Get next filename when files are ordered in format <filename>.<integer>

file-next-integer.js

const fs = require('fs');

const RegExp_escape = function(s) {
  return (s ? s + '' : '').replace(/[-\/\\^$*+?.()|[\]{}]/g, '\\$&');
};

/**
 * @prop  {Object}   options      - Options Object or String if Filename
 * @param {String}   filename     - File name
 * @param {String}   dir          - Directory to look for filename pattern
 * @param {String}   max_files    - Max files you want to create. Non positive means unlimited
 */
function FileNextInteger(options) {
  if ( !options || !options.filename || !options.dir) {
    throw new Error('Filename and dir required')
  }

  this.filename = options.filename.toString();
  this.dir = options.dir.toString();

  this.delimeter = '.';
  this.max_files = parseInt(options.max_files, 10) || 0;
}

// FileNextInteger.prototype. = function() { }
FileNextInteger.prototype._generateFileName = function(index) {
  if (index) {
    return this.filename + this.delimeter + index;
  }
  return this.filename;
}

FileNextInteger.prototype._getRegexString = function() {
  return '^' + RegExp_escape(this.filename) + '(' + RegExp_escape(this.delimeter) + '\\d+)?$';
}

FileNextInteger.prototype.getNext = function() {
  var regex = new RegExp(this._getRegexString());
  var self = this;
  var maxUsedNum = -1;
  var files = fs.readdirSync(this.dir).every(function(file) {
    result = regex.exec(file);
    // No match
    if (result === null) {
      return true;
    }

    // Consider example:
    // A) filename: 'a.log.12'
    //      - result    = [ 'a.log.12', '.12' ]
    //      - numberStr = '.12'
    //      - number    = '.12'.substring('.'.length) = '12'
    // B) filename: 'a.log'
    //      - result    = [ 'a.log.12', undefined ] // undefined as no group match
    //      - numberStr = undefined
    //      - number    = 0
    var number = -1;
    if (!result[result.length - 1]) {
      number = 0;
    } else {
      number = parseInt(result[result.length - 1].substring(self.delimeter.length), 10);
    }

    if (number > maxUsedNum) {
      maxUsedNum = number;
    }

    if (self.max_files && self.max_files <= maxUsedNum) {
      return false;
    }
    return true;
  });

  // Maximum files used
  if (self.max_files && self.max_files <= maxUsedNum) {
    return null;
  }

  // No file present
  if (maxUsedNum < 0) {
    return this._generateFileName();
  }
  return this._generateFileName(maxUsedNum + 1);
}

/**
 * For files under directory say '/tmp/abc', we have following files:
 *  - a.log
 *  - a.log.1
 *  - a.log.2
 *  - other.log
 *  - other.log.1
 * 
 * require('./file-next-integer')({
 *   dir: '/tmp/abc',
 *   filename: 'a.log',
 * }).getNext()
 * 
 * Above script will return filename as 'a.log.3'
 * 
 */
module.exports = function(options) {
  return new FileNextInteger(options);
}