nodejs-async: How to use a loop containing an asynchronous call within an "async.series" function block

Question

While using an async.series based design, I have to use a loop within a function block as shown below:

var someFile = filePath;
async.series([
    function(cb) {
         _.forEach (
               someDataList,
               function (item) {
                   //call that executes out of sync
                   writeToFile(someFile,item)
                   //how to provide callback here
                   cb(); //<- is this correct - won't this take control out of this block in the very first loop iteration?
               }
         );
    }
],
//callback function
function (err) {...}
);

you need to complete the loop and go to the next function?

zabusa
– zabusa

2018-01-09 10:52:43 +00:00
Commented Jan 9, 2018 at 10:52 — zabusa
– zabusa, Commented Jan 9, 2018 at 10:52

Nitin Bhapkar · Accepted Answer · 2018-01-10 06:44:09Z

2

You can use forEachSeries instead.

var someFile = filePath;
async.forEachSeries(someDataList, function(item, cb) {
   writeToFile(someFile, item)
   cb();
},
//callback function
function () {...}
);

Updated after comment:

Unfortunately there is no way of getting index of iteratee in forEachSeries. if you want index of iteratee, you can use eachOfSeries

var someFile = filePath;
async.eachOfSeries(someDataList, function(value, key, callback) {
    //Do something with key
    writeToFile(someFile, item)
    callback();
}, //callback function
function () {...}
);

edited Jan 10, 2018 at 6:44

answered Jan 9, 2018 at 11:14

Nitin Bhapkar

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1 Comment

Ulysses Over a year ago

Thanks - could you also please let me know how to obtain the index of the iteratee within forEachSeries - I looked up online but couldn't find a clear example

Yevhenii Herasymchuk · Accepted Answer · 2018-01-09 11:16:35Z

To explore more about async, you can use their method each.

Applies the function iteratee to each item in coll, in parallel. The iteratee is called with an item from the list, and a callback for when it has finished. If the iteratee passes an error to its callback, the main callback (for the each function) is immediately called with the error.

Your code can be transformed to this:

var someFile = filePath;
async.series([
    function (cb) {
      async.each(someDataList, function (item, callback) {
        //call that executes out of sync
        writeToFile(someFile,item)

        // Execute internal callback for `async.each`
        callback()
      }, function (error) {
        if (error) {
          cb(error)
        } else {
          cb()
        }
      })
    }
  ],
//callback function
  function (err) {...
  }
);

But, of course, you need to take a look at your writeToFile function. If it is sync you need to wrap it in try {} catch(e){} construction and run internal callback() with error callback(error) in catch section.

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