An implementation of the Windows 10 "duplicate filename" algorithm

win10_dupe_fname.js

// the following is an exact implementation of the algorithm Windows 10 uses when
// duplicating filenames

// dummy data
const allFiles = [
  'test',
  'test - Copy (2)',
  'test - Copy',
  'name',
  'othername'
];

// our test case
let filename = 'test';

// does the name end with '- Copy (n)'?
const isNthCopy = name => /- Copy \([\d]+\)$/.test(name);

// does the name end with '- Copy'?
const isSecondCopy = name => /- Copy$/.test(name);

// the Windows 'duplicate file' naming algorithm

// we append '- Copy' to the name of the file we are copying 
// but if this new name would be a duplicate of yet another file's name, we add a (2)
// if (2) is similarly taken, we increment until we get the nest lowest number that does not result in a duplicate
// this is all contingent on the name of the file we are duplicating: copying 'a - Copy (2)' would result in 'a - Copy (2) - Copy'

// incrementer - we always start at 1 and use prefix notation (so 1 is never used)
let i = 1;

// was the file using a duplicate name before we started iterating?
const isOriginalCopy = isSecondCopy(filename) || isNthCopy(filename);

do {
  // if this is true, `isSecondCopy` only became true after an iteration
  if (isSecondCopy(filename) && !isOriginalCopy) {
    filename = filename += ' (2)';

  // likewise, `isNthCopy` only became true after iterating
  } else if (isNthCopy(filename) && !isOriginalCopy) {
    // increment the numeric suffix
    filename = filename.substring(0, filename.length - 2) + ++i + ')';
  } else {
    filename += ' - Copy'; 
  }

  var duplicateExtant = allFiles.find(f => f === filename);
} while (duplicateExtant);

console.log(filename); // test - Copy (3)