Map to object with a mapping description

object-mapper.js

const f1 = a => a + 1
const f2 = (...args) => args.join(', ')

const src = { a: 1, b: 2, c: 3, d: 4, e: 5 }

const mapDesc = [
  'a',
  ['b', 'f'],
  [[f1, 'c'], 'g'],
  [[f2, 'd', 'e'], 'h']
]

const dest = map(src, mapDesc)

/*
 * Representation of the mapping:
 * dest = {
 *   'a': src['a'],
 *   'f': src['b'],
 *   'g': f1(src['c']),
 *   'h': f2(src['d'], src['e'])
 * }
 *
 * The object created:
 * {
 *   a: 1,
 *   f: 2,
 *   g: 5,
 *   h: '5, 6'
 * }
 */

// The `map` function as a oneliner:
function map (a, d) {
  return d.reduce(
    (r, v) => (
      (Array.isArray(v)
       ? (Array.isArray(v[0])
          ? r[v[1]] = v[0][0](...v[0].slice(1).map(k => a[k]))
          : r[v[1]] = a[v[0]])
       : r[v] = a[v])
      , r
  ), {})
}