enumerable.js

function Enumerable(arr) {
  this.toArray = function () {
    return arr
  }

  this.filter = function (fn) {
    return new Enumerable(arr.filter(fn))
  }
  
  this.map = function (fn) {
    return new Enumerable(arr.map(fn))
  }
  
  this.flatMap = function (fn) {
    return new Enumerable(_.flatMap(arr, fn))
  }
}

var a = [ [1,2], [3,4,5], [6,7] ]
var a = [ [11,12], [13,14,15], [16,17] ]

new Enumerable(a)
  .filter(function (a) { return a.length % 2 == 0 })
  .flatMap()     //=> [1, 2, 6, 7]


// Or, just add _.flatMap to Array's prototype:
// Array.prototype.flatMap = function (fn) { return _.flatMap(this, fn) }

// Build a pipeline that can be applied to different inputs
var pipeline = Enumerable
  .filter(function (a) { return a.length % 2 == 0 })
  .flatMap()

// Mostly equivalent to:
// pipeline = function (input) {
//   return Enumerable(input)
//   .filter(function (a) { return a.length % 2 == 0 })
//   .flatMap()
// }

pipeline.toString()   //=> filter(fn.name) | flatMap()
pipeline(a)           //=> [1, 2, 6, 7]
pipeline(b)           //=> [11, 12, 16, 17]