deepFlatten.js

const deepFlatten = function(arr) {
  const flatten = function(arr) {
    return [].concat(...arr)
  }
  return flatten(           // return shalowly flattened array
    arr.map(x=>             // with each x in array
      Array.isArray(x)      // is x an array?
        ? deepFlatten(x)    // if yes, return deeply flattened x
        : x                 // if no, return just x
    )
  )
}

const dimensionalArray = [
  [
    [
      [1, 2],
      [3, 4]
    ],
    [
      [5, 6],
      [7, 8]
    ]
  ],
  [
    [
      [9, 10],
      [11, 12]
    ],
    [
      [13, 14],
      [15, 16, [17]],
      [18, 19]
    ]
  ],
  [
    20
  ]
];

console.log(deepFlatten(dimensionalArray))