Flatten Integers

flatten.js

// FLATTEN INTEGERS /////////////////////////////////////////////////////////////////////

const flattenIntegers = ints =>
///// Transforms nested lists of integers into a flat list of integers.
///// e.g.   1           -> error
///// e.g. [ 1         ] -> [1      ]
///// e.g. [[1,2,[3]],4] -> [1,2,3,4]
{
  const concatLists = lists =>
  ///// Transforms a list of lists into a single list.
  ///// e.g. [ 1              ] -> error
  ///// e.g. [[1,2],[3],[],[4]] -> [1,2,3,4]
    [].concat.apply( [], lists );

  const flattenInteger = int =>
  ///// Transforms a single integer or nested list of integers into a list of integers.
  ///// e.g.  "1"         -> error
  ///// e.g.   1.1        -> error
  ///// e.g.   1.0        -> [ 1    ]
  ///// e.g.   1          -> [ 1    ]
  ///// e.g. [ 1        ] -> [ 1    ]
  ///// e.g. [ 1, [ 2 ] ] -> [ 1, 2 ]
  {
    if( Number.isInteger( int ) )
      return [ int ];
      ////// This is our recursion base-case!
      ////// We ALWAYS want to return a list of integers.
    else if( Array.isArray( int ) )
      return concatLists( int.map( flattenInteger ) );
      ////// We transform each member of the list into a list of integers, and then concatenate them all together.
    else
      throw `flattenIntegers expected an array whose members are integers or lists of integers, but it received ${ JSON.stringify( int ) }`;
  };

  if( Array.isArray( ints ) )
    return concatLists( ints.map( flattenInteger ) );
    ////// We transform each member of the list into a list of integers, and then concatenate them all together.
  else
    throw `flattenIntegers expected an array, but it received ${ JSON.stringify( ints ) }`;
};


// VERIFICATION /////////////////////////////////////////////////////////////////////////

console.log( flattenIntegers( [[1,2,[3]],4] ) );