Vfrdtyky

I have a function that writes data to a file then uploads that file to cloud storage. The file isn't finished writing before it starts uploading so I am getting a partial file in cloud storage. I found that fs.writeFileSync(path, data[, options]) could help, but I am not exactly sure how it works.

It is my understanding that node runs asynchronously and I have several async processes running prior to this portion of code. I understand what synchronous vs asynchronous means, but I am having a little trouble understanding how it plays in this example. Here are my questions if I replace the below code with fs.writeFileSync(path, data[, options])

1. What do the docs mean by "Synchronously append data to a file"
   
   
   
   
   • a. Will the next lines of code be halted until the fs.writeFileSync(path, data) is finished?
   
   
   • b. Are previous asynchronous processes halted by this line of code?
   
   
   
   



2. If other async processes are not affected how is writeFileSync different the writeFile?




3. Is there a callback feature in writeFileSync that I am misunderstanding?

Code for reference

outCsv = "x","y","z"

filename = "file.csv"

fs.writeFile(filename, outCsv, function (err) {

        if (err) {

            return console.log(err);

        }

        console.log('The file was saved!');

        bucket.upload(filename, (err, file) => {

            if (err) {

                return console.log(err);

            }

            console.log('The file was uploaded!');

        });

    });

Will the next lines of code be halted until the fs.writeFileSync(path, data) is finished?

Yes. It is a blocking operation. Note that you're assuming that fs.writeFileSync does finish.

Are previous asynchronous processes halted by this line of code?

Kinda. Since JavaScript is single-threaded, they will also not be running while the file is writing but will queue up at the next tick of the event loop.

If other async processes are not affected how is writeFileSync different the writeFile?

It blocks any code that comes after it. For an easier example consider the following:

setTimeout(() => console.log('3'), 5);

console.log('1'); // fs.writeFileSync

console.log('2');

vs

setTimeout(() => console.log('3'), 5);

setTimeout(() => console.log('1'), 0); // fs.writeFile

console.log('2');

The first will print 1 2 3 because the call to console.log blocks what comes after. The second will print 2 1 3 because the setTimeout is non-blocking. The code that prints 3 isn't affected either way: 3 will always come last.

Is there a callback feature in writeFileSync that I am misunderstanding?

Don't know. You didn't post enough code for us to say.

This all begs the question of why to prefer fs.writeFile over the alternative. The answer is this:

The sync version blocks.

While it's taking however long it takes your e.g. webserver isn't handling requests.