Resolve promise when multiple writeFile functions finished after for-loop in node.js

Question

First of all, here is a part of my current code:

return new Promise(function(resolve, reject) {
        for (var i = 1; i <= variables; i++) {
            fs.writeFile("file-" + i + ".txt", dataString + ' ' + i + ' 0', "utf-8");
        }   
        if (i == variables)
            resolve('Files have been written');
        else
            reject('Some error occured');
    })

My problem is that I don't know how to tell my promise to resolve after all files have been written as writeFile() is async. What I want to do is to use these files in my next then() when they are completely written to my drive. Can anybody help me out here? I'm kind of lost right now and also new to node.js so please bear with me :)

Thanks in advance!

#UPDATE 1:

    var promises = [];
    for (var i = 1; i <= variables; i++) {
        promises.push(Promise.resolve(
            new Promise(function(resolve, reject) {
                fs.writeFile('file-' + i + '.cnf', dimacs2 + ' ' + i + ' 0', 'utf-8', function(err) {
                    if (err) {
                        reject(err);
                    }
                    else {
                        resolve();
                    }
                });
            }
        )));
    }
    Promise.all(promises).then(function() {
        // code that runs after files have been written
    });

Answer 1

Maybe you're looking for something like this:

Promise.all(
  variables.map(function (variable, i) {
    return new Promise(function (resolve, reject) {
      fs.writeFile("file-" + i + ".txt", dataString + ' ' + i + ' 0', "utf-8", function (err) {
        if (err) {
          reject(err);
        } else {
          resolve();
        }
      });
    });
  })
)
.then(function () {
  // Files have been written
})
.catch(function (err) {
  // Some errors occured
});