Bathroom Security

{
  'U': [-1,0],
  'R': [0,1],
  'D': [1,0],
  'L': [0,-1]
}

[
  [1,2,3],
  [4,5,6],
  [7,8,9]
]

For each line from the input
  Accumulate a code - starting as an empty string
  For each character in the line
    Accumulate a 2-element array of coordinates
    If the next location is a valid cell
      Return the coordinate of the next location
    Else
      Return the coordinate of the current location
  Add the value at the final cell to the string

input.reduce(
  (code, line) => {
    key = line.reduce(
      (coords, path) => 
        keypad[coords[0] + map[path][0]] &&
        keypad[coords[0] + map[path][0]][coords[1] + map[path][1]]
        ? [ coords[0] + map[path][0],
            coords[1] + map[path][1] ]
        : coords,
        key.slice()
    )
    return code += keypad[key[0]][key[1]]
  }, ""
)

[
  [0,  0,  1,  0,  0],
  [0,  2,  3,  4,  0],
  [5,  6,  7,  8,  9],
  [0, 'A','B','C', 0],
  [0,  0, 'D', 0,  0]
]

input.reduce(
  (code, line) => {
    key = line.reduce(
      (coords, path) => 
        keypad[coords[0] + map[path][0]] &&
        keypad[coords[0] + map[path][0]][coords[1] + map[path][1]] &&
        keypad[coords[0] + map[path][0]][coords[1] + map[path][1]] !== 0
        ? [ coords[0] + map[path][0],
            coords[1] + map[path][1] ]
        : coords,
        key.slice()
    )
    return code += keypad[key[0]][key[1]]
  }, ""
)