如何使用回调函数修复错误？

Question

I am trying to make an application that helps a user find a friend based on certain questions that they answered. I keep coming up with an error that says "TypeError [ERR_INVALID_CALLBACK]: Callback must be a function" when the code gets to line 46 of the file, which uses fs to alter the contents of another file to add the information that the user inputed on the webpage for this application and I have no idea why it is doing this.

const fs = require('fs');

module.exports = function(app, path) {

    app.get('api/friends', function(req, res) {
        fs.readFile("app/data/friends.js", "utf8", function(err, data) {
            if (err) throw err;

            else {
                res.json(JSON.parse(data));
            }
        });
    });

    app.post('/api/friends', function(req, res) {
        let results = [];

        const postResponse = JSON.stringify(req.body);

        fs.readFile('app/data/friends.js', function (err, data) {
            if (err) throw err; 

            let friendFile = JSON.parse(data);
            console.log(friendFile[0].answers);
            let closestMatch = 0;
            let matchScore = 999999999999999;

            for (let i = 0; i < friendFile.length; i++) {
                console.log(friendFile.length);
                let spaceBetween = 0;
                for (let j = 0; j < friendFile[i].answers.length; j++) {
                    // ['answers[]'][j]
                    console.log(req.body.answers[j]);
                    spaceBetween += Math.abs((parseInt(req.body.answers[j]) - parseInt(friendFile[i].answers[j])));
                }
                if (spaceBetween <= matchScore) {
                    matchScore = spaceBetween;
                    closestMatch == i;
                } 
            }

            results.push(friendFile[closestMatch]);

            friendFile.push(JSON.parse(postResponse));

            fs.writeFile("app/data/friends.js", JSON.stringify(friendFile));
                res.send(results[0]);
        })
    })
}

I expect this to edit the friends.js file to add all of the information from the responses that the user gave on the survey and for it to post the user's closest friend match on the page based on the answers that they gave.

Answer 1

Just add the callback function when calling writeFile

let callback = function(err) {
 if (err) throw err;
  console.log('The file has been saved!');
};
fs.writeFile("app/data/friends.js", JSON.stringify(friendFile), callback);

Answer 2

I'm guessing this is an express app yeah? Just change..

fs.writeFile("app/data/friends.js", JSON.stringify(friendFile));

...to...

fs.writeFile("app/data/friends.js", JSON.stringify(friendFile), function (err) {
  if (err) {
    res.sendStatus(500) // Let the client know we died
    throw err; // Or do something else.
  }

  // If you want to add something for after the file is written to disk put it here.
  // Code execution won't reach this point if the error above throws.
})