bluebird each vs map vs map (with concurrency) vs mapSeries

gistfile1.txt

const bluebird = require('bluebird');

function mapItem(item) {
  console.log('mapItem called for ' + item);
  return bluebird.delay(1000).then(() => {
    console.log('returning for ' + item);
    return item * 10;
  });
}

// bluebird.each prints the following:
// > mapItem called for 1
// > returning for 1
// > mapItem called for 2
// > returning for 2
// > mapItem called for 3
// > returning for 3
// > mapItem called for 4
// > returning for 4
// > [ 1, 2, 3, 4 ]
bluebird.each([1,2,3,4], mapItem).then(console.log);

// bluebird.map prints the following:
// > mapItem called for 1
// > mapItem called for 2
// > mapItem called for 3
// > mapItem called for 4
// > returning for 1
// > returning for 2
// > returning for 3
// > returning for 4
// > [ 10, 20, 30, 40 ]
bluebird.map([1,2,3,4], mapItem).then(console.log);

// bluebird.map with concurrency prints the following:
// > mapItem called for 1
// > returning for 1
// > mapItem called for 4
// > returning for 4
// > mapItem called for 3
// > returning for 3
// > mapItem called for 2
// > returning for 2
// > [ 10, 20, 30, 40 ]
bluebird.map([1,2,3,4], mapItem, {concurrency: 1}).then(console.log);

// bluebird.mapSeries prints the following:
// > mapItem called for 1
// > returning for 1
// > mapItem called for 2
// > returning for 2
// > mapItem called for 3
// > returning for 3
// > mapItem called for 4
// > returning for 4
// > [ 10, 20, 30, 40 ]
bluebird.mapSeries([1,2,3,4], mapItem).then(console.log);