How to chain q.nfcall syncronous

Question

I read some tutorials but I don't understand how to chain various promises to read files but one after the other, I think its really easy, but I dont get it.

The idea is open one file, read the filename of the other file, open that file and then start the express server. I got something like this but I think I'm doing not really good, its working but I dont like it.

My code looks like this:

Q.nfcall(fs.readFile, path.resolve(__dirname, './config/db/default.connection.json'), "utf-8")
    .then(function(data){   ...
          /*HERE I OPEN OTHER 'Q' PROMISE FOR READ OTHER FILE AND START SERVER*/
             Q.nfcall(..). then(function(data2){ ... })
                   .catch(function(err){.. });
                  }). fail(function(err){...})
                    .done();
})
        .catch(function(err) {  ...});
})
       .fail(function(err) {..  })
        .done();

Answer 1

You can chain promises by returning a new promise from a .then() handler,

Q.nfcall(fs.readFile, 'file1.txt').then(function(data1) {
  console.log('file 1 read');
  return Q.nfcall(fs.readFile, 'file2.txt'); // return a new promise
}).then(function(data2) {
  console.log('file 2 read');
}).catch(function(err) {
  ...
}).done();

Errors/exceptions will be propagated along the chain as well.

EDIT: in case you want to errors reading file1.txt to not propagate, but instead handled in a different way (create the file instead), you could use something like this:

Q.nfcall(fs.readFile, 'test1.txt', 'utf-8').then(null, function(err) {
  // create an empty file (or whatever)
  return Q.nfcall(fs.writeFile, 'test1.txt', '').then(function() {
    return Q.nfcall(fs.readFile, 'test1.txt', 'utf-8');
  });
}).then(function(data) {
  console.log('data 1', data);
  return Q.nfcall(fs.readFile, 'test2.txt', 'utf-8');
}).then(function(data) {
  console.log('data 2', data);
}).catch(function(err) {
  ...
});

(which can probably be done even more concise)