Will I take advange of the Asynchronous using this variables? NODE.js

Question

I created these variables and to each of them assigned the text of each txt file. If i stored them in the the variables, will they still run in the background while executing the file or would this be Synchronously?

const getGiraffe = fs.readFile("./txt/giraffe.txt", "utf-8", (err, data) => {
  return data;
});

const getRhino = fs.readFileSync("./txt/rhino.txt", "utf-8", (err, data) => {
  return data;
});

It is not really clear what you mean. For `fs.readFile` the `getGiraffe` and `return data;` do not make any sense. For `fs.readFileSync` the complete callback passed to it does not make sense. What do you `i stored them in the the variables` which variable and what do you mean with `them`? `would this be Synchronously` the `Sync` in `readFileSync` means Synchronously. ... — t.niese, Feb 21 '22 at 23:22
Neither of your snippets make any sense. `fs.readFile` doesn't return a value and `fs.readFileSync` doesn't take a callback — derpirscher, Feb 21 '22 at 23:43
And there is no "reading in background" in javascript, as it is single threaded ... — derpirscher, Feb 21 '22 at 23:44
@derpirscher - That is not actually true. `fs.readFile()` does work in the background. In the native C++ code inside of nodejs, the implementation of `fs.readFile()` uses a thread pool to run the file operation in the background and that allows it to make `fs.readFile()` both asynchronous and non-blocking. — jfriend00, Feb 22 '22 at 01:31
@derpirscher This is actually not correct. There are a few functions in JS which use a C thread pool. (Using libuv) I think this video talks about it: https://www.youtube.com/watch?v=gMtchRodC2I — Ben Rauzi, Feb 22 '22 at 01:35

jfriend00 · Answer 1 · 2022-02-22T01:32:25.650

Neither of your code examples is correct.

In the first code example:

const getGiraffe = fs.readFile("./txt/giraffe.txt", "utf-8", (err, data) => {
  return data;
});

You are reading the file (with no error handling) and then you do return data back to the callback, but that doesn't go anywhere. So, getGiraffe will simply be undefined.

To use the asynchronous version of fs.readFile(), you must use the data value inside the callback or call some function from within that callback and pass it the data. You cannot simply return the data out.

fs.readFile("./txt/giraffe.txt", "utf-8", (err, data) => {
    if (err) {
        console.log(err);
    } else {
        console.log(data);
    }
});

In the second code example:

const getRhino = fs.readFileSync("./txt/rhino.txt", "utf-8", (err, data) => {
  return data;
});

You are passing a callback to a function fs.readFileSync() that does not accept a callback. That callback will be ignored.

The calling signature for fs.readFileSync() is this:

fs.readFileSync(path[, options])

It takes only two arguments, neither is a callback.

Your question

If i stored them in the the variables, will they still run in the background while executing the file or would this be Synchronously?

The first code example will run in the background asynchronously, but getGiraffe will be undefined. It will not have the data from your file in it. That is just not how fs.readFile() works. You must use the data it delivers INSIDE the callback itself.

Your second code example, is synchronous and blocking. It will block the event loop and getRhino will contain the data from the file, but your callback will be ignored and never called because that's not a supported parameter for fs.readFileSync().

For more information on communicating back a result to the caller from an asynchronous function like fs.readFile() see this reference:

How to return the response from an asynchronous call

Will I take advange of the Asynchronous using this variables? NODE.js

1 Answers1