word_length.js

function length(xs) {
  var max = 0
  for(var i = 0; i < xs.length; i++) {
    for(var j = i+1; j < xs.length; j++) {

      if (card(xs[i], xs[j]) == xs[i].length + xs[j].length) {
        var mul = xs[i].length * xs[j].length
        max = mul > max ? mul : max
      }
    }
  }
  return max
}

function card(xs, ys) {
  var seen = {}
  ;[xs,ys].forEach(xs => [].slice.call(xs).forEach(x => seen[x] = true))
  return Object.keys(seen).length
}

console.log(length(["abcw", "baz", "foo", "bar", "xtfn", "abcdef"]), 16)

console.log(length(["a", "ab", "abc", "d", "cd", "bcd", "abcd"]), 4)

console.log(length(["a", "aa", "aaa", "aaaa"]), 0)