Calculate reachable area

robotGridProblem.js

// Helper function
function countUpUntil(endNum) { 
  return 1 === endNum
    ? [1]
    : [...countUpUntil(endNum - 1), endNum ];
};


function calculateReachableArea(maxSide){
  const axisArea = maxSide * 4 + 1 // + 1 is for the [0,0] cell
  
  const areaInsideQuarter = countUpUntil(maxSide - 1) // generate inner columns values
    .reduce((sum, current) => sum + current, 0) // sum columns area

  return axisArea + (4 * areaInsideQuarter)
}


console.log(calculateReachableArea(23))