zipmerge.js

function zipmerge(/* ...lists */) {
  var lists = Array.prototype.slice.call(arguments)

  // find longest list
  var nlists = lists.length, maxlen = 0, i
  for (i = 0; i < nlists; i++) {
    maxlen = Math.max(arguments[i].length, maxlen)
  }

  // populate result
  var res = new Array(maxlen * nlists), L, listi, x, maxi = 0
  for (listi = 0; listi < nlists; listi++) {
    L = lists[listi]
    for (i = 0; i < L.length; ++i) {
      x = listi + (i * nlists)
      res[x] = L[i]
      if (x > maxi) {
        maxi = x
      }
    }
  }

  res.length = maxi + 1
  return res
}

console.log(
  zipmerge(
    [1,  2,  3,  4],
    [10, 20, 30, 40],
    ['a','b','c','d']
  )
)
// [1, 10, "a", 2, 20, "b", 3, 30, "c", 4, 40, "d"]

console.log(
  zipmerge(
    [1,  2,  3,  4],
    ['a']
  )
)
// [1, "a", 2, empty, 3, empty, 4]

console.log(
  zipmerge(
    [1,  2,  3,  4],
    [10, 20, 30     ],
    ['a','b','c','d']
  )
)
// [1, 10, "a", 2, 20, "b", 3, 30, "c", 4, <empty>, "d"]