Check if file is completely written with node.js

I need a little bit help how I can handle following task in JavaScript: I have an app, that use Jimp for image processing and node-sprite-generator. That all runs in node.js context. I load a few images to Jimp, make something with the image then write it back to my file system with the nodejs filemodule. Then I would take the new created images and pasted it to node-sprite-generator. The problem is, that not all images are created/written at this time. While the code for create the spritesheet runs immediately after Jimp returns, I think Jimp processing all of the images and return a promise. The result is, that the code for creating the spritesheet get executed but the stack is not done.

I tried to test if the file is written, with fs.stat() and the propertie mtime like

 if (stat.mtime.getTime() === prev.mtime.getTime())

But then it can happens, that an error occurred, when the file is not created at this time. Also: I need a way to check if the image is written completely with handling, when the path to the image is current not available.

function resize(img) {
    Jimp.read(img.path).then(function (err, file) {
        if (err) throw err;
        file.resize(200, 200)
            .quality(70)
            .write(newPath); //newPath for simplicity
    });
}


function rec(imgObjArray) {
    if(_.isEmpty(imgObjArray)) return;  
    resize(imgObjArray.pop());
//correct mistake in function call from fn() to rec()
    rec(imgObjArray);
}

rec(imgObjArray); //imgObjArray === [img,img,img....]

//nsg() does not work because not all images are written at this time
     nsg({
            src: [
                'out/images/desktop/*.jpg'
            ],
            spritePath: 'out/images/desktop/sprite.jpg',,
            compositor: 'jimp'
        }, function (err) {
            console.log('Sprite generated!');
        })

I think first I must check if the image exist at a given path and then check if written is finished. But when I make a fn with fs.access(path[, mode], callback) and the file is not created at this time, I get an error.

Answer

You’ve got a mix of synchronous and asynchronous code in here. I’ll try to describe what’s happening in comments:

First, your function definitions – you’re firing off asynchronous actions without properly handling their completions

// I've re-spaced the code slightly and removed your comments so mine stand out
function resize(img) {
    Jimp.read(img.path).then(function (err, file) {
        // this code only executes after the file is done reading, but this
        // is an asynchronous action - it doesn't hold up execution
        if (err) throw err;
        file.resize(200, 200).quality(70).write(newPath);
        // .write() is presumably *also* an asynchronous action - if you want
        // something to happen only *after* it's been written, it needs to be in
        // a callback or promise on the write method
    });

    // I added this explicitly - after you *start* your Jimp.read, you *immediately*
    // return from this function, *before* the read is completed.  If you want
    // something to happen only *after* your read and write, you either need to
    // return the promise so you can act on it, or put the further actions in a callback
    return undefined;
}

function rec(imgObjArray) {
    if(_.isEmpty(imgObjArray)) return; 
    // resize() runs and returns *before* the file is read, resized, and written
    resize(imgObjArray.pop());
    // I don't know what fn() is, it's not defined here - presumably it's not important
    fn(imgObjArray);
}

… then, your procedural calls:

// this fires off and completes immediately, having initiated the asynchronous methods
rec(imgObjArray);

// you call this on the assumption that all of your code above has completed, but since
// it's asynchronous, that's not true, you get here with *none* of your images completed
nsg({
    src: [
        'out/images/desktop/*.jpg'
    ],
    spritePath: 'out/images/desktop/sprite.jpg',
    compositor: 'jimp'
}, function (err) {
    console.log('Sprite generated!');
});

You have two options:

If file.write() is a synchronous call, you can just return the promise and act on it:

function resize(img) {
    // by *returning* this call, we're actually returning the promise, we can act on
    // in the future
    return Jimp.read(img.path).then(function (err, file) {
        if (err) throw err;
        file.resize(200, 200).quality(70).write(newPath);
    });
}

function rec(imgObjArray) {
    if(_.isEmpty(imgObjArray)) return; 
    // the result of resize is now a promise
    return resize(imgObjArray.pop()).then(function(err) {;
        // again, assuming `fn()` is synchronous...
        fn(imgObjArray);
    });
}

// now the result of *this* call is a promise, which you can use to control
// the timing of your next call
rec(imgObjArray).then(function(err) {
    // now this will only run after all of the previous calls have executed
    nsg({
        src: [
            'out/images/desktop/*.jpg'
        ],
        spritePath: 'out/images/desktop/sprite.jpg',
        compositor: 'jimp'
    }, function (err) {
        console.log('Sprite generated!');
    });
});

… apologies if the promise syntax is incorrect, I haven’t used node actively since they became ubiquitous.

There is likely a way to use promises in the same way even if your sub calls are asynchronous, I just don’t have that ready.

Otherwise, you can pass a callback into your functions:

function resize(img, cb) {
    // ... you get the idea...
        file.resize(200, 300).quality(70).write(newPath, cb);
}

function rec(imgObjArray, cb) {
    // ... you get the idea...
    resize(imgObjArray.pop(), cb);
}

rec(imgObjArray, function(err, response) {
    nsg({
        src: [
            'out/images/desktop/*.jpg'
        ],
        spritePath: 'out/images/desktop/sprite.jpg',
        compositor: 'jimp'
    }, function (err) {
        console.log('Sprite generated!');
    });
});

Hope this helps!