How to write data from asynchronous function?

Question

JavaScript

For example, there is such a function

function getFiles(directoryPath, arrFiles, callback) {
    fs.readdir(directoryPath, (err, files) => {
        if (err) console.log(err);
        ... //здесь добавляю в массив файлы
        callback(arrFiles);
    });
});

And here I want to get the data, but it outputs undefined

var file;
getFiles("./",[], function(result){
    file = result;
    return file;
});

How to write data to a variable?

Answer 1

Since two answers cannot be added, this answer will have two parts.
Most likely you are doing something like this:

var file;
getFiles("./",[], function(result){
    file = result;
    return file;
});
console.log(file);

If yes, then this is a common mistake for those who are starting to deal with asynchrony.
If you do this, then this is what will happen :
1. A variable has been declared
2. "Create a request to receive files. When they are received, call the callback" (very roughly)

getFiles("./",[], function(result){
    file = result;
    return file;
});

3. Output file (empty)
4. Files received, callback is called. Return doesn't get anywhere from here.

file = result;
return file;

Option 1:

getFiles("./",[], function(result){
    var file = result;
    // Дальше работаем с file здесь
});

Option 2:

function getFiles(directoryPath) {
    return new Promise((resolve, reject) => {
    	fs.readdir(directoryPath, (err, files) => {
         let arrFiles = [];
         // ... Добавляете в arrFiles нужную информацию
               if (err) {
          	console.log(err);
          	reject(err);
          }
          resolve(arrFiles);
      });
    })
};

// где-то выше
async function someFunction() { // Функция, где находится var file и остальное
  // ...

  try {
    var file = await getFiles("./"); // Здесь await заставит ваш код "ждать", пока из Promise в getFiles не будет вызван resolve
    console.log(file);
  } catch (e) {
    // Сделать что-то с ошибкой
  }

  // ...
}

Add the debugger keyword to your code before calling getFiles, like this:

var file;
debugger;
getFiles("./",[], function(result){
    file = result;
    return file;
});

Before running the script, open chrome://inspect. Click on "Open dedicated DevTools for Node".
Run the script with the --inspect option, like this:
After running the script, it will stop and you can execute it gradually, understanding where the error is. In the debugger on the right-top there will be several buttons with different transition options, you need "Step", this is a dot and a straight arrow to the right. When you click on it, the current operation will be performed and the code will stop again at the next one.