diff --git a/index.js b/index.js
index dedcf936f..383755e2b 100644
--- a/index.js
+++ b/index.js
@@ -7,24 +7,6 @@ var spawn = require('child_process').spawn;
 var path = require('path');
 var fs = require('fs');
 
-/**
- * Inherit `Command` from `EventEmitter.prototype`.
- */
-
-require('util').inherits(Command, EventEmitter);
-
-/**
- * Expose the root command.
- */
-
-exports = module.exports = new Command();
-
-/**
- * Expose `Command`.
- */
-
-exports.Command = Command;
-
 class Option {
   /**
    * Initialize a new `Option` with the given `flags` and `description`.
@@ -82,12 +64,6 @@ class Option {
   };
 }
 
-/**
- * Expose `Option`.
- */
-
-exports.Option = Option;
-
 /**
  * CommanderError class
  * @class
@@ -110,1384 +86,1399 @@ class CommanderError extends Error {
   }
 }
 
-exports.CommanderError = CommanderError;
-
-/**
- * Initialize a new `Command`.
- *
- * @param {String} [name]
- * @api public
- */
-
-function Command(name) {
-  this.commands = [];
-  this.options = [];
-  this._allowUnknownOption = false;
-  this._args = [];
-  this._name = name || '';
-  this._optionValues = {};
-  this._storeOptionsAsProperties = true; // backwards compatible by default
-  this._passCommandToAction = true; // backwards compatible by default
-  this._actionResults = [];
-  this._actionHandler = null;
-  this._executableHandler = false;
-  this._executableFile = null; // custom name for executable
-  this._defaultCommandName = null;
-  this._exitCallback = null;
-
-  this._noHelp = false;
-  this._helpFlags = '-h, --help';
-  this._helpDescription = 'display help for command';
-  this._helpShortFlag = '-h';
-  this._helpLongFlag = '--help';
-  this._hasImplicitHelpCommand = undefined; // Deliberately undefined, not decided whether true or false
-  this._helpCommandName = 'help';
-  this._helpCommandnameAndArgs = 'help [command]';
-  this._helpCommandDescription = 'display help for command';
-}
-
-/**
- * Define a command.
- *
- * There are two styles of command: pay attention to where to put the description.
- *
- * Examples:
- *
- *      // Command implemented using action handler (description is supplied separately to `.command`)
- *      program
- *        .command('clone <source> [destination]')
- *        .description('clone a repository into a newly created directory')
- *        .action((source, destination) => {
- *          console.log('clone command called');
- *        });
- *
- *      // Command implemented using separate executable file (description is second parameter to `.command`)
- *      program
- *        .command('start <service>', 'start named service')
- *        .command('stop [service]', 'stop named service, or all if no name supplied');
- *
- * @param {string} nameAndArgs - command name and arguments, args are `<required>` or `[optional]` and last may also be `variadic...`
- * @param {Object|string} [actionOptsOrExecDesc] - configuration options (for action), or description (for executable)
- * @param {Object} [execOpts] - configuration options (for executable)
- * @return {Command} returns new command for action handler, or top-level command for executable command
- * @api public
- */
-
-Command.prototype.command = function(nameAndArgs, actionOptsOrExecDesc, execOpts) {
-  var desc = actionOptsOrExecDesc;
-  var opts = execOpts;
-  if (typeof desc === 'object' && desc !== null) {
-    opts = desc;
-    desc = null;
-  }
-  opts = opts || {};
-  var args = nameAndArgs.split(/ +/);
-  var cmd = new Command(args.shift());
+class Command extends EventEmitter {
+  /**
+   * Initialize a new `Command`.
+   *
+   * @param {String} [name]
+   * @api public
+   */
 
-  if (desc) {
-    cmd.description(desc);
-    cmd._executableHandler = true;
+  constructor(name) {
+    super();
+    this.commands = [];
+    this.options = [];
+    this._allowUnknownOption = false;
+    this._args = [];
+    this._name = name || '';
+    this._optionValues = {};
+    this._storeOptionsAsProperties = true; // backwards compatible by default
+    this._passCommandToAction = true; // backwards compatible by default
+    this._actionResults = [];
+    this._actionHandler = null;
+    this._executableHandler = false;
+    this._executableFile = null; // custom name for executable
+    this._defaultCommandName = null;
+    this._exitCallback = null;
+
+    this._noHelp = false;
+    this._helpFlags = '-h, --help';
+    this._helpDescription = 'display help for command';
+    this._helpShortFlag = '-h';
+    this._helpLongFlag = '--help';
+    this._hasImplicitHelpCommand = undefined; // Deliberately undefined, not decided whether true or false
+    this._helpCommandName = 'help';
+    this._helpCommandnameAndArgs = 'help [command]';
+    this._helpCommandDescription = 'display help for command';
   }
-  if (opts.isDefault) this._defaultCommandName = cmd._name;
-
-  cmd._noHelp = !!opts.noHelp;
-  cmd._helpFlags = this._helpFlags;
-  cmd._helpDescription = this._helpDescription;
-  cmd._helpShortFlag = this._helpShortFlag;
-  cmd._helpLongFlag = this._helpLongFlag;
-  cmd._helpCommandName = this._helpCommandName;
-  cmd._helpCommandnameAndArgs = this._helpCommandnameAndArgs;
-  cmd._helpCommandDescription = this._helpCommandDescription;
-  cmd._exitCallback = this._exitCallback;
-  cmd._storeOptionsAsProperties = this._storeOptionsAsProperties;
-  cmd._passCommandToAction = this._passCommandToAction;
-
-  cmd._executableFile = opts.executableFile || null; // Custom name for executable file, set missing to null to match constructor
-  this.commands.push(cmd);
-  cmd._parseExpectedArgs(args);
-  cmd.parent = this;
-
-  if (desc) return this;
-  return cmd;
-};
-
-/**
- * Add a prepared subcommand.
- *
- * @param {Command} cmd - new subcommand
- * @return {Command} parent command for chaining
- * @api public
- */
 
-Command.prototype.addCommand = function(cmd) {
-  if (!cmd._name) throw new Error('Command passed to .AddCommand must have a name');
-
-  // To keep things simple, block automatic name generation for deeply nested executables.
-  // Fail fast and detect when adding rather than later when parsing.
-  function checkExplicitNames(commandArray) {
-    commandArray.forEach((cmd) => {
-      if (cmd._executableHandler && !cmd._executableFile) {
-        throw new Error(`Must specify executableFile for deeply nested executable: ${cmd.name()}`);
-      }
-      checkExplicitNames(cmd.commands);
-    });
-  }
-  checkExplicitNames(cmd.commands);
+  /**
+   * Define a command.
+   *
+   * There are two styles of command: pay attention to where to put the description.
+   *
+   * Examples:
+   *
+   *      // Command implemented using action handler (description is supplied separately to `.command`)
+   *      program
+   *        .command('clone <source> [destination]')
+   *        .description('clone a repository into a newly created directory')
+   *        .action((source, destination) => {
+   *          console.log('clone command called');
+   *        });
+   *
+   *      // Command implemented using separate executable file (description is second parameter to `.command`)
+   *      program
+   *        .command('start <service>', 'start named service')
+   *        .command('stop [service]', 'stop named service, or all if no name supplied');
+   *
+   * @param {string} nameAndArgs - command name and arguments, args are `<required>` or `[optional]` and last may also be `variadic...`
+   * @param {Object|string} [actionOptsOrExecDesc] - configuration options (for action), or description (for executable)
+   * @param {Object} [execOpts] - configuration options (for executable)
+   * @return {Command} returns new command for action handler, or top-level command for executable command
+   * @api public
+   */
 
-  this.commands.push(cmd);
-  cmd.parent = this;
-  return this;
-};
+  command(nameAndArgs, actionOptsOrExecDesc, execOpts) {
+    var desc = actionOptsOrExecDesc;
+    var opts = execOpts;
+    if (typeof desc === 'object' && desc !== null) {
+      opts = desc;
+      desc = null;
+    }
+    opts = opts || {};
+    var args = nameAndArgs.split(/ +/);
+    var cmd = new Command(args.shift());
 
-/**
- * Define argument syntax for the top-level command.
- *
- * @api public
- */
+    if (desc) {
+      cmd.description(desc);
+      cmd._executableHandler = true;
+    }
+    if (opts.isDefault) this._defaultCommandName = cmd._name;
+
+    cmd._noHelp = !!opts.noHelp;
+    cmd._helpFlags = this._helpFlags;
+    cmd._helpDescription = this._helpDescription;
+    cmd._helpShortFlag = this._helpShortFlag;
+    cmd._helpLongFlag = this._helpLongFlag;
+    cmd._helpCommandName = this._helpCommandName;
+    cmd._helpCommandnameAndArgs = this._helpCommandnameAndArgs;
+    cmd._helpCommandDescription = this._helpCommandDescription;
+    cmd._exitCallback = this._exitCallback;
+    cmd._storeOptionsAsProperties = this._storeOptionsAsProperties;
+    cmd._passCommandToAction = this._passCommandToAction;
+
+    cmd._executableFile = opts.executableFile || null; // Custom name for executable file, set missing to null to match constructor
+    this.commands.push(cmd);
+    cmd._parseExpectedArgs(args);
+    cmd.parent = this;
+
+    if (desc) return this;
+    return cmd;
+  };
 
-Command.prototype.arguments = function(desc) {
-  return this._parseExpectedArgs(desc.split(/ +/));
-};
+  /**
+   * Add a prepared subcommand.
+   *
+   * @param {Command} cmd - new subcommand
+   * @return {Command} parent command for chaining
+   * @api public
+   */
 
-/**
- * Override default decision whether to add implicit help command.
- *
- *    addHelpCommand() // force on
- *    addHelpCommand(false); // force off
- *    addHelpCommand('help [cmd]', 'display help for [cmd]'); // force on with custom detais
- *
- * @return {Command} for chaining
- * @api public
- */
+  addCommand(cmd) {
+    if (!cmd._name) throw new Error('Command passed to .AddCommand must have a name');
 
-Command.prototype.addHelpCommand = function(enableOrNameAndArgs, description) {
-  if (enableOrNameAndArgs === false) {
-    this._hasImplicitHelpCommand = false;
-  } else {
-    this._hasImplicitHelpCommand = true;
-    if (typeof enableOrNameAndArgs === 'string') {
-      this._helpCommandName = enableOrNameAndArgs.split(' ')[0];
-      this._helpCommandnameAndArgs = enableOrNameAndArgs;
+    // To keep things simple, block automatic name generation for deeply nested executables.
+    // Fail fast and detect when adding rather than later when parsing.
+    function checkExplicitNames(commandArray) {
+      commandArray.forEach((cmd) => {
+        if (cmd._executableHandler && !cmd._executableFile) {
+          throw new Error(`Must specify executableFile for deeply nested executable: ${cmd.name()}`);
+        }
+        checkExplicitNames(cmd.commands);
+      });
     }
-    this._helpCommandDescription = description || this._helpCommandDescription;
-  }
-  return this;
-};
+    checkExplicitNames(cmd.commands);
 
-/**
- * @return {boolean}
- * @api private
- */
+    this.commands.push(cmd);
+    cmd.parent = this;
+    return this;
+  };
 
-Command.prototype._lazyHasImplicitHelpCommand = function() {
-  if (this._hasImplicitHelpCommand === undefined) {
-    this._hasImplicitHelpCommand = this.commands.length && !this._actionHandler && !this._findCommand('help');
-  }
-  return this._hasImplicitHelpCommand;
-};
+  /**
+   * Define argument syntax for the top-level command.
+   *
+   * @api public
+   */
 
-/**
- * Parse expected `args`.
- *
- * For example `["[type]"]` becomes `[{ required: false, name: 'type' }]`.
- *
- * @param {Array} args
- * @return {Command} for chaining
- * @api private
- */
+  arguments(desc) {
+    return this._parseExpectedArgs(desc.split(/ +/));
+  };
 
-Command.prototype._parseExpectedArgs = function(args) {
-  if (!args.length) return;
-  var self = this;
-  args.forEach(function(arg) {
-    var argDetails = {
-      required: false,
-      name: '',
-      variadic: false
-    };
+  /**
+   * Override default decision whether to add implicit help command.
+   *
+   *    addHelpCommand() // force on
+   *    addHelpCommand(false); // force off
+   *    addHelpCommand('help [cmd]', 'display help for [cmd]'); // force on with custom detais
+   *
+   * @return {Command} for chaining
+   * @api public
+   */
 
-    switch (arg[0]) {
-      case '<':
-        argDetails.required = true;
-        argDetails.name = arg.slice(1, -1);
-        break;
-      case '[':
-        argDetails.name = arg.slice(1, -1);
-        break;
+  addHelpCommand(enableOrNameAndArgs, description) {
+    if (enableOrNameAndArgs === false) {
+      this._hasImplicitHelpCommand = false;
+    } else {
+      this._hasImplicitHelpCommand = true;
+      if (typeof enableOrNameAndArgs === 'string') {
+        this._helpCommandName = enableOrNameAndArgs.split(' ')[0];
+        this._helpCommandnameAndArgs = enableOrNameAndArgs;
+      }
+      this._helpCommandDescription = description || this._helpCommandDescription;
     }
+    return this;
+  };
 
-    if (argDetails.name.length > 3 && argDetails.name.slice(-3) === '...') {
-      argDetails.variadic = true;
-      argDetails.name = argDetails.name.slice(0, -3);
-    }
-    if (argDetails.name) {
-      self._args.push(argDetails);
+  /**
+   * @return {boolean}
+   * @api private
+   */
+
+  _lazyHasImplicitHelpCommand() {
+    if (this._hasImplicitHelpCommand === undefined) {
+      this._hasImplicitHelpCommand = this.commands.length && !this._actionHandler && !this._findCommand('help');
     }
-  });
-  return this;
-};
+    return this._hasImplicitHelpCommand;
+  };
 
-/**
- * Register callback to use as replacement for calling process.exit.
- *
- * @param {Function} [fn] optional callback which will be passed a CommanderError, defaults to throwing
- * @return {Command} for chaining
- * @api public
- */
+  /**
+   * Parse expected `args`.
+   *
+   * For example `["[type]"]` becomes `[{ required: false, name: 'type' }]`.
+   *
+   * @param {Array} args
+   * @return {Command} for chaining
+   * @api private
+   */
 
-Command.prototype.exitOverride = function(fn) {
-  if (fn) {
-    this._exitCallback = fn;
-  } else {
-    this._exitCallback = function(err) {
-      if (err.code !== 'commander.executeSubCommandAsync') {
-        throw err;
-      } else {
-        // Async callback from spawn events, not useful to throw.
-      }
-    };
-  }
-  return this;
-};
+  _parseExpectedArgs(args) {
+    if (!args.length) return;
+    var self = this;
+    args.forEach(function(arg) {
+      var argDetails = {
+        required: false,
+        name: '',
+        variadic: false
+      };
 
-/**
- * Call process.exit, and _exitCallback if defined.
- *
- * @param {Number} exitCode exit code for using with process.exit
- * @param {String} code an id string representing the error
- * @param {String} message human-readable description of the error
- * @return never
- * @api private
- */
+      switch (arg[0]) {
+        case '<':
+          argDetails.required = true;
+          argDetails.name = arg.slice(1, -1);
+          break;
+        case '[':
+          argDetails.name = arg.slice(1, -1);
+          break;
+      }
 
-Command.prototype._exit = function(exitCode, code, message) {
-  if (this._exitCallback) {
-    this._exitCallback(new CommanderError(exitCode, code, message));
-    // Expecting this line is not reached.
-  }
-  process.exit(exitCode);
-};
+      if (argDetails.name.length > 3 && argDetails.name.slice(-3) === '...') {
+        argDetails.variadic = true;
+        argDetails.name = argDetails.name.slice(0, -3);
+      }
+      if (argDetails.name) {
+        self._args.push(argDetails);
+      }
+    });
+    return this;
+  };
 
-/**
- * Register callback `fn` for the command.
- *
- * Examples:
- *
- *      program
- *        .command('help')
- *        .description('display verbose help')
- *        .action(function() {
- *           // output help here
- *        });
- *
- * @param {Function} fn
- * @return {Command} for chaining
- * @api public
- */
+  /**
+   * Register callback to use as replacement for calling process.exit.
+   *
+   * @param {Function} [fn] optional callback which will be passed a CommanderError, defaults to throwing
+   * @return {Command} for chaining
+   * @api public
+   */
 
-Command.prototype.action = function(fn) {
-  var self = this;
-  var listener = function(args) {
-    // The .action callback takes an extra parameter which is the command or options.
-    var expectedArgsCount = self._args.length;
-    var actionArgs = args.slice(0, expectedArgsCount);
-    if (self._passCommandToAction) {
-      actionArgs[expectedArgsCount] = self;
+  exitOverride(fn) {
+    if (fn) {
+      this._exitCallback = fn;
     } else {
-      actionArgs[expectedArgsCount] = self.opts();
-    }
-    // Add the extra arguments so available too.
-    if (args.length > expectedArgsCount) {
-      actionArgs.push(args.slice(expectedArgsCount));
+      this._exitCallback = function(err) {
+        if (err.code !== 'commander.executeSubCommandAsync') {
+          throw err;
+        } else {
+          // Async callback from spawn events, not useful to throw.
+        }
+      };
     }
+    return this;
+  };
 
-    const actionResult = fn.apply(self, actionArgs);
-    // Remember result in case it is async. Assume parseAsync getting called on root.
-    let rootCommand = self;
-    while (rootCommand.parent) {
-      rootCommand = rootCommand.parent;
+  /**
+   * Call process.exit, and _exitCallback if defined.
+   *
+   * @param {Number} exitCode exit code for using with process.exit
+   * @param {String} code an id string representing the error
+   * @param {String} message human-readable description of the error
+   * @return never
+   * @api private
+   */
+
+  _exit(exitCode, code, message) {
+    if (this._exitCallback) {
+      this._exitCallback(new CommanderError(exitCode, code, message));
+      // Expecting this line is not reached.
     }
-    rootCommand._actionResults.push(actionResult);
+    process.exit(exitCode);
   };
-  this._actionHandler = listener;
-  return this;
-};
 
-/**
- * Internal implementation shared by .option() and .requiredOption()
- *
- * @param {Object} config
- * @param {String} flags
- * @param {String} description
- * @param {Function|*} [fn] - custom option processing function or default vaue
- * @param {*} [defaultValue]
- * @return {Command} for chaining
- * @api private
- */
+  /**
+   * Register callback `fn` for the command.
+   *
+   * Examples:
+   *
+   *      program
+   *        .command('help')
+   *        .description('display verbose help')
+   *        .action(function() {
+   *           // output help here
+   *        });
+   *
+   * @param {Function} fn
+   * @return {Command} for chaining
+   * @api public
+   */
 
-Command.prototype._optionEx = function(config, flags, description, fn, defaultValue) {
-  var self = this,
-    option = new Option(flags, description),
-    oname = option.name(),
-    name = option.attributeName();
-  option.mandatory = !!config.mandatory;
-
-  // default as 3rd arg
-  if (typeof fn !== 'function') {
-    if (fn instanceof RegExp) {
-      // This is a bit simplistic (especially no error messages), and probably better handled by caller using custom option processing.
-      // No longer documented in README, but still present for backwards compatibility.
-      var regex = fn;
-      fn = function(val, def) {
-        var m = regex.exec(val);
-        return m ? m[0] : def;
-      };
-    } else {
-      defaultValue = fn;
-      fn = null;
-    }
-  }
+  action(fn) {
+    var self = this;
+    var listener = function(args) {
+      // The .action callback takes an extra parameter which is the command or options.
+      var expectedArgsCount = self._args.length;
+      var actionArgs = args.slice(0, expectedArgsCount);
+      if (self._passCommandToAction) {
+        actionArgs[expectedArgsCount] = self;
+      } else {
+        actionArgs[expectedArgsCount] = self.opts();
+      }
+      // Add the extra arguments so available too.
+      if (args.length > expectedArgsCount) {
+        actionArgs.push(args.slice(expectedArgsCount));
+      }
 
-  // preassign default value for --no-*, [optional], <required>, or plain flag if boolean value
-  if (option.negate || option.optional || option.required || typeof defaultValue === 'boolean') {
-    // when --no-foo we make sure default is true, unless a --foo option is already defined
-    if (option.negate) {
-      const positiveLongFlag = option.long.replace(/^--no-/, '--');
-      defaultValue = self._findOption(positiveLongFlag) ? self._getOptionValue(name) : true;
-    }
-    // preassign only if we have a default
-    if (defaultValue !== undefined) {
-      self._setOptionValue(name, defaultValue);
-      option.defaultValue = defaultValue;
-    }
-  }
+      const actionResult = fn.apply(self, actionArgs);
+      // Remember result in case it is async. Assume parseAsync getting called on root.
+      let rootCommand = self;
+      while (rootCommand.parent) {
+        rootCommand = rootCommand.parent;
+      }
+      rootCommand._actionResults.push(actionResult);
+    };
+    this._actionHandler = listener;
+    return this;
+  };
 
-  // register the option
-  this.options.push(option);
+  /**
+   * Internal implementation shared by .option() and .requiredOption()
+   *
+   * @param {Object} config
+   * @param {String} flags
+   * @param {String} description
+   * @param {Function|*} [fn] - custom option processing function or default vaue
+   * @param {*} [defaultValue]
+   * @return {Command} for chaining
+   * @api private
+   */
 
-  // when it's passed assign the value
-  // and conditionally invoke the callback
-  this.on('option:' + oname, function(val) {
-    // coercion
-    if (val !== null && fn) {
-      val = fn(val, self._getOptionValue(name) === undefined ? defaultValue : self._getOptionValue(name));
+  _optionEx(config, flags, description, fn, defaultValue) {
+    var self = this,
+      option = new Option(flags, description),
+      oname = option.name(),
+      name = option.attributeName();
+    option.mandatory = !!config.mandatory;
+
+    // default as 3rd arg
+    if (typeof fn !== 'function') {
+      if (fn instanceof RegExp) {
+        // This is a bit simplistic (especially no error messages), and probably better handled by caller using custom option processing.
+        // No longer documented in README, but still present for backwards compatibility.
+        var regex = fn;
+        fn = function(val, def) {
+          var m = regex.exec(val);
+          return m ? m[0] : def;
+        };
+      } else {
+        defaultValue = fn;
+        fn = null;
+      }
     }
 
-    // unassigned or boolean value
-    if (typeof self._getOptionValue(name) === 'boolean' || typeof self._getOptionValue(name) === 'undefined') {
-      // if no value, negate false, and we have a default, then use it!
-      if (val == null) {
-        self._setOptionValue(name, option.negate
-          ? false
-          : defaultValue || true);
-      } else {
-        self._setOptionValue(name, val);
+    // preassign default value for --no-*, [optional], <required>, or plain flag if boolean value
+    if (option.negate || option.optional || option.required || typeof defaultValue === 'boolean') {
+      // when --no-foo we make sure default is true, unless a --foo option is already defined
+      if (option.negate) {
+        const positiveLongFlag = option.long.replace(/^--no-/, '--');
+        defaultValue = self._findOption(positiveLongFlag) ? self._getOptionValue(name) : true;
+      }
+      // preassign only if we have a default
+      if (defaultValue !== undefined) {
+        self._setOptionValue(name, defaultValue);
+        option.defaultValue = defaultValue;
       }
-    } else if (val !== null) {
-      // reassign
-      self._setOptionValue(name, option.negate ? false : val);
     }
-  });
 
-  return this;
-};
+    // register the option
+    this.options.push(option);
 
-/**
- * Define option with `flags`, `description` and optional
- * coercion `fn`.
- *
- * The `flags` string should contain both the short and long flags,
- * separated by comma, a pipe or space. The following are all valid
- * all will output this way when `--help` is used.
- *
- *    "-p, --pepper"
- *    "-p|--pepper"
- *    "-p --pepper"
- *
- * Examples:
- *
- *     // simple boolean defaulting to undefined
- *     program.option('-p, --pepper', 'add pepper');
- *
- *     program.pepper
- *     // => undefined
- *
- *     --pepper
- *     program.pepper
- *     // => true
- *
- *     // simple boolean defaulting to true (unless non-negated option is also defined)
- *     program.option('-C, --no-cheese', 'remove cheese');
- *
- *     program.cheese
- *     // => true
- *
- *     --no-cheese
- *     program.cheese
- *     // => false
- *
- *     // required argument
- *     program.option('-C, --chdir <path>', 'change the working directory');
- *
- *     --chdir /tmp
- *     program.chdir
- *     // => "/tmp"
- *
- *     // optional argument
- *     program.option('-c, --cheese [type]', 'add cheese [marble]');
- *
- * @param {String} flags
- * @param {String} description
- * @param {Function|*} [fn] - custom option processing function or default vaue
- * @param {*} [defaultValue]
- * @return {Command} for chaining
- * @api public
- */
+    // when it's passed assign the value
+    // and conditionally invoke the callback
+    this.on('option:' + oname, function(val) {
+      // coercion
+      if (val !== null && fn) {
+        val = fn(val, self._getOptionValue(name) === undefined ? defaultValue : self._getOptionValue(name));
+      }
 
-Command.prototype.option = function(flags, description, fn, defaultValue) {
-  return this._optionEx({}, flags, description, fn, defaultValue);
-};
+      // unassigned or boolean value
+      if (typeof self._getOptionValue(name) === 'boolean' || typeof self._getOptionValue(name) === 'undefined') {
+        // if no value, negate false, and we have a default, then use it!
+        if (val == null) {
+          self._setOptionValue(name, option.negate
+            ? false
+            : defaultValue || true);
+        } else {
+          self._setOptionValue(name, val);
+        }
+      } else if (val !== null) {
+        // reassign
+        self._setOptionValue(name, option.negate ? false : val);
+      }
+    });
 
-/*
- * Add a required option which must have a value after parsing. This usually means
- * the option must be specified on the command line. (Otherwise the same as .option().)
- *
- * The `flags` string should contain both the short and long flags, separated by comma, a pipe or space.
- *
- * @param {String} flags
- * @param {String} description
- * @param {Function|*} [fn] - custom option processing function or default vaue
- * @param {*} [defaultValue]
- * @return {Command} for chaining
- * @api public
- */
+    return this;
+  };
 
-Command.prototype.requiredOption = function(flags, description, fn, defaultValue) {
-  return this._optionEx({ mandatory: true }, flags, description, fn, defaultValue);
-};
+  /**
+   * Define option with `flags`, `description` and optional
+   * coercion `fn`.
+   *
+   * The `flags` string should contain both the short and long flags,
+   * separated by comma, a pipe or space. The following are all valid
+   * all will output this way when `--help` is used.
+   *
+   *    "-p, --pepper"
+   *    "-p|--pepper"
+   *    "-p --pepper"
+   *
+   * Examples:
+   *
+   *     // simple boolean defaulting to undefined
+   *     program.option('-p, --pepper', 'add pepper');
+   *
+   *     program.pepper
+   *     // => undefined
+   *
+   *     --pepper
+   *     program.pepper
+   *     // => true
+   *
+   *     // simple boolean defaulting to true (unless non-negated option is also defined)
+   *     program.option('-C, --no-cheese', 'remove cheese');
+   *
+   *     program.cheese
+   *     // => true
+   *
+   *     --no-cheese
+   *     program.cheese
+   *     // => false
+   *
+   *     // required argument
+   *     program.option('-C, --chdir <path>', 'change the working directory');
+   *
+   *     --chdir /tmp
+   *     program.chdir
+   *     // => "/tmp"
+   *
+   *     // optional argument
+   *     program.option('-c, --cheese [type]', 'add cheese [marble]');
+   *
+   * @param {String} flags
+   * @param {String} description
+   * @param {Function|*} [fn] - custom option processing function or default vaue
+   * @param {*} [defaultValue]
+   * @return {Command} for chaining
+   * @api public
+   */
 
-/**
- * Allow unknown options on the command line.
- *
- * @param {Boolean} arg if `true` or omitted, no error will be thrown
- * for unknown options.
- * @api public
- */
-Command.prototype.allowUnknownOption = function(arg) {
-  this._allowUnknownOption = arguments.length === 0 || arg;
-  return this;
-};
+  option(flags, description, fn, defaultValue) {
+    return this._optionEx({}, flags, description, fn, defaultValue);
+  };
 
-/**
-  * Whether to store option values as properties on command object,
-  * or store separately (specify false). In both cases the option values can be accessed using .opts().
+  /*
+  * Add a required option which must have a value after parsing. This usually means
+  * the option must be specified on the command line. (Otherwise the same as .option().)
   *
-  * @param {boolean} value
-  * @return {Command} Command for chaining
-  * @api public
-  */
-
-Command.prototype.storeOptionsAsProperties = function(value) {
-  this._storeOptionsAsProperties = (value === undefined) || value;
-  if (this.options.length) {
-    // This is for programmer, not end user.
-    console.error('Commander usage error: call storeOptionsAsProperties before adding options');
-  }
-  return this;
-};
-
-/**
-  * Whether to pass command to action handler,
-  * or just the options (specify false).
+  * The `flags` string should contain both the short and long flags, separated by comma, a pipe or space.
   *
-  * @param {boolean} value
-  * @return {Command} Command for chaining
+  * @param {String} flags
+  * @param {String} description
+  * @param {Function|*} [fn] - custom option processing function or default vaue
+  * @param {*} [defaultValue]
+  * @return {Command} for chaining
   * @api public
   */
 
-Command.prototype.passCommandToAction = function(value) {
-  this._passCommandToAction = (value === undefined) || value;
-  return this;
-};
+  requiredOption(flags, description, fn, defaultValue) {
+    return this._optionEx({ mandatory: true }, flags, description, fn, defaultValue);
+  };
 
-/**
- * Store option value
- *
- * @param {String} key
- * @param {Object} value
- * @api private
- */
+  /**
+   * Allow unknown options on the command line.
+   *
+   * @param {Boolean} arg if `true` or omitted, no error will be thrown
+   * for unknown options.
+   * @api public
+   */
+  allowUnknownOption(arg) {
+    this._allowUnknownOption = arguments.length === 0 || arg;
+    return this;
+  };
 
-Command.prototype._setOptionValue = function(key, value) {
-  if (this._storeOptionsAsProperties) {
-    this[key] = value;
-  } else {
-    this._optionValues[key] = value;
-  }
-};
+  /**
+    * Whether to store option values as properties on command object,
+    * or store separately (specify false). In both cases the option values can be accessed using .opts().
+    *
+    * @param {boolean} value
+    * @return {Command} Command for chaining
+    * @api public
+    */
+
+  storeOptionsAsProperties(value) {
+    this._storeOptionsAsProperties = (value === undefined) || value;
+    if (this.options.length) {
+      // This is for programmer, not end user.
+      console.error('Commander usage error: call storeOptionsAsProperties before adding options');
+    }
+    return this;
+  };
 
-/**
- * Retrieve option value
- *
- * @param {String} key
- * @return {Object} value
- * @api private
- */
+  /**
+    * Whether to pass command to action handler,
+    * or just the options (specify false).
+    *
+    * @param {boolean} value
+    * @return {Command} Command for chaining
+    * @api public
+    */
+
+  passCommandToAction(value) {
+    this._passCommandToAction = (value === undefined) || value;
+    return this;
+  };
 
-Command.prototype._getOptionValue = function(key) {
-  if (this._storeOptionsAsProperties) {
-    return this[key];
-  }
-  return this._optionValues[key];
-};
+  /**
+   * Store option value
+   *
+   * @param {String} key
+   * @param {Object} value
+   * @api private
+   */
 
-/**
- * Parse `argv`, setting options and invoking commands when defined.
- *
- * @param {Array} argv
- * @return {Command} for chaining
- * @api public
- */
+  _setOptionValue(key, value) {
+    if (this._storeOptionsAsProperties) {
+      this[key] = value;
+    } else {
+      this._optionValues[key] = value;
+    }
+  };
 
-Command.prototype.parse = function(argv) {
-  // store raw args
-  this.rawArgs = argv;
+  /**
+   * Retrieve option value
+   *
+   * @param {String} key
+   * @return {Object} value
+   * @api private
+   */
 
-  // Guess name, used in usage in help.
-  this._name = this._name || path.basename(argv[1], path.extname(argv[1]));
+  _getOptionValue(key) {
+    if (this._storeOptionsAsProperties) {
+      return this[key];
+    }
+    return this._optionValues[key];
+  };
 
-  this._parseCommand([], argv.slice(2));
+  /**
+   * Parse `argv`, setting options and invoking commands when defined.
+   *
+   * @param {Array} argv
+   * @return {Command} for chaining
+   * @api public
+   */
 
-  return this;
-};
+  parse(argv) {
+    // store raw args
+    this.rawArgs = argv;
 
-/**
- * Parse `argv`, setting options and invoking commands when defined.
- *
- * Use parseAsync instead of parse if any of your action handlers are async. Returns a Promise.
- *
- * @param {Array} argv
- * @return {Promise}
- * @api public
- */
+    // Guess name, used in usage in help.
+    this._name = this._name || path.basename(argv[1], path.extname(argv[1]));
 
-Command.prototype.parseAsync = function(argv) {
-  this.parse(argv);
-  return Promise.all(this._actionResults);
-};
+    this._parseCommand([], argv.slice(2));
 
-/**
- * Execute a sub-command executable.
- *
- * @api private
- */
+    return this;
+  };
 
-Command.prototype._executeSubCommand = function(subcommand, args) {
-  args = args.slice();
-  let launchWithNode = false; // Use node for source targets so do not need to get permissions correct, and on Windows.
-  const sourceExt = ['.js', '.ts', '.mjs'];
+  /**
+   * Parse `argv`, setting options and invoking commands when defined.
+   *
+   * Use parseAsync instead of parse if any of your action handlers are async. Returns a Promise.
+   *
+   * @param {Array} argv
+   * @return {Promise}
+   * @api public
+   */
 
-  // Not checking for help first. Unlikely to have mandatory and executable, and can't robustly test for help flags in external command.
-  this._checkForMissingMandatoryOptions();
+  parseAsync(argv) {
+    this.parse(argv);
+    return Promise.all(this._actionResults);
+  };
 
-  // Want the entry script as the reference for command name and directory for searching for other files.
-  const scriptPath = this.rawArgs[1];
+  /**
+   * Execute a sub-command executable.
+   *
+   * @api private
+   */
 
-  let baseDir;
-  try {
-    const resolvedLink = fs.realpathSync(scriptPath);
-    baseDir = path.dirname(resolvedLink);
-  } catch (e) {
-    baseDir = '.'; // dummy, probably not going to find executable!
-  }
+  _executeSubCommand(subcommand, args) {
+    args = args.slice();
+    let launchWithNode = false; // Use node for source targets so do not need to get permissions correct, and on Windows.
+    const sourceExt = ['.js', '.ts', '.mjs'];
 
-  // name of the subcommand, like `pm-install`
-  let bin = path.basename(scriptPath, path.extname(scriptPath)) + '-' + subcommand._name;
-  if (subcommand._executableFile) {
-    bin = subcommand._executableFile;
-  }
+    // Not checking for help first. Unlikely to have mandatory and executable, and can't robustly test for help flags in external command.
+    this._checkForMissingMandatoryOptions();
 
-  const localBin = path.join(baseDir, bin);
-  if (fs.existsSync(localBin)) {
-    // prefer local `./<bin>` to bin in the $PATH
-    bin = localBin;
-  } else {
-    // Look for source files.
-    sourceExt.forEach((ext) => {
-      if (fs.existsSync(`${localBin}${ext}`)) {
-        bin = `${localBin}${ext}`;
-      }
-    });
-  }
-  launchWithNode = sourceExt.includes(path.extname(bin));
+    // Want the entry script as the reference for command name and directory for searching for other files.
+    const scriptPath = this.rawArgs[1];
 
-  let proc;
-  if (process.platform !== 'win32') {
-    if (launchWithNode) {
-      args.unshift(bin);
-      // add executable arguments to spawn
-      args = incrementNodeInspectorPort(process.execArgv).concat(args);
+    let baseDir;
+    try {
+      const resolvedLink = fs.realpathSync(scriptPath);
+      baseDir = path.dirname(resolvedLink);
+    } catch (e) {
+      baseDir = '.'; // dummy, probably not going to find executable!
+    }
+
+    // name of the subcommand, like `pm-install`
+    let bin = path.basename(scriptPath, path.extname(scriptPath)) + '-' + subcommand._name;
+    if (subcommand._executableFile) {
+      bin = subcommand._executableFile;
+    }
 
-      proc = spawn(process.argv[0], args, { stdio: 'inherit' });
+    const localBin = path.join(baseDir, bin);
+    if (fs.existsSync(localBin)) {
+      // prefer local `./<bin>` to bin in the $PATH
+      bin = localBin;
     } else {
-      proc = spawn(bin, args, { stdio: 'inherit' });
+      // Look for source files.
+      sourceExt.forEach((ext) => {
+        if (fs.existsSync(`${localBin}${ext}`)) {
+          bin = `${localBin}${ext}`;
+        }
+      });
     }
-  } else {
-    args.unshift(bin);
-    // add executable arguments to spawn
-    args = incrementNodeInspectorPort(process.execArgv).concat(args);
-    proc = spawn(process.execPath, args, { stdio: 'inherit' });
-  }
+    launchWithNode = sourceExt.includes(path.extname(bin));
 
-  const signals = ['SIGUSR1', 'SIGUSR2', 'SIGTERM', 'SIGINT', 'SIGHUP'];
-  signals.forEach(function(signal) {
-    process.on(signal, function() {
-      if (proc.killed === false && proc.exitCode === null) {
-        proc.kill(signal);
+    let proc;
+    if (process.platform !== 'win32') {
+      if (launchWithNode) {
+        args.unshift(bin);
+        // add executable arguments to spawn
+        args = incrementNodeInspectorPort(process.execArgv).concat(args);
+
+        proc = spawn(process.argv[0], args, { stdio: 'inherit' });
+      } else {
+        proc = spawn(bin, args, { stdio: 'inherit' });
       }
-    });
-  });
+    } else {
+      args.unshift(bin);
+      // add executable arguments to spawn
+      args = incrementNodeInspectorPort(process.execArgv).concat(args);
+      proc = spawn(process.execPath, args, { stdio: 'inherit' });
+    }
 
-  // By default terminate process when spawned process terminates.
-  // Suppressing the exit if exitCallback defined is a bit messy and of limited use, but does allow process to stay running!
-  const exitCallback = this._exitCallback;
-  if (!exitCallback) {
-    proc.on('close', process.exit.bind(process));
-  } else {
-    proc.on('close', () => {
-      exitCallback(new CommanderError(process.exitCode || 0, 'commander.executeSubCommandAsync', '(close)'));
+    const signals = ['SIGUSR1', 'SIGUSR2', 'SIGTERM', 'SIGINT', 'SIGHUP'];
+    signals.forEach(function(signal) {
+      process.on(signal, function() {
+        if (proc.killed === false && proc.exitCode === null) {
+          proc.kill(signal);
+        }
+      });
     });
-  }
-  proc.on('error', function(err) {
-    if (err.code === 'ENOENT') {
-      console.error('error: %s(1) does not exist', bin);
-    } else if (err.code === 'EACCES') {
-      console.error('error: %s(1) not executable', bin);
-    }
+
+    // By default terminate process when spawned process terminates.
+    // Suppressing the exit if exitCallback defined is a bit messy and of limited use, but does allow process to stay running!
+    const exitCallback = this._exitCallback;
     if (!exitCallback) {
-      process.exit(1);
+      proc.on('close', process.exit.bind(process));
     } else {
-      const wrappedError = new CommanderError(1, 'commander.executeSubCommandAsync', '(error)');
-      wrappedError.nestedError = err;
-      exitCallback(wrappedError);
+      proc.on('close', () => {
+        exitCallback(new CommanderError(process.exitCode || 0, 'commander.executeSubCommandAsync', '(close)'));
+      });
     }
-  });
-
-  // Store the reference to the child process
-  this.runningCommand = proc;
-};
+    proc.on('error', function(err) {
+      if (err.code === 'ENOENT') {
+        console.error('error: %s(1) does not exist', bin);
+      } else if (err.code === 'EACCES') {
+        console.error('error: %s(1) not executable', bin);
+      }
+      if (!exitCallback) {
+        process.exit(1);
+      } else {
+        const wrappedError = new CommanderError(1, 'commander.executeSubCommandAsync', '(error)');
+        wrappedError.nestedError = err;
+        exitCallback(wrappedError);
+      }
+    });
 
-/**
- * @api private
- */
-Command.prototype._dispatchSubcommand = function(commandName, operands, unknown) {
-  const subCommand = this._findCommand(commandName);
-  if (!subCommand) this._helpAndError();
-
-  if (subCommand._executableHandler) {
-    this._executeSubCommand(subCommand, operands.concat(unknown));
-  } else {
-    subCommand._parseCommand(operands, unknown);
-  }
-};
+    // Store the reference to the child process
+    this.runningCommand = proc;
+  };
 
-/**
- * Process arguments in context of this command.
- *
- * @api private
- */
+  /**
+   * @api private
+   */
+  _dispatchSubcommand(commandName, operands, unknown) {
+    const subCommand = this._findCommand(commandName);
+    if (!subCommand) this._helpAndError();
 
-Command.prototype._parseCommand = function(operands, unknown) {
-  const parsed = this.parseOptions(unknown);
-  operands = operands.concat(parsed.operands);
-  unknown = parsed.unknown;
-  this.args = operands.concat(unknown);
-
-  if (operands && this._findCommand(operands[0])) {
-    this._dispatchSubcommand(operands[0], operands.slice(1), unknown);
-  } else if (this._lazyHasImplicitHelpCommand() && operands[0] === this._helpCommandName) {
-    if (operands.length === 1) {
-      this.help();
+    if (subCommand._executableHandler) {
+      this._executeSubCommand(subCommand, operands.concat(unknown));
     } else {
-      this._dispatchSubcommand(operands[1], [], [this._helpLongFlag]);
-    }
-  } else if (this._defaultCommandName) {
-    outputHelpIfRequested(this, unknown); // Run the help for default command from parent rather than passing to default command
-    this._dispatchSubcommand(this._defaultCommandName, operands, unknown);
-  } else {
-    if (this.commands.length && this.args.length === 0 && !this._actionHandler && !this._defaultCommandName) {
-      // probaby missing subcommand and no handler, user needs help
-      this._helpAndError();
+      subCommand._parseCommand(operands, unknown);
     }
+  };
 
-    outputHelpIfRequested(this, parsed.unknown);
-    this._checkForMissingMandatoryOptions();
-    if (parsed.unknown.length > 0) {
-      this.unknownOption(parsed.unknown[0]);
-    }
+  /**
+   * Process arguments in context of this command.
+   *
+   * @api private
+   */
 
-    if (this._actionHandler) {
-      const self = this;
-      const args = this.args.slice();
-      this._args.forEach(function(arg, i) {
-        if (arg.required && args[i] == null) {
-          self.missingArgument(arg.name);
-        } else if (arg.variadic) {
-          if (i !== self._args.length - 1) {
-            self.variadicArgNotLast(arg.name);
-          }
+  _parseCommand(operands, unknown) {
+    const parsed = this.parseOptions(unknown);
+    operands = operands.concat(parsed.operands);
+    unknown = parsed.unknown;
+    this.args = operands.concat(unknown);
+
+    if (operands && this._findCommand(operands[0])) {
+      this._dispatchSubcommand(operands[0], operands.slice(1), unknown);
+    } else if (this._lazyHasImplicitHelpCommand() && operands[0] === this._helpCommandName) {
+      if (operands.length === 1) {
+        this.help();
+      } else {
+        this._dispatchSubcommand(operands[1], [], [this._helpLongFlag]);
+      }
+    } else if (this._defaultCommandName) {
+      outputHelpIfRequested(this, unknown); // Run the help for default command from parent rather than passing to default command
+      this._dispatchSubcommand(this._defaultCommandName, operands, unknown);
+    } else {
+      if (this.commands.length && this.args.length === 0 && !this._actionHandler && !this._defaultCommandName) {
+        // probaby missing subcommand and no handler, user needs help
+        this._helpAndError();
+      }
 
-          args[i] = args.splice(i);
-        }
-      });
+      outputHelpIfRequested(this, parsed.unknown);
+      this._checkForMissingMandatoryOptions();
+      if (parsed.unknown.length > 0) {
+        this.unknownOption(parsed.unknown[0]);
+      }
 
-      this._actionHandler(args);
-      this.emit('command:' + this.name(), operands, unknown);
-    } else if (operands.length) {
-      if (this._findCommand('*')) {
-        this._dispatchSubcommand('*', operands, unknown);
-      } else if (this.listenerCount('command:*')) {
-        this.emit('command:*', operands, unknown);
+      if (this._actionHandler) {
+        const self = this;
+        const args = this.args.slice();
+        this._args.forEach(function(arg, i) {
+          if (arg.required && args[i] == null) {
+            self.missingArgument(arg.name);
+          } else if (arg.variadic) {
+            if (i !== self._args.length - 1) {
+              self.variadicArgNotLast(arg.name);
+            }
+
+            args[i] = args.splice(i);
+          }
+        });
+
+        this._actionHandler(args);
+        this.emit('command:' + this.name(), operands, unknown);
+      } else if (operands.length) {
+        if (this._findCommand('*')) {
+          this._dispatchSubcommand('*', operands, unknown);
+        } else if (this.listenerCount('command:*')) {
+          this.emit('command:*', operands, unknown);
+        } else if (this.commands.length) {
+          this.unknownCommand();
+        }
       } else if (this.commands.length) {
-        this.unknownCommand();
+        // This command has subcommands and nothing hooked up at this level, so display help.
+        this._helpAndError();
+      } else {
+        // fall through for caller to handle after calling .parse()
       }
-    } else if (this.commands.length) {
-      // This command has subcommands and nothing hooked up at this level, so display help.
-      this._helpAndError();
-    } else {
-      // fall through for caller to handle after calling .parse()
     }
-  }
-};
+  };
 
-/**
- * Find matching command.
- *
- * @api private
- */
-Command.prototype._findCommand = function(name) {
-  if (!name) return undefined;
-  return this.commands.find(cmd => cmd._name === name || cmd._alias === name);
-};
+  /**
+   * Find matching command.
+   *
+   * @api private
+   */
+  _findCommand(name) {
+    if (!name) return undefined;
+    return this.commands.find(cmd => cmd._name === name || cmd._alias === name);
+  };
 
-/**
- * Return an option matching `arg` if any.
- *
- * @param {String} arg
- * @return {Option}
- * @api private
- */
+  /**
+   * Return an option matching `arg` if any.
+   *
+   * @param {String} arg
+   * @return {Option}
+   * @api private
+   */
 
-Command.prototype._findOption = function(arg) {
-  return this.options.find(option => option.is(arg));
-};
+  _findOption(arg) {
+    return this.options.find(option => option.is(arg));
+  };
 
-/**
- * Display an error message if a mandatory option does not have a value.
- * Lazy calling after checking for help flags from leaf subcommand.
- *
- * @api private
- */
+  /**
+   * Display an error message if a mandatory option does not have a value.
+   * Lazy calling after checking for help flags from leaf subcommand.
+   *
+   * @api private
+   */
 
-Command.prototype._checkForMissingMandatoryOptions = function() {
-  // Walk up hierarchy so can call in subcommand after checking for displaying help.
-  for (var cmd = this; cmd; cmd = cmd.parent) {
-    cmd.options.forEach((anOption) => {
-      if (anOption.mandatory && (cmd._getOptionValue(anOption.attributeName()) === undefined)) {
-        cmd.missingMandatoryOptionValue(anOption);
-      }
-    });
-  }
-};
+  _checkForMissingMandatoryOptions() {
+    // Walk up hierarchy so can call in subcommand after checking for displaying help.
+    for (var cmd = this; cmd; cmd = cmd.parent) {
+      cmd.options.forEach((anOption) => {
+        if (anOption.mandatory && (cmd._getOptionValue(anOption.attributeName()) === undefined)) {
+          cmd.missingMandatoryOptionValue(anOption);
+        }
+      });
+    }
+  };
 
-/**
- * Parse options from `argv` removing known options,
- * and return argv split into operands and unknown arguments.
- *
- * Examples:
- *
- *    argv => operands, unknown
- *    --known kkk op => [op], []
- *    op --known kkk => [op], []
- *    sub --unknown uuu op => [sub], [--unknown uuu op]
- *    sub -- --unknown uuu op => [sub --unknown uuu op], []
- *
- * @param {String[]} argv
- * @return {{operands: String[], unknown: String[]}}
- * @api public
- */
+  /**
+   * Parse options from `argv` removing known options,
+   * and return argv split into operands and unknown arguments.
+   *
+   * Examples:
+   *
+   *    argv => operands, unknown
+   *    --known kkk op => [op], []
+   *    op --known kkk => [op], []
+   *    sub --unknown uuu op => [sub], [--unknown uuu op]
+   *    sub -- --unknown uuu op => [sub --unknown uuu op], []
+   *
+   * @param {String[]} argv
+   * @return {{operands: String[], unknown: String[]}}
+   * @api public
+   */
 
-Command.prototype.parseOptions = function(argv) {
-  const operands = []; // operands, not options or values
-  const unknown = []; // first unknown option and remaining unknown args
-  let dest = operands;
-  const args = argv.slice();
+  parseOptions(argv) {
+    const operands = []; // operands, not options or values
+    const unknown = []; // first unknown option and remaining unknown args
+    let dest = operands;
+    const args = argv.slice();
 
-  function maybeOption(arg) {
-    return arg.length > 1 && arg[0] === '-';
-  }
+    function maybeOption(arg) {
+      return arg.length > 1 && arg[0] === '-';
+    }
 
-  // parse options
-  while (args.length) {
-    const arg = args.shift();
+    // parse options
+    while (args.length) {
+      const arg = args.shift();
 
-    // literal
-    if (arg === '--') {
-      if (dest === unknown) dest.push(arg);
-      dest.push(...args);
-      break;
-    }
+      // literal
+      if (arg === '--') {
+        if (dest === unknown) dest.push(arg);
+        dest.push(...args);
+        break;
+      }
 
-    if (maybeOption(arg)) {
-      const option = this._findOption(arg);
-      // recognised option, call listener to assign value with possible custom processing
-      if (option) {
-        if (option.required) {
-          const value = args.shift();
-          if (value === undefined) this.optionMissingArgument(option);
-          this.emit(`option:${option.name()}`, value);
-        } else if (option.optional) {
-          let value = null;
-          // historical behaviour is optional value is following arg unless an option
-          if (args.length > 0 && !maybeOption(args[0])) {
-            value = args.shift();
+      if (maybeOption(arg)) {
+        const option = this._findOption(arg);
+        // recognised option, call listener to assign value with possible custom processing
+        if (option) {
+          if (option.required) {
+            const value = args.shift();
+            if (value === undefined) this.optionMissingArgument(option);
+            this.emit(`option:${option.name()}`, value);
+          } else if (option.optional) {
+            let value = null;
+            // historical behaviour is optional value is following arg unless an option
+            if (args.length > 0 && !maybeOption(args[0])) {
+              value = args.shift();
+            }
+            this.emit(`option:${option.name()}`, value);
+          } else { // boolean flag
+            this.emit(`option:${option.name()}`);
           }
-          this.emit(`option:${option.name()}`, value);
-        } else { // boolean flag
-          this.emit(`option:${option.name()}`);
+          continue;
         }
-        continue;
       }
-    }
 
-    // Look for combo options following single dash, eat first one if known.
-    if (arg.length > 2 && arg[0] === '-' && arg[1] !== '-') {
-      const option = this._findOption(`-${arg[1]}`);
-      if (option) {
-        if (option.required || option.optional) {
-          // option with value following in same argument
-          this.emit(`option:${option.name()}`, arg.slice(2));
-        } else {
-          // boolean option, emit and put back remainder of arg for further processing
-          this.emit(`option:${option.name()}`);
-          args.unshift(`-${arg.slice(2)}`);
+      // Look for combo options following single dash, eat first one if known.
+      if (arg.length > 2 && arg[0] === '-' && arg[1] !== '-') {
+        const option = this._findOption(`-${arg[1]}`);
+        if (option) {
+          if (option.required || option.optional) {
+            // option with value following in same argument
+            this.emit(`option:${option.name()}`, arg.slice(2));
+          } else {
+            // boolean option, emit and put back remainder of arg for further processing
+            this.emit(`option:${option.name()}`);
+            args.unshift(`-${arg.slice(2)}`);
+          }
+          continue;
         }
-        continue;
       }
-    }
 
-    // Look for known long flag with value, like --foo=bar
-    if (/^--[^=]+=/.test(arg)) {
-      const index = arg.indexOf('=');
-      const option = this._findOption(arg.slice(0, index));
-      if (option && (option.required || option.optional)) {
-        this.emit(`option:${option.name()}`, arg.slice(index + 1));
-        continue;
+      // Look for known long flag with value, like --foo=bar
+      if (/^--[^=]+=/.test(arg)) {
+        const index = arg.indexOf('=');
+        const option = this._findOption(arg.slice(0, index));
+        if (option && (option.required || option.optional)) {
+          this.emit(`option:${option.name()}`, arg.slice(index + 1));
+          continue;
+        }
       }
-    }
 
-    // looks like an option but unknown, unknowns from here
-    if (arg.length > 1 && arg[0] === '-') {
-      dest = unknown;
-    }
+      // looks like an option but unknown, unknowns from here
+      if (arg.length > 1 && arg[0] === '-') {
+        dest = unknown;
+      }
 
-    // add arg
-    dest.push(arg);
-  }
+      // add arg
+      dest.push(arg);
+    }
 
-  return { operands, unknown };
-};
+    return { operands, unknown };
+  };
 
-/**
- * Return an object containing options as key-value pairs
- *
- * @return {Object}
- * @api public
- */
-Command.prototype.opts = function() {
-  if (this._storeOptionsAsProperties) {
-    // Preserve original behaviour so backwards compatible when still using properties
-    var result = {},
-      len = this.options.length;
-
-    for (var i = 0; i < len; i++) {
-      var key = this.options[i].attributeName();
-      result[key] = key === this._versionOptionName ? this._version : this[key];
+  /**
+   * Return an object containing options as key-value pairs
+   *
+   * @return {Object}
+   * @api public
+   */
+  opts() {
+    if (this._storeOptionsAsProperties) {
+      // Preserve original behaviour so backwards compatible when still using properties
+      var result = {},
+        len = this.options.length;
+
+      for (var i = 0; i < len; i++) {
+        var key = this.options[i].attributeName();
+        result[key] = key === this._versionOptionName ? this._version : this[key];
+      }
+      return result;
     }
-    return result;
-  }
 
-  return this._optionValues;
-};
-
-/**
- * Argument `name` is missing.
- *
- * @param {String} name
- * @api private
- */
+    return this._optionValues;
+  };
 
-Command.prototype.missingArgument = function(name) {
-  const message = `error: missing required argument '${name}'`;
-  console.error(message);
-  this._exit(1, 'commander.missingArgument', message);
-};
+  /**
+   * Argument `name` is missing.
+   *
+   * @param {String} name
+   * @api private
+   */
 
-/**
- * `Option` is missing an argument, but received `flag` or nothing.
- *
- * @param {Option} option
- * @param {String} [flag]
- * @api private
- */
+  missingArgument(name) {
+    const message = `error: missing required argument '${name}'`;
+    console.error(message);
+    this._exit(1, 'commander.missingArgument', message);
+  };
 
-Command.prototype.optionMissingArgument = function(option, flag) {
-  let message;
-  if (flag) {
-    message = `error: option '${option.flags}' argument missing, got '${flag}'`;
-  } else {
-    message = `error: option '${option.flags}' argument missing`;
-  }
-  console.error(message);
-  this._exit(1, 'commander.optionMissingArgument', message);
-};
+  /**
+   * `Option` is missing an argument, but received `flag` or nothing.
+   *
+   * @param {Option} option
+   * @param {String} [flag]
+   * @api private
+   */
 
-/**
- * `Option` does not have a value, and is a mandatory option.
- *
- * @param {Option} option
- * @api private
- */
+  optionMissingArgument(option, flag) {
+    let message;
+    if (flag) {
+      message = `error: option '${option.flags}' argument missing, got '${flag}'`;
+    } else {
+      message = `error: option '${option.flags}' argument missing`;
+    }
+    console.error(message);
+    this._exit(1, 'commander.optionMissingArgument', message);
+  };
 
-Command.prototype.missingMandatoryOptionValue = function(option) {
-  const message = `error: required option '${option.flags}' not specified`;
-  console.error(message);
-  this._exit(1, 'commander.missingMandatoryOptionValue', message);
-};
+  /**
+   * `Option` does not have a value, and is a mandatory option.
+   *
+   * @param {Option} option
+   * @api private
+   */
 
-/**
- * Unknown option `flag`.
- *
- * @param {String} flag
- * @api private
- */
+  missingMandatoryOptionValue(option) {
+    const message = `error: required option '${option.flags}' not specified`;
+    console.error(message);
+    this._exit(1, 'commander.missingMandatoryOptionValue', message);
+  };
 
-Command.prototype.unknownOption = function(flag) {
-  if (this._allowUnknownOption) return;
-  const message = `error: unknown option '${flag}'`;
-  console.error(message);
-  this._exit(1, 'commander.unknownOption', message);
-};
+  /**
+   * Unknown option `flag`.
+   *
+   * @param {String} flag
+   * @api private
+   */
 
-/**
- * Unknown command.
- *
- * @param {String} flag
- * @api private
- */
+  unknownOption(flag) {
+    if (this._allowUnknownOption) return;
+    const message = `error: unknown option '${flag}'`;
+    console.error(message);
+    this._exit(1, 'commander.unknownOption', message);
+  };
 
-Command.prototype.unknownCommand = function() {
-  const message = `error: unknown command '${this.args[0]}'`;
-  console.error(message);
-  this._exit(1, 'commander.unknownCommand', message);
-};
+  /**
+   * Unknown command.
+   *
+   * @param {String} flag
+   * @api private
+   */
 
-/**
- * Variadic argument with `name` is not the last argument as required.
- *
- * @param {String} name
- * @api private
- */
+  unknownCommand() {
+    const message = `error: unknown command '${this.args[0]}'`;
+    console.error(message);
+    this._exit(1, 'commander.unknownCommand', message);
+  };
 
-Command.prototype.variadicArgNotLast = function(name) {
-  const message = `error: variadic arguments must be last '${name}'`;
-  console.error(message);
-  this._exit(1, 'commander.variadicArgNotLast', message);
-};
+  /**
+   * Variadic argument with `name` is not the last argument as required.
+   *
+   * @param {String} name
+   * @api private
+   */
 
-/**
- * Set the program version to `str`.
- *
- * This method auto-registers the "-V, --version" flag
- * which will print the version number when passed.
- *
- * You can optionally supply the  flags and description to override the defaults.
- *
- * @param {String} str
- * @param {String} [flags]
- * @param {String} [description]
- * @return {Command} for chaining
- * @api public
- */
+  variadicArgNotLast(name) {
+    const message = `error: variadic arguments must be last '${name}'`;
+    console.error(message);
+    this._exit(1, 'commander.variadicArgNotLast', message);
+  };
 
-Command.prototype.version = function(str, flags, description) {
-  if (arguments.length === 0) return this._version;
-  this._version = str;
-  flags = flags || '-V, --version';
-  description = description || 'output the version number';
-  var versionOption = new Option(flags, description);
-  this._versionOptionName = versionOption.long.substr(2) || 'version';
-  this.options.push(versionOption);
-  var self = this;
-  this.on('option:' + this._versionOptionName, function() {
-    process.stdout.write(str + '\n');
-    self._exit(0, 'commander.version', str);
-  });
-  return this;
-};
+  /**
+   * Set the program version to `str`.
+   *
+   * This method auto-registers the "-V, --version" flag
+   * which will print the version number when passed.
+   *
+   * You can optionally supply the  flags and description to override the defaults.
+   *
+   * @param {String} str
+   * @param {String} [flags]
+   * @param {String} [description]
+   * @return {Command} for chaining
+   * @api public
+   */
 
-/**
- * Set the description to `str`.
- *
- * @param {String} str
- * @param {Object} [argsDescription]
- * @return {String|Command}
- * @api public
- */
+  version(str, flags, description) {
+    if (arguments.length === 0) return this._version;
+    this._version = str;
+    flags = flags || '-V, --version';
+    description = description || 'output the version number';
+    var versionOption = new Option(flags, description);
+    this._versionOptionName = versionOption.long.substr(2) || 'version';
+    this.options.push(versionOption);
+    var self = this;
+    this.on('option:' + this._versionOptionName, function() {
+      process.stdout.write(str + '\n');
+      self._exit(0, 'commander.version', str);
+    });
+    return this;
+  };
 
-Command.prototype.description = function(str, argsDescription) {
-  if (arguments.length === 0) return this._description;
-  this._description = str;
-  this._argsDescription = argsDescription;
-  return this;
-};
+  /**
+   * Set the description to `str`.
+   *
+   * @param {String} str
+   * @param {Object} [argsDescription]
+   * @return {String|Command}
+   * @api public
+   */
 
-/**
- * Set an alias for the command
- *
- * @param {String} alias
- * @return {String|Command}
- * @api public
- */
+  description(str, argsDescription) {
+    if (arguments.length === 0) return this._description;
+    this._description = str;
+    this._argsDescription = argsDescription;
+    return this;
+  };
 
-Command.prototype.alias = function(alias) {
-  var command = this;
-  if (this.commands.length !== 0) {
-    command = this.commands[this.commands.length - 1];
-  }
+  /**
+   * Set an alias for the command
+   *
+   * @param {String} alias
+   * @return {String|Command}
+   * @api public
+   */
 
-  if (arguments.length === 0) return command._alias;
+  alias(alias) {
+    var command = this;
+    if (this.commands.length !== 0) {
+      command = this.commands[this.commands.length - 1];
+    }
 
-  if (alias === command._name) throw new Error('Command alias can\'t be the same as its name');
+    if (arguments.length === 0) return command._alias;
 
-  command._alias = alias;
-  return this;
-};
+    if (alias === command._name) throw new Error('Command alias can\'t be the same as its name');
 
-/**
- * Set / get the command usage `str`.
- *
- * @param {String} [str]
- * @return {String|Command}
- * @api public
- */
+    command._alias = alias;
+    return this;
+  };
 
-Command.prototype.usage = function(str) {
-  var args = this._args.map(function(arg) {
-    return humanReadableArgName(arg);
-  });
+  /**
+   * Set / get the command usage `str`.
+   *
+   * @param {String} [str]
+   * @return {String|Command}
+   * @api public
+   */
 
-  var usage = '[options]' +
-    (this.commands.length ? ' [command]' : '') +
-    (this._args.length ? ' ' + args.join(' ') : '');
+  usage(str) {
+    var args = this._args.map(function(arg) {
+      return humanReadableArgName(arg);
+    });
 
-  if (arguments.length === 0) return this._usage || usage;
-  this._usage = str;
+    var usage = '[options]' +
+      (this.commands.length ? ' [command]' : '') +
+      (this._args.length ? ' ' + args.join(' ') : '');
 
-  return this;
-};
+    if (arguments.length === 0) return this._usage || usage;
+    this._usage = str;
 
-/**
- * Get or set the name of the command
- *
- * @param {String} [str]
- * @return {String|Command}
- * @api public
- */
+    return this;
+  };
 
-Command.prototype.name = function(str) {
-  if (arguments.length === 0) return this._name;
-  this._name = str;
-  return this;
-};
+  /**
+   * Get or set the name of the command
+   *
+   * @param {String} [str]
+   * @return {String|Command}
+   * @api public
+   */
 
-/**
- * Return prepared commands.
- *
- * @return {Array}
- * @api private
- */
+  name(str) {
+    if (arguments.length === 0) return this._name;
+    this._name = str;
+    return this;
+  };
 
-Command.prototype.prepareCommands = function() {
-  const commandDetails = this.commands.filter(function(cmd) {
-    return !cmd._noHelp;
-  }).map(function(cmd) {
-    var args = cmd._args.map(function(arg) {
-      return humanReadableArgName(arg);
-    }).join(' ');
+  /**
+   * Return prepared commands.
+   *
+   * @return {Array}
+   * @api private
+   */
 
-    return [
-      cmd._name +
-        (cmd._alias ? '|' + cmd._alias : '') +
-        (cmd.options.length ? ' [options]' : '') +
-        (args ? ' ' + args : ''),
-      cmd._description
-    ];
-  });
+  prepareCommands() {
+    const commandDetails = this.commands.filter(function(cmd) {
+      return !cmd._noHelp;
+    }).map(function(cmd) {
+      var args = cmd._args.map(function(arg) {
+        return humanReadableArgName(arg);
+      }).join(' ');
+
+      return [
+        cmd._name +
+          (cmd._alias ? '|' + cmd._alias : '') +
+          (cmd.options.length ? ' [options]' : '') +
+          (args ? ' ' + args : ''),
+        cmd._description
+      ];
+    });
 
-  if (this._lazyHasImplicitHelpCommand()) {
-    commandDetails.push([this._helpCommandnameAndArgs, this._helpCommandDescription]);
-  }
-  return commandDetails;
-};
+    if (this._lazyHasImplicitHelpCommand()) {
+      commandDetails.push([this._helpCommandnameAndArgs, this._helpCommandDescription]);
+    }
+    return commandDetails;
+  };
 
-/**
- * Return the largest command length.
- *
- * @return {Number}
- * @api private
- */
+  /**
+   * Return the largest command length.
+   *
+   * @return {Number}
+   * @api private
+   */
 
-Command.prototype.largestCommandLength = function() {
-  var commands = this.prepareCommands();
-  return commands.reduce(function(max, command) {
-    return Math.max(max, command[0].length);
-  }, 0);
-};
+  largestCommandLength() {
+    var commands = this.prepareCommands();
+    return commands.reduce(function(max, command) {
+      return Math.max(max, command[0].length);
+    }, 0);
+  };
 
-/**
- * Return the largest option length.
- *
- * @return {Number}
- * @api private
- */
+  /**
+   * Return the largest option length.
+   *
+   * @return {Number}
+   * @api private
+   */
 
-Command.prototype.largestOptionLength = function() {
-  var options = [].slice.call(this.options);
-  options.push({
-    flags: this._helpFlags
-  });
+  largestOptionLength() {
+    var options = [].slice.call(this.options);
+    options.push({
+      flags: this._helpFlags
+    });
 
-  return options.reduce(function(max, option) {
-    return Math.max(max, option.flags.length);
-  }, 0);
-};
+    return options.reduce(function(max, option) {
+      return Math.max(max, option.flags.length);
+    }, 0);
+  };
 
-/**
- * Return the largest arg length.
- *
- * @return {Number}
- * @api private
- */
+  /**
+   * Return the largest arg length.
+   *
+   * @return {Number}
+   * @api private
+   */
 
-Command.prototype.largestArgLength = function() {
-  return this._args.reduce(function(max, arg) {
-    return Math.max(max, arg.name.length);
-  }, 0);
-};
+  largestArgLength() {
+    return this._args.reduce(function(max, arg) {
+      return Math.max(max, arg.name.length);
+    }, 0);
+  };
 
-/**
- * Return the pad width.
- *
- * @return {Number}
- * @api private
- */
+  /**
+   * Return the pad width.
+   *
+   * @return {Number}
+   * @api private
+   */
 
-Command.prototype.padWidth = function() {
-  var width = this.largestOptionLength();
-  if (this._argsDescription && this._args.length) {
-    if (this.largestArgLength() > width) {
-      width = this.largestArgLength();
+  padWidth() {
+    var width = this.largestOptionLength();
+    if (this._argsDescription && this._args.length) {
+      if (this.largestArgLength() > width) {
+        width = this.largestArgLength();
+      }
     }
-  }
 
-  if (this.commands && this.commands.length) {
-    if (this.largestCommandLength() > width) {
-      width = this.largestCommandLength();
+    if (this.commands && this.commands.length) {
+      if (this.largestCommandLength() > width) {
+        width = this.largestCommandLength();
+      }
     }
-  }
 
-  return width;
-};
+    return width;
+  };
 
-/**
- * Return help for options.
- *
- * @return {String}
- * @api private
- */
+  /**
+   * Return help for options.
+   *
+   * @return {String}
+   * @api private
+   */
 
-Command.prototype.optionHelp = function() {
-  var width = this.padWidth();
+  optionHelp() {
+    var width = this.padWidth();
 
-  var columns = process.stdout.columns || 80;
-  var descriptionWidth = columns - width - 4;
+    var columns = process.stdout.columns || 80;
+    var descriptionWidth = columns - width - 4;
 
-  // Append the help information
-  return this.options.map(function(option) {
-    const fullDesc = option.description +
-      ((!option.negate && option.defaultValue !== undefined) ? ' (default: ' + JSON.stringify(option.defaultValue) + ')' : '');
-    return pad(option.flags, width) + '  ' + optionalWrap(fullDesc, descriptionWidth, width + 2);
-  }).concat([pad(this._helpFlags, width) + '  ' + optionalWrap(this._helpDescription, descriptionWidth, width + 2)])
-    .join('\n');
-};
+    // Append the help information
+    return this.options.map(function(option) {
+      const fullDesc = option.description +
+        ((!option.negate && option.defaultValue !== undefined) ? ' (default: ' + JSON.stringify(option.defaultValue) + ')' : '');
+      return pad(option.flags, width) + '  ' + optionalWrap(fullDesc, descriptionWidth, width + 2);
+    }).concat([pad(this._helpFlags, width) + '  ' + optionalWrap(this._helpDescription, descriptionWidth, width + 2)])
+      .join('\n');
+  };
 
-/**
- * Return command help documentation.
- *
- * @return {String}
- * @api private
- */
+  /**
+   * Return command help documentation.
+   *
+   * @return {String}
+   * @api private
+   */
 
-Command.prototype.commandHelp = function() {
-  if (!this.commands.length && !this._lazyHasImplicitHelpCommand()) return '';
+  commandHelp() {
+    if (!this.commands.length && !this._lazyHasImplicitHelpCommand()) return '';
 
-  var commands = this.prepareCommands();
-  var width = this.padWidth();
+    var commands = this.prepareCommands();
+    var width = this.padWidth();
 
-  var columns = process.stdout.columns || 80;
-  var descriptionWidth = columns - width - 4;
+    var columns = process.stdout.columns || 80;
+    var descriptionWidth = columns - width - 4;
 
-  return [
-    'Commands:',
-    commands.map(function(cmd) {
-      var desc = cmd[1] ? '  ' + cmd[1] : '';
-      return (desc ? pad(cmd[0], width) : cmd[0]) + optionalWrap(desc, descriptionWidth, width + 2);
-    }).join('\n').replace(/^/gm, '  '),
-    ''
-  ].join('\n');
-};
+    return [
+      'Commands:',
+      commands.map(function(cmd) {
+        var desc = cmd[1] ? '  ' + cmd[1] : '';
+        return (desc ? pad(cmd[0], width) : cmd[0]) + optionalWrap(desc, descriptionWidth, width + 2);
+      }).join('\n').replace(/^/gm, '  '),
+      ''
+    ].join('\n');
+  };
 
-/**
- * Return program help documentation.
- *
- * @return {String}
- * @api private
- */
+  /**
+   * Return program help documentation.
+   *
+   * @return {String}
+   * @api private
+   */
 
-Command.prototype.helpInformation = function() {
-  var desc = [];
-  if (this._description) {
-    desc = [
-      this._description,
+  helpInformation() {
+    var desc = [];
+    if (this._description) {
+      desc = [
+        this._description,
+        ''
+      ];
+
+      var argsDescription = this._argsDescription;
+      if (argsDescription && this._args.length) {
+        var width = this.padWidth();
+        var columns = process.stdout.columns || 80;
+        var descriptionWidth = columns - width - 5;
+        desc.push('Arguments:');
+        desc.push('');
+        this._args.forEach(function(arg) {
+          desc.push('  ' + pad(arg.name, width) + '  ' + wrap(argsDescription[arg.name], descriptionWidth, width + 4));
+        });
+        desc.push('');
+      }
+    }
+
+    var cmdName = this._name;
+    if (this._alias) {
+      cmdName = cmdName + '|' + this._alias;
+    }
+    var parentCmdNames = '';
+    for (var parentCmd = this.parent; parentCmd; parentCmd = parentCmd.parent) {
+      parentCmdNames = parentCmd.name() + ' ' + parentCmdNames;
+    }
+    var usage = [
+      'Usage: ' + parentCmdNames + cmdName + ' ' + this.usage(),
       ''
     ];
 
-    var argsDescription = this._argsDescription;
-    if (argsDescription && this._args.length) {
-      var width = this.padWidth();
-      var columns = process.stdout.columns || 80;
-      var descriptionWidth = columns - width - 5;
-      desc.push('Arguments:');
-      desc.push('');
-      this._args.forEach(function(arg) {
-        desc.push('  ' + pad(arg.name, width) + '  ' + wrap(argsDescription[arg.name], descriptionWidth, width + 4));
-      });
-      desc.push('');
+    var cmds = [];
+    var commandHelp = this.commandHelp();
+    if (commandHelp) cmds = [commandHelp];
+
+    var options = [
+      'Options:',
+      '' + this.optionHelp().replace(/^/gm, '  '),
+      ''
+    ];
+
+    return usage
+      .concat(desc)
+      .concat(options)
+      .concat(cmds)
+      .join('\n');
+  };
+
+  /**
+   * Output help information for this command.
+   *
+   * When listener(s) are available for the helpLongFlag
+   * those callbacks are invoked.
+   *
+   * @api public
+   */
+
+  outputHelp(cb) {
+    if (!cb) {
+      cb = function(passthru) {
+        return passthru;
+      };
     }
-  }
+    const cbOutput = cb(this.helpInformation());
+    if (typeof cbOutput !== 'string' && !Buffer.isBuffer(cbOutput)) {
+      throw new Error('outputHelp callback must return a string or a Buffer');
+    }
+    process.stdout.write(cbOutput);
+    this.emit(this._helpLongFlag);
+  };
 
-  var cmdName = this._name;
-  if (this._alias) {
-    cmdName = cmdName + '|' + this._alias;
-  }
-  var parentCmdNames = '';
-  for (var parentCmd = this.parent; parentCmd; parentCmd = parentCmd.parent) {
-    parentCmdNames = parentCmd.name() + ' ' + parentCmdNames;
-  }
-  var usage = [
-    'Usage: ' + parentCmdNames + cmdName + ' ' + this.usage(),
-    ''
-  ];
-
-  var cmds = [];
-  var commandHelp = this.commandHelp();
-  if (commandHelp) cmds = [commandHelp];
-
-  var options = [
-    'Options:',
-    '' + this.optionHelp().replace(/^/gm, '  '),
-    ''
-  ];
-
-  return usage
-    .concat(desc)
-    .concat(options)
-    .concat(cmds)
-    .join('\n');
-};
+  /**
+   * You can pass in flags and a description to override the help
+   * flags and help description for your command.
+   *
+   * @param {String} [flags]
+   * @param {String} [description]
+   * @return {Command}
+   * @api public
+   */
 
-/**
- * Output help information for this command.
- *
- * When listener(s) are available for the helpLongFlag
- * those callbacks are invoked.
- *
- * @api public
- */
+  helpOption(flags, description) {
+    this._helpFlags = flags || this._helpFlags;
+    this._helpDescription = description || this._helpDescription;
 
-Command.prototype.outputHelp = function(cb) {
-  if (!cb) {
-    cb = function(passthru) {
-      return passthru;
-    };
-  }
-  const cbOutput = cb(this.helpInformation());
-  if (typeof cbOutput !== 'string' && !Buffer.isBuffer(cbOutput)) {
-    throw new Error('outputHelp callback must return a string or a Buffer');
-  }
-  process.stdout.write(cbOutput);
-  this.emit(this._helpLongFlag);
-};
+    var splitFlags = this._helpFlags.split(/[ ,|]+/);
 
-/**
- * You can pass in flags and a description to override the help
- * flags and help description for your command.
- *
- * @param {String} [flags]
- * @param {String} [description]
- * @return {Command}
- * @api public
- */
+    if (splitFlags.length > 1) this._helpShortFlag = splitFlags.shift();
 
-Command.prototype.helpOption = function(flags, description) {
-  this._helpFlags = flags || this._helpFlags;
-  this._helpDescription = description || this._helpDescription;
+    this._helpLongFlag = splitFlags.shift();
 
-  var splitFlags = this._helpFlags.split(/[ ,|]+/);
+    return this;
+  };
 
-  if (splitFlags.length > 1) this._helpShortFlag = splitFlags.shift();
+  /**
+   * Output help information and exit.
+   *
+   * @param {Function} [cb]
+   * @api public
+   */
 
-  this._helpLongFlag = splitFlags.shift();
+  help(cb) {
+    this.outputHelp(cb);
+    // exitCode: preserving original behaviour which was calling process.exit()
+    // message: do not have all displayed text available so only passing placeholder.
+    this._exit(process.exitCode || 0, 'commander.help', '(outputHelp)');
+  };
 
-  return this;
+  /**
+   * Output help information and exit. Display for error situations.
+   *
+   * @api private
+   */
+
+  _helpAndError() {
+    this.outputHelp();
+    // message: do not have all displayed text available so only passing placeholder.
+    this._exit(1, 'commander.help', '(outputHelp)');
+  };
 };
 
 /**
- * Output help information and exit.
- *
- * @param {Function} [cb]
- * @api public
+ * Expose the root command.
  */
 
-Command.prototype.help = function(cb) {
-  this.outputHelp(cb);
-  // exitCode: preserving original behaviour which was calling process.exit()
-  // message: do not have all displayed text available so only passing placeholder.
-  this._exit(process.exitCode || 0, 'commander.help', '(outputHelp)');
-};
+exports = module.exports = new Command();
 
 /**
- * Output help information and exit. Display for error situations.
- *
- * @api private
+ * Expose classes
  */
 
-Command.prototype._helpAndError = function() {
-  this.outputHelp();
-  // message: do not have all displayed text available so only passing placeholder.
-  this._exit(1, 'commander.help', '(outputHelp)');
-};
+exports.Command = Command;
+exports.Option = Option;
+exports.CommanderError = CommanderError;
 
 /**
  * Camel-case the given `flag`