JavaScript / Node.js + Express-函数在完成之前返回未定义

Question

So I'm having a lot of trouble getting my module to return the value I want it to. Basically it looks into my filesystem and returns the last created GIF file. If there is no GIF, it looks for a PPM (another file type) and generates a GIF and returns that. If there's no PPM, it'll look into the MySQL database and generate a GIF based on what's in there. All of this works fine.

This module is called with a HTTP request in Express, where currentImg() is calling the module:

//REQUEST CURRENT IMAGE
app.get('/img', function(req, res) {
    console.log(currentImg());
});

This request always returns 'undefined'. I've tried a bit of debugging and the module is returning 'undefined' before it finishes getting the return from the 'queryDatabase(returnFile)' line in the module.exports function.

// CurrentImg

module.exports = function() {
    imageFile = "";
    queryDatabase(returnFile);
    return imageFile;
}

function queryDatabase(callback) {
    imageFile = "";
    client.query("SHOW TABLES FROM mathsDB", function(err, result) {
            if (err)
                    console.log(err);
            else
                    currentImage = result[result.length - 1]["Tables_in_mathsDB"];
                    currentImage = currentImage.substring(5);
                    callback;
    });
}

function returnFile() {
    fs.stat("./img/image" + currentImage + ".gif", function(err, stat) {
            if (err==null) {
                    imageFile = "/img/image" + currentImage + ".gif";
                    return imageFile;
            }
            else {
                    fs.stat("./img/image" + currentImage + ".ppm", function(err, stat) {
                            if (err==null) {
                                    convert.convert("image" + currentImage);
                                    imageFile = "/img/image" + currentImage + ".gif";
                                    return imageFile;
                            }

                            else {
                                    exportPPM.make();
                                    setTimeout(waitConvert, 500);
                                    function waitConvert() {
                                            convert.convert("image" + currentImage);
                                            imageFile = "/img/image" + currentImage + ".gif";
                                            return imageFile;
                                    }
                            }
                    });
            }
    });
}

I've tried doing some reading into callbacks, as you can see in my code, however that didn't seem to solve anything. I've read a bit in to promises too but they don't seem to provide a solid solution either.

Can anyone provide me with a pointer in the right direction to get this running in a non-blocking way?

Thanks.

Answer 1

Use async waterfall to run function in series one after another. https://github.com/caolan/async#waterfall . So something like this.

async.waterfall([
function (callback) {
imageFile = "";
client.query("SHOW TABLES FROM mathsDB", function(err, result) {
        if (err)
                console.log(err);
        else
                currentImage = result[result.length - 1]["Tables_in_mathsDB"];
                currentImage = currentImage.substring(5);
                callback(currentImage)
});

},

    function(currentImage, callback) {
      // arg1 now equals 'one' and arg2 now equals 'two'
      fs.stat("./img/image" + currentImage + ".gif", function(err, stat) {
            if (err==null) {
                    imageFile = "/img/image" + currentImage + ".gif";
                    return imageFile;
            }
            else {
                    fs.stat("./img/image" + currentImage + ".ppm", function(err, stat) {
                            if (err==null) {
                                    convert.convert("image" + currentImage);
                                    imageFile = "/img/image" + currentImage + ".gif";
                                    return imageFile;
                            }

                            else {
                                    exportPPM.make();
                                    setTimeout(waitConvert, 500);
                                    function waitConvert() {
                                            convert.convert("image" + currentImage);
                                            imageFile = "/img/image" + currentImage + ".gif";
                                            return imageFile;
                                    }
                            }
                    })
            }
  callback(null,currentImage)  })


    }

], function (err, currentImage) {
    console.log(currentImage);
});