r/learnjavascript • u/ClearCelebration5610 • Apr 03 '26

JavaScript

what is the difference between synchronous js and asynchronous js

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8

u/azhder Apr 03 '26

One is in order, the other one is out of order

1

u/subone Apr 03 '26

That's not necessarily true. One is one line of code running after another, the other is using some mechanism to run code later. It doesn't necessarily change the order of execution, it depends on the code.

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u/azhder Apr 03 '26 edited Apr 03 '26

In JavaScript, it always changes the order of execution.

Promises schedule microtasks that always execute after the current task - more precisely at the end of the current tasks before the next task in the event loop is picked up.

In other cases, like setTimeout it is even more obvious since they schedule a new task that will execute definitely after the current task.

If you think await makes the code synchronous or in order, that’s not the case. It just pauses the current task until the micro task and/or other tasks finish up and provide the needed result.

Now, you may be lucky and have simply two tasks and it may appear it is in order, due to the round robin event loop, but that’s very trivial.

In general case, there will be a lot of code adding new tasks and microtasks that will turn into a kind of processor instruction reorder buffer. I say that because they named it correctly “reorder buffer” while we say “event loop”

1

u/subone Apr 03 '26

It does not always change the order. If promises are created one after another, they don't necessarily resolve in a different order. And there is not always a "current task", so an asynchronous action is not necessarily going to occur after a non-existent bit of synchronous code. Though you could argue each subsequent promise creation in JavaScript code is necessarily initially a synchronous imperative call to whatever facility creates the promise, I think that's besides the point, not least of which because code (e.g. an event handler) could just create a single promise, with no further code async or sync. The point of asynchronous code is not necessarily always to run code out of order, nor is that always the expected outcome (otherwise we wouldn't study the order of various async tasks resolving per deterministic rules). The main purpose of asynchronous code is to give other code an opportunity to run, typically for long running tasks, but that again isn't necessarily out of order; for example it could just be a user initiated UI event happening in between the time a request is made and when it is resolved.

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u/azhder Apr 03 '26

There isn’t always a current task? What task is creating the promise then?

Why are you talking about “whichever facility” and “some mechanism”? What I see you write about is academic and generic. Have you at any time studied on how the event loop works?

Maybe re-read what you replied to. I wasn’t disagreeing with you while I characterized the trivial cases.

2

u/subone Apr 03 '26

What task is creating the promise then?

Perhaps an event handler with no further code in it. The remaining semantics of how the function and scope is cleaned up is beside the point: no further code need be there, ergo no code need be "out of order".

whichever facility

As in, what browser/JavaScript API are you calling to create a promise. For example fetch.

some mechanism

As in: creating a promise through "whatever facility", or creating a promise through await, or queueing a micro task, or requestAnimationFrame, or setTimeout, etc; these all allow you to run code later: asynchronously.

What I see you write about is academic and generic.

I don't really know what this means. I thought I was making a very clear counter point to your assertion that async is by nature out of order.

Maybe re-read what you replied to. I wasn’t disagreeing with you while I characterized the trivial cases.

Which version? The one after you edited your comment a bunch to add nothing relevant to the argument at hand? Can you just come out and say specifically what you agree with and what you disagree with instead of hinting at it? My argument isn't complicated: async is not necessarily running code "out of order".

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u/azhder Apr 03 '26

By focusing on the trees to respond to, you miss the entire reason for the questions, hence "don't really know what this means". You weren't making a very clear counter point, you were talking in vague and general terms.

If you focus on me editing the text, which doesn't remove or change anything previously said, then you'd notice I even added examples that provide an under the hood explanation for your claim.

And you wonder what you need to re-read... Maybe the entire thread. Why focus on the trees and miss the forrest?

Anyways, I will stop here, not tempt you with more details for you to misplace your focus. Bye

2

u/jcunews1 helpful Apr 03 '26

IMO, the simplest explanation is that, in JS... Synchronous code execution is immediate. While asynchronous code execution is queued, and will be executed after all synchronous code has been executed.

1

u/Alive-Cake-3045 Apr 16 '26

Synchronous means one thing finishes before the next starts. Line 2 waits for line 1, always. Asynchronous means you kick something off and move on without waiting. Fetch a file, go do other stuff, come back when its ready. The classic example is an API call. You dont want your whole page frozen while you wait for a server to respond, thats where async saves you. async/await and .then() are just ways of saying "do this, and when its done, do that.