const objectsArray =
[
  {
    "id": 1,
    "first_name": "Mark",
    "last_name": "Harrison",
    "email": "mharrison0@g.co",
    "gender": "Male",
    "ip_address": "247.64.88.56"
  }, {
    "id": 2,
    "first_name": "Carol",
    "last_name": "Bradley",
    "email": "cbradley1@edublogs.org",
    "gender": "Female",
    "ip_address": "30.85.44.102"
  }, {
    "id": 3,
    "first_name": "Carolyn",
    "last_name": "Webb",
    "email": "cwebb2@alexa.com",
    "gender": "Female",
    "ip_address": "204.43.122.95"
  }, {
    "id": 4,
    "first_name": "Todd",
    "last_name": "Rodriguez",
    "email": "trodriguez3@zdnet.com",
    "gender": "Male",
    "ip_address": "115.218.218.156"
  }, {
    "id": 5,
    "first_name": "Scott",
    "last_name": "Simpson",
    "email": "ssimpson4@marketwatch.com",
    "gender": "Male",
    "ip_address": "95.33.253.249"
  }, {
    "id": 6,
    "first_name": "Frank",
    "last_name": "Johnston",
    "email": "fjohnston5@marketwatch.com",
    "gender": "Male",
    "ip_address": "153.46.102.52"
  }, {
    "id": 7,
    "first_name": "John",
    "last_name": "Moore",
    "email": "jmoore6@blogs.com",
    "gender": "Male",
    "ip_address": "105.173.113.212"
  }, {
    "id": 8,
    "first_name": "Jacqueline",
    "last_name": "Parker",
    "email": "jparker7@go.com",
    "gender": "Female",
    "ip_address": "167.105.134.135"
  }, {
    "id": 9,
    "first_name": "Aaron",
    "last_name": "Lawrence",
    "email": "alawrence8@google.pl",
    "gender": "Male",
    "ip_address": "52.8.77.22"
  }, {
    "id": 10,
    "first_name": "Mary",
    "last_name": "Thomas",
    "email": "mthomas9@washington.edu",
    "gender": "Female",
    "ip_address": "119.66.85.117"
  }
]

const countCharacters = function (arr, prop) {

  const count =  arr.filter(item => item.hasOwnProperty(prop))
    .map(item => item[prop])
    .reduce((acc, val) => {
      val = String(val).toLowerCase()
      for (const item of val) {
        acc.hasOwnProperty(item)
        ? acc[item]++
        : acc[item] = 1
      }
      return acc
    }, {})
  console.log(count)

  let mostFrequent
  for (const prop in count) {
    if (!mostFrequent) {
      mostFrequent = {character: prop, number: count[prop]}
    } else {
      if (count[prop] > mostFrequent.number) {
        mostFrequent = {character: prop, number: count[prop]}
      }
    }
  }
  return mostFrequent.character
}

countCharacters(objectsArray, 'first_name')