Map by hand

function map(arr, mapCallback) {
  if (!Array.isArray(arr) || !arr.length || typeof mapCallback !== 'function') { 
    return [];
  } else {
    let result = [];
    for (let i = 0, len = arr.length; i < len; i++) {
      result.push(mapCallback(arr[i], i, arr));
    }
    return result;
  }
}