Quick and dirty functions to replicate macOS behaviour if filename is already taken.

macOS-naming.js

var folderName = 'untitledfolder';

var nameTests = [
  {
    nr: 1,
    fileSystemIn: [],
    fileSystemOut: ['untitledfolder'],
  },
  {
    nr: 2,
    fileSystemIn: ['untitledfolder'],
    fileSystemOut: ['untitledfolder', 'untitledfolder 2'],
  },
  {
    nr: 3,
    fileSystemIn: ['untitledfolder', 'untitledfolder 2', 'untitledfolder 4', 'untitledfolder 5'],
    fileSystemOut: ['untitledfolder', 'untitledfolder 2', 'untitledfolder 3', 'untitledfolder 4', 'untitledfolder 5'],
  },
  {
    nr: 4,
    fileSystemIn: ['untitledfolder 2', 'untitledfolder 3', 'untitledfolder 4', 'untitledfolder 5'],
    fileSystemOut: ['untitledfolder', 'untitledfolder 2', 'untitledfolder 3', 'untitledfolder 4', 'untitledfolder 5'],
  },
  {
    nr: 5,
    fileSystemIn: ['untitledfolder', 'untitledfolder 2', 'untitledfolder 3', 'untitledfolder 4'],
    fileSystemOut: ['untitledfolder', 'untitledfolder 2', 'untitledfolder 3', 'untitledfolder 4', 'untitledfolder 5'],
  },
  {
    nr: 6,
    fileSystemIn: ['untitledfolder', 'untitledfolder 2', 'untitledfolder 3.'],
    fileSystemOut: ['untitledfolder', 'untitledfolder 2', 'untitledfolder 3', 'untitledfolder 3.'],
  },
  {
    nr: 7,
    fileSystemIn: ['untitledfolder', 'untitledfolder -2'],
    fileSystemOut: ['untitledfolder', 'untitledfolder -2', 'untitledfolder 2'],
  }
];

nameTests.forEach(function(test) {
  var inn = test.fileSystemIn;
  inn.push(createNewFolder(folderName, inn));
  inn.sort();
  if (JSON.stringify(inn) === JSON.stringify(test.fileSystemOut)) {
    console.log(test.nr, 'Test pass');
  } else {
    console.log(test.nr, 'Test failed', JSON.stringify(inn));
  }

});


// Add an incrementing number to the end of the new name, starting from number 2.
// If there is a gap it fills in the gap.
function createNewFolder(folderName, fileSystem) {
  var isPositiveInteger = function(s) {
    return /^\+?[1-9][\d]*$/.test(s);
  };

  var newName = folderName;
  while(fileSystem.includes(newName)) {
    var p = newName.split(' ');
    if (p.length >= 2) {
      var last = p[p.length - 1]
      if (isPositiveInteger(last)) {
        newName = newName.substr(0, newName.length - last.length);
        newName = newName + (Number(last) + 1);
        continue
      }
    }
    newName = newName + ' 2';
  }
  return newName;
}
a
var duplicateTest = [
  {
    nr: 1,
    fileSystemIn: ['untitledfolder'],
    fileSystemOut: ['untitledfolder', 'untitledfolder copy'],
  },
  {
    nr: 2,
    fileSystemIn: ['untitledfolder copy'],
    fileSystemOut: ['untitledfolder copy', 'untitledfolder copy 2'],
  },
  {
    nr: 3,
    fileSystemIn: ['untitledfolder copy 2', 'untitledfolder copy 3'],
    fileSystemOut: ['untitledfolder copy 2', 'untitledfolder copy 3', 'untitledfolder copy 4'],
  },
  {
    nr: 4,
    fileSystemIn: ['untitledfolder  copy '],
    fileSystemOut: ['untitledfolder  copy ', 'untitledfolder  copy  copy'],
  },
  {
    nr: 5,
    fileSystemIn: ['untitledfolder 1 copy 100000'],
    fileSystemOut: ['untitledfolder 1 copy 100000', 'untitledfolder 1 copy 100001'],
  },
];

duplicateTest.forEach(function(test) {
  var inn = test.fileSystemIn;
  var folderName = inn[inn.length - 1];
  inn.push(duplicateName(folderName, inn));
  inn.sort();
  if (JSON.stringify(inn) === JSON.stringify(test.fileSystemOut)) {
    console.log(test.nr, 'Test pass');
  } else {
    console.log(test.nr, 'Test failed', JSON.stringify(inn));
  }

});

// duplicateName returns a new name that is not yet in the list.
// If the name is already taken add copy to the end.
// If the name is already taken add an incrementing number at the end, starting from number 2.
// If the user duplicates a folder with the name ' copy %d' then it should increment the number.
function duplicateName(name, nameList, copyWord) {
  copyWord = copyWord === undefined ? 'copy' : copyWord;

  var isPositiveInteger = function(s) {
    return /^\+?[1-9][\d]*$/.test(s);
  };

  var newName = name;
  while(nameList.includes(newName)) {
    if (newName.endsWith(' ' + copyWord)) {
      newName = newName + ' 2';
      continue;
    }
    var p = newName.split(' ');
    if (p.length >= 3) {
      var secondLast = p[p.length - 2];
      var last = p[p.length - 1];
      if (secondLast === copyWord && isPositiveInteger(last)) {
        newName = newName.substr(0, newName.length - last.length);
        newName = newName + (Number(last) + 1);
        continue;
      }
    }
    newName = newName + ' ' + copyWord;
  }
  return newName;
}