CODEWITHSUNDEEP
javascript
user3784950
user3784950

Reputation: 85

callback of the callback function node js

So in Node we use async functions. And to do that we use callback functions as parameters. But how do I execute the final function after which I want to terminate the code? Just pass the empty function? Here's example:

fs.mkdir('stuff', function(){
    fs.readFile('readMe.txt', 'utf8', function(err, data) {
        fs.writeFile('./stuff/writeMe.txt', data);
    });
});

mkdir has callback function - all fine

readFile has callback function - all fine

writeFile has NOT callback function because that's the last thing I want to do, but then I get an error in console: "DeprecationWarning: Calling an asynchronous function without callback is deprecated."

Should I every time I do that, pass an empty function as a callback to avoid the error? What's the best practice for this?

Upvotes: 0

Views: 581

Answers (2)

T.J. Crowder
T.J. Crowder

Reputation: 1073968

Should I every time I do that, pass an empty function as a callback to avoid the error?

No.

What's the best practice for this?

Pass in a function and handle any errors it reports. You also need to handle errors from mkdir and readFile, which currently you're ignoring.

E.g.:

fs.mkdir('stuff', function(err) {
    if (err) {
        // Do something with the fact mkdir failed
    } else {
        fs.readFile('readMe.txt', 'utf8', function(err, data) {
            if (err) {
                // Do something with the fact readFile failed
            } else {
                fs.writeFile('./stuff/writeMe.txt', data, function(err) {
                    if (err) {
                        // Do something with the fact writeFile failed
                    }
                });
            }
        });
    }
});

...which is a fair example of callback hell, which is part of the motivation of using promises instead. You could promisify the fs API (using any of several libs, such as promisify) and do it like this:

fsp.mkdir('stuff')
.then(() => fsp.readFile('readMe.txt', 'utf8'))
.then(data => fsp.writeFile('./stuff/writeMe.txt', data))
.catch(err => {
    // Do something with the fact something failed
});

...where fsp is a placeholder for the promisified fs API.

In Node 8.x+, you could use async/await to write synchronous-looking code with those promisified APIs:

// This must be inside an `async` function
try {
    fsp.mkdir('stuff');
    const data = await fsp.readFile('readMe.txt', 'utf8');
    await fsp.writeFile('./stuff/writeMe.txt', data);
} catch (err) {
    // Do something with the fact something failed
}

Upvotes: 3

Reza Karami
Reza Karami

Reputation: 515

You can use writeFileSync instead.

fs.mkdir('stuff', function(){
    fs.readFile('readMe.txt', 'utf8', function(err, data) {
        fs.writeFileSync('./stuff/writeMe.txt', data);
    });
});

Upvotes: 0

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