如何从回调函数获取值到外部作用域？

Question

I have a function getHighScores which I want to go into a JSON file and get score objects for a given game , sort them, and return the top amount of them.

This is my (broken) code:

function getHighScores(game, amount) {
    var highScores = null;
    fs.readFile('scores.json', 'utf8', function (error, data) {
        var scoresObj = JSON.parse(data);
        var gameScores = scoresObj.games[game];
        sortedScores = gameScores.sort(compareScoreObjects);
        highScores = sortedScores.slice(0, amount);
    });
    return highScores;
}

console.log(getHighScores('snake', 10));

This is logging null , because highScores cannot be accessed within the scope of the callback function for fs.readFile .

Is there any way to do this while maintaining the logic of my code - where I can simply have getHighScores return the data? If not, how should I be thinking about this problem?

Answer 1

The problem here is you're using readFile, which is an asynchronous method. Meaning by the time this method resolves, highScores is already returned.

Checkout the readFileSync method ( https://nodejs.org/api/fs.html#fs_fs_readfilesync_path_options ) which is a synchronous version of readFile.

You can then change your code as follows.

function getHighScores(game, amount) {
  var scoresObj = fs.readFileSync('scores.json', 'utf8');
  var parsedScoresObj = JSON.parse(data);
  var gameScores = parsedScoresObj.games[game];
  var sortedScores = gameScores.sort(compareScoreObjects);
  var highScores = sortedScores.slice(0, amount);
  return highScores;
}

Answer 2

From what I have read, fs.readfile is async. So highScores is returning immediately instead of wating to read the file. You could change the code to be synchronous.

function getHighScores(game, amount) {
var highScores = null;
var data = fs.readFileSync('scores.json', 'utf8')
var scoresObj = JSON.parse(data);
var gameScores = scoresObj.games[game];
sortedScores = gameScores.sort(compareScoreObjects);
highScores = sortedScores.slice(0, amount);
return highScores;
}

console.log(getHighScores('snake', 10));

If you want to keep it asynchronous though then you should check this link

How do I return the response from an asynchronous call?