node.js force async processes to run in series

Node.js is asynchronous I/O that other processes can start and doesn’t have to wait for some long running input/output processes such read and write from files or databases. For example

queryDatabase(); //assume this will take some time, few seconds
console.log('This print statement will run before the previous database query function');

For whatever reason if you want to force the long running process to run one after another in series. Here is an example for it

// A list of items need to be processed in series
var items = [ 1, 2, 3, 4, 5],
    startIndex = 0,
    endIndex = items.length,
    asyncResults = [],
    seriesResults = [];

// An long time async process, multiple the arg by 2 and pass it to the callback 
// after 2 seconds has passed
// This could be I/O process such as read/write from file or database
function longTimeProcess(arg, callback) {
  setTimeout(function() { 
    callback(arg * 2); 
  }, 5000);
}

// This is called when all items are processed.
function done() { 
  console.log('series results: ', seriesResults); 
}

//recursive call to force each process to run after the previous one is finished.
function runAsyncProcessesInSeries(itemIndex) {
  if( itemIndex < items.length ) {
    longTimeProcess( items[itemIndex], function(result) {
      console.log(result);
      seriesResults.push(result);
      return runAsyncProcessesInSeries( ++itemIndex );
    });
  } else {
    return done();
  }
}

runAsyncProcessesInSeries(startIndex);