Callback not returning data

Question

I am trying to make a convolutional neural network and activate it using a single function which returns the data. I am having trouble with the asynchronous part because when I try to callback the data and log it, it returns an empty object.

const convolute = (callback) => {
let inputData = {};

  for(let p = 0; p < files.length; p++){
    let fileData = [];
    let pixels = [];
    let dimensions = [];
    let image, iterationArr;
    let FinalVector = [];
    fs.readFile("./imgs/" + files[p], "utf8", (err, data) => {
      if(err) console.log(err);

      fileData = data.split("\n");
      dimensions = fileData[0].split(" ").map(l => parseInt(l));
      pixels = fileData[1].split(" ").map(p => parseInt(p));
      pixels.splice(pixels[pixels.length-1], 1);
      pixels = MM.convertToMatrix(pixels, dimensions[0]);

      image = new ImageProcess(pixels);
      for(let i = 0; i < edges.length; i++){
        iterationArr = image.pixels;
        let iteration = 0
        while(iterationArr.length > 30){
          iteration++;
          iterationArr = image.start(iterationArr, edges[i]);
        }
        let iterationVector = MM.convertToVector(iterationArr)
        for(let i = 0; i < iterationVector.length; i++){
          FinalVector.push(iterationVector);
        }
      }//end of each file
      let fileName = files[p].substring(0, files[p].length -4);
      inputData[fileName] = FinalVector;
    });//end of reading file
  }//new file reading starts here

  callback(inputData)
}//takes roughly around 5 seconds to execute

convolute((data) => {
  console.log(data);
})

files is an array that stores all the file names. FinalVector is the array that contains 2000+ items. I've done some testing and the code works, I know this because it works when I log each files vector separately it returns the data, but there is trouble when I use a callback to return the data from the function.

PS. I've only included the relevant parts of my code that may contribute to my problem.

Jérôme Migné · Accepted Answer · 2018-07-15 19:54:23Z

inputData is empty because readFile is asynchronous : callback(inputData) is called before the callbacks of each readFile where inputData is filled.

So callback(inputData) should be called after all the (asynchronous) readFile are processed : inside the callback of readFile once all the files are read :

const convolute = (callback) => {
    let inputData = {};
    let remainings = files.length;
    if (!remainings) {
        callback(inputData);
        return;
    }
    for (let p = 0; p < files.length; p++) {
        fs.readFile("./imgs/" + files[p], "utf8", (err, data) => {
            if (err) {
                console.log(err);
            }
            else {
                /* process data simplified */
                inputData[files[p]] = data;
            }

            if (--remainings <= 0) {
                callback(inputData);
            }
        });
    }
};

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