NODEJS: Return array of arrays from dialog openDirectory

I am working in a electron desktop app, and what I need to do is:

1. Open a directory using ‘dialog.showOpenDialog’.
2. Filter the files by their extension.
3. Read filtered files (they have no header).
4. Parse them into columns and return only columns 4 and 6 (coordinates).
5. Return an array of arrays of all the files (Output example at the end).

With my little knowledge in js, this is my code so far, I’m stuck at point 4:

document.getElementById('btn-readfile').addEventListener('click', () => {

    dialog.showOpenDialog({
        properties: ['openDirectory']
    }).then(function(response) {
        if (!response.canceled) {
            dirname = response.filePaths[0] + '\'
            var fs = require('fs');
            var path = require('path');

            function readFiles(dirname, onFileContent, onError) {
                fs.readdir(dirname, function(err, files) {
                    if (err) {
                        onError(err);
                        return;
                    }
                    filesList = files.filter(function(e) {
                        return path.extname(e).toLowerCase() === '.txt' // ==> Filter files by extension
                    });
                    filesList.forEach(function(filesList) {
                        fs.readFile(dirname + filesList, 'utf-8', function(err, content) {
                            if (err) {
                                onError(err);
                                return;
                            }

                            onFileContent(filesList, content);

                        });
                    });
                });
            }
            var data = {};
            readFiles(dirname, function(filesList, content) {
                data[filesList] = content;
                console.log(data[filesList]);
            }, function(err) {
                throw err;
            });
        } else {
            console.log("no file selected");
        }
    });
}, false);

Raw file:

-1  2021-01-20  08:11:19    43.30981408167  N   13.73270596167  E   1.08    M   4
-1  2021-01-20  08:11:20    43.30981406000  N   13.73270596333  E   1.07    M   4
-1  2021-01-20  08:11:21    43.30981403667  N   13.73270598333  E   1.07    M   4
-1  2021-01-20  08:11:22    43.30981403833  N   13.73270598500  E   1.07    M   4
1   2021-01-20  08:11:23    43.30981406333  N   13.73270597333  E   1.07    M   4
2   2021-01-20  08:11:24    43.30981404833  N   13.73270598167  E   1.07    M   4
3   2021-01-20  08:11:25    43.30981459167  N   13.73270569667  E   1.08    M   4
9   2021-01-20  08:11:26    43.30981820000  N   13.73270345667  E   1.07    M   4

Desired Output: an array of arrays, where every array represent columns 4 and 6 from every file in the folder.

var latlng = [
                [
                    [ 45.64172279, 10.19579398],
                    [ 45.64193714, 10.1958776],
                    [ 45.64220345, 10.19598908],
                    [ 45.6423983, 10.19606341],
                    [ 45.6429504, 10.19632354],
                    [ 45.64329464, 10.19658367],
                    [ 45.64341805, 10.19758703]
                ],
                [
                    [ 45.64339856, 10.19838601],
                    [ 45.64313876, 10.1987855],
                    [ 45.64244377, 10.19869259],
                    [ 45.6418527, 10.19879479],
                    [ 45.6415669, 10.19715967],
                    [ 45.64170331, 10.19648147],
                    [ 45.64189167, 10.19615631]
                ]
            ];

Answer

Don’t cram everything into the event handler, that’s not reusable and has horrible maintainability. Make functions that take over the fundamental parts of your task.

First, top-level dependencies go to the top.

const fs = require('fs');
const path = require('path');

A function that reads a directory and returns a promise for an array of filenames:

function getFilesAsync(dirname) {
    return new Promise((resolve, reject) => {
        fs.readdir(dirname, function(err, files) {
            if (err) reject(err); else resolve(files);
        });
    });
}

A function that takes a filename and an optional encoding, and returns a promise for the file content:

function getFileContentAsync(filename, encoding) {
    return new Promise((resolve, reject) => {
        fs.readFile(filename, {encoding: encoding}, function (err, content) {
            if (err) reject (err); else resolve(content);
        });
    });
}

A function that takes a block of text and splits it into rows and columns at certain positions (since your data uses fixed-width columns):

function splitFixedColumnData(text, positions) {
    return text.split('n').map(line => 
        positions.concat(line.length).map( (pos, i) => 
            line.substring(positions[i-1] || 0, pos).trim() // from the previous to the current column pos
        )
    );
}

And a function that picks out certain elements from an array, so you can pick the columns you want to work with from the larger set of columns the previous function returns:

function pluckArray(arr, indexes) {
    return arr.reduce((result, v, i) => {
        if (indexes.includes(i)) result.push(v);
        return result;
    }, []);
}

And with all these defined, we can combine them to do something useful:

document.getElementById('btn-readfile').addEventListener('click', async () => {
    let dlg = await dialog.showOpenDialog({
        properties: ['openDirectory']
    });
    if (dlg.canceled) {
        console.log("no file selected");
        return;
    }

    try {
        let txtFiles = (await getFilesAsync(root))
            .filter(fn => path.extname(fn).toLowerCase() === '.txx')
            .map(fn => path.join(root, fn));
        let pendingContents = txtFiles.map(fn => getFileContentAsync(fn, 'utf-8'));
        let contents = await Promise.all(pendingContents);
        let columnData = contents.map(text => splitFixedColumnData(text, [4, 16, 28, 44, 48, 64, 68, 76, 80]));
        let latlng = columnData.map(rows => rows.map(row => pluckArray(row, [3, 5])));

        for (let i = 0; i < txtFiles.length; i++) {
            console.log(txtFiles[i], latlng[i]);
        }
    } catch (err) {
        console.log(err);
    }
}, false);