operations, Opening and closing connections, setTimeouts are the example of such operations.
Event loop in Node Js has different phases which has FIFO queue of callbacks to execute. When event loop enters a given phase it operates callbacks in that phase queue until the queue has been exhausted and maximum number of callbacks has executed and then moves to next phase.

The Event Loop is an endless loop which waits for the tasks, executes them and then sleeps until it receives more tasks. The event loop executes tasks from queue only when stack is empty. It processes the oldest task first and allows us to use callbacks and promises.

Difference between both the event loops?

First difference is node uses a thread pool to manage disk I/O. It executes the I/O and other timer API's asynchronously.
Browser does not have setImmediate() function. This function execute once the I/O operation is done, if particular code is inside this it will be executed first. Whereas in setTimeout() callback function is executed after given minimum threshold value in milliseconds.
Node Js event loop has multiple phases and each phase handle specific type of tasks whereas browser has micro task and macro task queue within which all the tasks are processed in order they were placed into the queue.
In a browser when you open a page in a tab, you actually create a process in which there can be multiple threads, such as JS engine, page rendering, HTTP request threads and many more. Whereas in Node JS when you initiate a Client Request which performs Blocking I/O operations, event loop picks up a thread and assign the client request to that thread as Event loop is single threaded.

These are some major differences between the event loops for Node JS and Browser. Let me know if I missed something 😅

Happy Learning! 👩🏻‍💻

Top comments (25)

Dhanush N • Mar 29 '22

Well explained 🙂

Adding on

In a browser when you open a page in a tab, you actually create a process in which there can be multiple threads, such as js engine, page rendering, HTTP request threads and many more. Whereas in nodejs you initiate a request, you actually create a thread that may be destroyed when the request is completed.

Jasmin Virdi • Mar 29 '22

Thanks for adding up☺️

Do you mind if I add this to the difference list in the post?

Dhanush N • Mar 29 '22

Yeah, you can add 🙂

Comment marked as low quality/non-constructive by the community. View Code of Conduct

theoldman • Oct 7 '22 • Edited

what can you even say to a person, who is writing article about event loop and at the same time agreeing with some stupid person who thinks node js creates a thread for every request. WOW, I have no words, the first line of the documentation of the Node JS says node js is single threaded..

Jasmin Virdi • Oct 9 '22 • Edited

Hey,

Thanks for pointing that out. It was incomplete as information and must have created confusion among the readers regarding the concept. I have updated the context. 🙌🏼 🙂

Friendly advice: While pointing out issues in someone's article we can try using polite language instead of calling someone stupid or making fun of them. It is really discouraging for people out there writing articles and others who are sharing their feedback.

This is platform where everyone is sharing their learnings and mistakes are bound to happen for which I apologise. We as developers should help each other by providing valuable feedback in a better way and make this community a better place. 🙂

Khaledinges • Mar 31 '22

"Whereas in Node JS you initiate a request, you actually create a thread that may be destroyed when the request is completed"

It is incomplete as information. This is actually correct for the first request on the server, when you will have other requests or simultaneous requests, Node.js will still have one thread async processing.

Other languages like rust, java, .net have those type of concepts, I mean multi-threading approach.

Rahul Ramteke • Mar 30 '22

Node doesn't always use a thread pool. At its core it relies on operating system's ability to intimate it about certain events, for example kequeue, epoll etc.

The burden of actually executing the async operation and notifying node lies with OS. But for cases where that's not possible, it falls back to the threadpool. For example dns resolution is handled by thread pool, but file and socket operations are mostly OS.

LG • Mar 30 '22 • Edited

Instead of forking (crude-cloning) processes into splitted (distinct) context for each child process,
we instead employ workers (CPU heavy computations e.g. for DNS lookup) within single-threaded-context (thread pool) . I see workers (a.k.a. threads) as some sort of virtualization (optimization) within same boundaries of memory (RAM) at time . This image helped me a lot to comprehend what I stated in my comment above ; I'll be honest I may be mistaken , always welcome to give alternative argument for that .

Rahul Ramteke • Mar 30 '22

That's a nice article! Although, just to re-iterate, I was talking about the threadpool which libuv maintains.
A very brief explaination :
As I mentioned, at its core, node expects the OS to do the heavylifting. And different Operating Systems have different mechanism of doing that, hence node folks built an abstraction library called libuv. This library abstracts event loop and the OS interactions(fun fact, you can use this lib stand alone).
Now, let's say there's an operation which node wants to do differently, or there's something which OS can't handle or doesn't support. To manage such scenarios libuv has its own threadpool, and this threadpool(default size 4) simulates the async behavior which node expects from OS.