CODEWITHSUNDEEP
node.jspromise
tonymx227
tonymx227

Reputation: 5451

Promises with NodeJS

I'd like to use promises with NodeJS but it seems not working as expected.

I'd like to see LOG 1, LOG 2 and LOG 3 for all files in this order. And LOG 4 is hidden...

But I get this :

LOG 1
LOG 3
LOG 1
LOG 3
LOG 1
LOG 3
SUCCESS LOG 2
SUCCESS LOG 2
SUCCESS LOG 2
SUCCESS LOG 2

My code :

var filePromise = _.map(files, function(file) {

var filePath = './sql/' + file;

fs.lstat(filePath, function(err, stats) {

  if (stats.isFile() && file !== '.gitignore' && file !== 'index.js') {

    fs.readFile(filePath, 'utf-8', function(err, data) {

      console.log('LOG 1');

      db.query(data).then(function() {
        console.log(colors.green('SUCCESS LOG 2'));
      }).catch(function() {
        console.log(colors.red('ERROR LOG 2'));
      });

      console.log('LOG 3');

    });
  }
});
});

Promise.all(filePromise).then(function() {
  console.log(colors.green('LOG 4'));
});

Upvotes: 0

Views: 92

Answers (1)

Luke Stoward
Luke Stoward

Reputation: 1540

In order for filePromise to contain a collection of promises, you need to return a promise for each iteration of your map call. Only then will the call to Promise.all be evaluated correctly.

var filePromise = _.map(files, function(file) {
    return new Promise(function(resolve, reject) {
        var filePath = './sql/' + file;

        fs.lstat(filePath, function(err, stats) {

            if (stats.isFile() && file !== '.gitignore' && file !== 'index.js') {

                fs.readFile(filePath, 'utf-8', function(err, data) {

                    console.log('LOG 1');

                    db.query(data).then(function() {
                        console.log(colors.green('SUCCESS LOG 2'));
                        resolve();
                    }).catch(function() {
                        console.log(colors.red('ERROR LOG 2'));
                        reject();
                    });

                    console.log('LOG 3');

                });
            } else {
                reject(new Error("failed condition"))
            }
        });
    })
});

Promise.all(filePromise).then(function() {
    console.log(colors.green('LOG 4'));
});

This does not fix the ordering of your console logs. That is a separate issue. Your console logs are not actually out of order, they are logging as expected. However, this is due to the async call you are making to db.query which, when triggered does not block waiting for a response. Instead the next line is executed immediately after the function call returns.

Side note: Make sure to handle your err arguments passed to the inline callback functions of fs.lstat and fs.readFile. Failure to do so will result in the promise never completing in the event of an error.

Upvotes: 1

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