承诺循环内等待

Question

I have this code:

var services = ['EC2', 'S3', 'RDS', 'IAM']
var promises = [];

for(var i = 0; i < services.length; i++) {
   var promise = awsStatus('us-east-1', services[i])
      promises.push(promise);
      console.log(promises);
}

q.all(promises).then(function(data){
   console.log(promises);
});

it's supposed to loop on the services array with the awsStatus method. The problem is that sometimes I get all the results I want:

{ service: 'IAM', status: 0 }
{ service: 'EC2', status: 0 }
{ service: 'RDS', status: 0 }

But sometimes I get incomplete results. I thought I needed .then after awsStatus but that also didn't resolve this. What else is wrong in this piece of code?

since now there's a comment, I've also tried this :

var services = ['EC2', 'S3', 'RDS', 'IAM']
var promises = [];

for(var i = 0; i < services.length; i++) {
   var promise = awsStatus('us-east-1', services[i]).then(function(promise){
      promises.push(promise);
      //console.log(promises);
});
}
q.all(promises).then(function(data){
   console.log(promises);
});

and it produces the same results.

Answer 1

Try using .each

Promise.each(services, function(service) {
  return awsStatus('us-east-1', service).then(function(promise){
      promises.push(promise);
      //console.log(promises);
  });
}).then(function() {
  console.log('Done with all instances');
});

Answer 2

I can suggest a solution using async library as follows:

var services = ['EC2', 'S3', 'RDS', 'IAM']
async.map(services, 
  function(item, done){
    awsStatus('us-east-1', item).then(
      function(){
        done(null, "Completed item " + item);
      }, 
      function(err){
        done(err + " in item " + item, null);
      }
    );
  }, 
  function(err, results){
    if(err) return console.log(err);
    console.log(results);
  }
);

It will fail and stop executing if one of the promises returns an error. If dont want to stop you can return null in the error function of promise instead of error message as the first argument of done method

Answer 3

There are two problems in your code:

1. Using q.all() does not guarantee that the code will wait for all the promises to resolve. If one of them is rejected, the then method would be called, not waiting for the rest of the promises. You can solve that by using q.allResolved() instead.
2. You are logging the wrong objects. You should be logging the results of the promises, and not the promise objects themselves.

Example of how to solve both problems:

q.allResolved(promises).then(function(results) {
  console.log(results);
}).catch(function(err) {
  console.error(err);
});