function consine(x, y) {
  var z = x.reduce(function(sum, cur, i) {
    return sum + x[i] * y[i]
  }, 0)
  var xSum = x.reduce(function(sum, cur) {
    return sum + Math.pow(cur, 2)
  }, 0)
  var ySum = y.reduce(function(sum, cur) {
    return sum + Math.pow(cur, 2)
  }, 0)
  var m = Math.sqrt(xSum * ySum)
  return z / m
}

console.log(consine([3, -1, 8], [-2, 3, -6]))