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JavaScript Build a Simple Dynamic Site with Node.js Creating a Basic Template Engine in Node.js Reading from Files

Thomas Santoro
Thomas Santoro
4,007 Points

Sync vs Async: please explain reason for differing syntax.

(1) "We don't need a call back because it is synchronous this time" - WHY?

(2) "let's set the variable as file contents", again presumably because it is synchronous this time. WHY would you execute the same task as a function (with a callback) when it is asynchronous and then put it into a variable when you refactor it to be synchronous?

(2.1) And related to that, why is so much stuff put into a variable in JavaScript any way? it seems a bit odd, for example, to require a library like "fs" into a variable.

Also, take a breath man. You do not save us time with this frenetic effort to get to the end because to follow along--without adequate explanation--we have to rewatch and decrypt your lesson. I'm sure it's easy stuff. Really obvious. And after I went back it wasn't hard to sort out. But why do I have to do this so much more with this guy than with other instructors?

JASON LEE
JASON LEE
13,615 Points

Another observation I made -- it's possible to use the original async version readFile and make it work.

In the router.js file where the functions are called, I tried delaying the response.end( ) using a setTimeout and delay for 1000ms or so. Obviously, not good practice, but just helpful to understand (and debug?) why the async version didn't work in the video example.

I believe the command response.end(), once executed, will prevent any further response.write() action.

2 Answers

Seth Kroger
Seth Kroger
56,403 Points

I recently got an answer request for this so I'll give it a go.

The functions in this library have asynchronous and synchronous versions. The synchronous versions have "Sync" added to their name. Being synchronous means then won't return until they have a result and will return the result directly. This is unlike the asynchronous version where you give it a callback that accepts the result and return immediately to continue on with other things. You'll will also hear this referred to as blocking I/O because you are waiting for it to finish.

Think of it like having an assistant, Bob. The synchronous version is you saying "Bob, I need you to go get a file for me from the file room. It's very important to me and can't continue work without it." vs. "Bob, I need you to go to the file room and retrieve this file. I have other things to do in the meantime so give it to Alice when you've got it. She'll know what to do with it."

You'll usually avoid blocking I/O as much as possible in NodeJS but using non-blocking asynchronous I/O would over complicate the current project.

Thank you, Seth Kroger.