let names = ['Tayfun', 'Mehmet', 'Ahmet', 'Gökhan', 'Tayfun']

let countedNames = names.reduce(function (allNames, name) { 
  if (name in allNames) {
    allNames[name]++
  } else {
    allNames[name] = 1
  }
  return allNames
}, {})

console.log(countedNames)
// { Tayfun: 2, Mehmet: 1, Ahmet: 1, 'Gökhan': 1 }