Batches of concurrent jobs with js promises

test.js

var http = require('httpism');
var _ = require('underscore');

var p1 = () => http.get('http://google.com').then(() => console.log('p1'))
var p2 = () => http.get('http://google.com').then(() => console.log('p2'))
var p3 = () => http.get('http://google.com').then(() => console.log('p3'))
var p4 = () => http.get('http://google.com').then(() => console.log('p4'))

var a1 = () => Promise.all([p1(), p2()]).then(() => console.log('a1'))
var a2 = () => Promise.all([p3(), p4()]).then(() => console.log('a2'))

_.reduce([a1, a2], (seq, p) => seq.then(p), Promise.resolve()).then(() => console.log("THE END"))

// `reduce` is the same as:
// Promise.resolve().then(a1).then(a2).then(() => console.log("THE END"))