for循环内的async.waterfall逃脱了for循环

Question

On a Form Action of type POST , we fetch all the values in Node.JS/Express and try saving it into MongoDB .

A hidden field determines the length of a property from the frontend javascript and it's value is updated as the hidden field's value.

This length is used in the backend (Node) to iterate over a list of items.

I have a async.waterfall function and a for loop running inside it like this.

async.waterfall([
function(callback){
       var itemLength = req.body.itemLength;
        var itemProp,itemComponent;
        var destination;
        var destinationsArray =[];

        for(var k=1; k<=itemLength; k++){

            destination = new Destination({
                name: req.body['destinationName'+k],
            });

            itemComponent = {
              "itemCompProp" : req.body['itemCompProp'+k]
            };


            itemProp = new ItemProp({
               itemComponent: itemComponent
            });

            itemProp.save(function(err,itemPropSaved){
              destination.newProperty = itemPropSaved._id

              destination.save(function(err,destinationSaved){
                if(err){
                  console.log("Error== " + err);
                }
                else{
                  destinationsArray.push(destinationSaved._id);
                }
              });

            });
         }// End of For
  callback(null,destinationsArray);
},
function(destinationsArray,callback){
   var brand = new Brand({
    name : req.body.brandName,
  });

  brand.save(function(err,brandSaved){
      if(err){
          console.log("Error== " + err);
        }else{
            console.log('Brand Saved');
        }
   });
   callback(null);
}
], function (err, status) {
  if(err){
    req.flash('error', {
          msg: 'Error Saving Brands'
      });

     console.log("Error : " + err); 
  }  
  else{
      console.log("Brand Saved."); 
      req.flash('success', {
          msg: 'Brand Successfully Added!'
      });
  }
});

res.redirect('/redirectSomewhere');

When we run this, The destinationsArray is returned first as null , as opposed to going through the for loop and then returning the proper value of destinationsArray over a length ( itemLength ) of destinations.

We want the process to be synchronous. We also tried using a closure wrapping the for Loop but to no avail.

We can't use a async.eachSeries instead of the for Loop as I am just iterating over a numeric property and we don't have any documents to iterate over

Any feasible solution to run a for Loop inside a async.waterfall ?

Cheers and Thanks in Advance.

Answer 1

There are few problems with the code you have there:

1. where the callbacks got called.
2. where res.redirect() got call.
3. the for loop.

save() is asynchronous. Regular for loop will just continue without waiting for all save() calls to finish. That's why destinationsArray is empty. As you said, you cannot use async.eachSeries() since you're iterating through numeric property. However, you're on the right track there. Async.whilst() does just that. Here is the revised code with Async.whilst() and proper calling locations of the callbacks:

async.waterfall([
  function(callback){
    var itemLength = req.body.itemLength;
    var itemProp,itemComponent;
    var destination;
    var destinationsArray =[];
    var k = 1;  // 1st part of for loop:  for(k=1; k<=itemLength; k++)

    async.whilst(
      function() {
        return k <= itemLength;  // 2nd part of for loop:  for(k=1; k<=itemLength; k++)
      },
      function(whilstCb) {
        destination = new Destination({
          name: req.body['destinationName'+k]
        });

        itemComponent = {
          "itemCompProp" : req.body['itemCompProp'+k]
        };

        itemProp = new ItemProp({
          itemComponent: itemComponent
        });

        itemProp.save(function(err,itemPropSaved){
          destination.newProperty = itemPropSaved._id

          destination.save(function(err,destinationSaved){
            if(err){
              console.log("Error== " + err);
            } else {
              destinationsArray.push(destinationSaved._id);
            }
            k++;  // 3rd part of for loop:  for(k=1; k<=itemLength; k++)
            whilstCb(null);
          });
        });
      },
      function(err) {
        // It gets here once the loop is done
        console.log(destinationsArray);  // This array should have all the values pushed
        callback(null, destinationsArray);
      }
    );
  },
  function(destinationsArray,callback){
    var brand = new Brand({
      name : req.body.brandName
    });

    brand.save(function(err,brandSaved){
      if(err){
        console.log("Error== " + err);
      } else {
        console.log('Brand Saved');
      }
      callback(null);
    });
  }
], function (err, status) {
  if(err){
    req.flash('error', {
      msg: 'Error Saving Brands'
    });
    console.log("Error : " + err);
  } else {
    console.log("Brand Saved.");
    req.flash('success', {
      msg: 'Brand Successfully Added!'
    });
  }
  res.redirect('/redirectSomewhere'); 
});

Answer 2

The issue has to do with callback(null, destinationsArray); getting called outside the for loop without checking first to see the loop has been finished.

Try replacing callback(null, destinationsArray); with something like this:

if (itemLength > 0 && destinationsArray.length === k - 1)  {
    callback(null, destinationsArray);
} else {
    callback(true);
}

The above checks to make sure the destination.save() gets completed the proper number of times successfully.

I actually prefer the method proposed by djskinner. However, because of the console.log() that occurs when there is a save() error, the callbacked destinationsArray could possibly hold the incorrect number of items. To fix this, you could make sure to replace the console.log("Error== " + err); with something like callback(err) to end the waterfall with the error returned. In addition, the k === itemLength check doesn't properly account for the correct number of items that should be saved. This should be replaced with k === destinationsArray.length .

I made modifications to fix this and posted an updated version below.

destination.save(function(err, destinationSaved){
    if (err) {
        callback(err);
    }
    else {
        destinationsArray.push(destinationSaved._id);
        if (k === destinationsArray.length) {
            callback(null, destinationsArray);
        }
    }
});

--EDIT-- I really like the solution that Ben posted using whilst() . This allows the creation of a loop where the iterations runs serially. For more info, view the npm page here .

Answer 3

Its not so much the for loop that is causing you problems but that save is an asynchronous operation. The for loop completes and the callback is executed before any of the save callbacks have had chance to complete.

What you want to do is call the async.waterfall callback after all the destination save callbacks have been executed. Something like:

         destination.save(function(err,destinationSaved){
            if(err){
              console.log("Error== " + err);
            } else {
              destinationsArray.push(destinationSaved._id);
              if (k === itemLength) {
                  // all destination callbacks have completed successfully
                  callback(null, destinationsArray);
              }
            }
          });