https://github.com/babel/babel-loader/issues/214#issuecomment-184297416

input.js

const nameBuilder = function (firstName = "John", lastName = "Doe") {
  console.log(firstName + " " + lastName);
};

nameBuilder();

output.js

/******/ (function(modules) { // webpackBootstrap
/******/ 	// The module cache
/******/ 	var installedModules = {};

/******/ 	// The require function
/******/ 	function __webpack_require__(moduleId) {

/******/ 		// Check if module is in cache
/******/ 		if(installedModules[moduleId])
/******/ 			return installedModules[moduleId].exports;

/******/ 		// Create a new module (and put it into the cache)
/******/ 		var module = installedModules[moduleId] = {
/******/ 			exports: {},
/******/ 			id: moduleId,
/******/ 			loaded: false
/******/ 		};

/******/ 		// Execute the module function
/******/ 		modules[moduleId].call(module.exports, module, module.exports, __webpack_require__);

/******/ 		// Flag the module as loaded
/******/ 		module.loaded = true;

/******/ 		// Return the exports of the module
/******/ 		return module.exports;
/******/ 	}


/******/ 	// expose the modules object (__webpack_modules__)
/******/ 	__webpack_require__.m = modules;

/******/ 	// expose the module cache
/******/ 	__webpack_require__.c = installedModules;

/******/ 	// __webpack_public_path__
/******/ 	__webpack_require__.p = "";

/******/ 	// Load entry module and return exports
/******/ 	return __webpack_require__(0);
/******/ })
/************************************************************************/
/******/ ([
/* 0 */
/***/ function(module, exports) {

	"use strict";

	var nameBuilder = function nameBuilder() {
	  var firstName = arguments.length <= 0 || arguments[0] === undefined ? "John" : arguments[0];
	  var lastName = arguments.length <= 1 || arguments[1] === undefined ? "Doe" : arguments[1];

	  console.log(firstName + " " + lastName);
	};

	nameBuilder();

/***/ }
/******/ ]);

package.json

{
  "name": "test",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "start": "webpack --config webpack.config.js"
  },
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "babel-core": "^6.5.2",
    "babel-loader": "^6.2.2",
    "babel-preset-es2015": "^6.5.0",
    "webpack": "^1.12.13"
  }
}

webpack.config.js

module.exports = {
  entry: './input.js',
  output: {
    filename: 'output.js'
  },
  module: {
    loaders: [
      {
        test: /\.js?$/,
        exclude: /node_modules/,
        loader: 'babel',
        query: {
          presets: ['es2015']
        }
      }
    ]
  }
};