//Functionality: Assert if two arrays are equal
function arraysEqual(a, b) {
  if (a === b) return true;
  if (a == null || b == null) throw new Error('Error: null array as an argument');
  if (a.length != b.length) throw new Error('Error: arrays of different length');
  for (var i = 0; i < a.length; ++i) {
    if (a[i] !== b[i]) throw new Error('Error: arrays are different');
  }
  return console.log("Passed!");
}

//Functionality: Flatten an array of arbitrarily nested arrays of integers into a flat array of integers
function flatten (arr, initalArray) {
    //Set default value of initalArray to: []
    var initalArray = arguments.length <= 1 || arguments[1] === undefined ? [] : arguments[1];
    return arr.reduce(function(prev,curr) {
        if (typeof curr === "object"){
            return flatten(curr,prev)
        }
        else {
          return prev.concat([curr]);
        }
    }, initalArray)
}
//Assert arrays. Given case
arraysEqual(flatten([[1,2,[3]],4]), [1,2,3,4])

//Assert arrays. Complex case of nested arrays
arraysEqual(flatten([[1,2,[3],[4,[5]]],[[[6]]]]), [1,2,3,4,5,6])

//Assert arrays. Empty arrays
arraysEqual(flatten([]), [])

//Assert arrays. Already as desired
arraysEqual(flatten([1,2,3,4]), [1,2,3,4])