CODEWITHSUNDEEP
node.js
Saad
Saad

Reputation: 28486

How does the stack work on NodeJs?

I've recently started reading up on NodeJS (and JS) and am a little confused about how call backs are working in NodeJs. Assume we do something like this:

setTimeout(function(){
    console.log("World");
},1000);
console.log("Hello");

output: "Hello World"

So far on what I've read JS is single threaded so the event loop is going though one big stack, and I've also been told not to put big calls in the callback functions.

1)

Ok, so my first question is assuming its one stack does the callback function get run by the main event loop thread? if so then if we have a site with that serves up content via callbacks (fetches from db and pushes request), and we have 1000 concurrent, are those 1000 users basically being served synchronously, where the main thread goes in each callback function does the computation and then continues the main event loop? If this is the case, how exactly is this concurrent?

2) How are the callback functions added to the stack, so lets say my code was like the following:

setTimeout(function(){
    console.log("Hello");
},1000);
setTimeout(function(){
    console.log("World");
},2000);

then does the callback function get added to the stack before the timeout even has occured? if so is there a flag that's set that notifies the main thread the callback function is ready (or is there another mechanism). If this is infact what is happening doesn't it just bloat the stack, especially for large web applications with callback functions, and the larger the stack the longer everything will take to run since the thread has to step through the entire thing.

Upvotes: 0

Views: 245

Answers (2)

Bret Copeland
Bret Copeland

Reputation: 24000

The event loop is not a stack. It could be better thought of as a queue (first in/first out). The most common usage is for performing I/O operations.

Imagine you want to read a file from disk. You start by saying hey, I want to read this file, and when you're done reading, call this callback. While the actual I/O operation is being performed in a separate process, the Node application is free to keep doing something else. When the file has finished being read, it adds an item to the event loop's queue indicating the completion. The node app might still be busy doing something else, or there may be other items waiting to be dequeued first, but eventually our completion notification will be dequeued by node's event loop. It will match this up with our callback, and then the callback will be called as expected. When the callback has returned, the event loop dequeues the next item and continues. When there is nothing left in node's event loop, the application exits.

That's a rough approximation of how the event loop works, not an exact technical description.

For the specific setTimeout case, think of it like a priority queue. Node won't consider dequeuing the item/job/callback until at least that amount of time has passed.

There are lots of great write-ups on the Node/JavaScript event loop which you probably want to read up on if you're confused.

Upvotes: 1

Andrey Sidorov
Andrey Sidorov

Reputation: 25456

callback functions are not added to caller stack. There is no recursion here. They are called from event loop. Try to replace console.log in your example and watch result - stack is not growing.

Upvotes: 1

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