Gulpfile.js

var gulp = require('gulp');

var sourcemaps = require('gulp-sourcemaps');
var source = require('vinyl-source-stream');
var buffer = require('vinyl-buffer');
var browserify = require('browserify');
var babel = require('babelify');
var uglify = require('gulp-uglify');
var eslint = require('gulp-eslint');

var postcss = require('gulp-postcss');
var autoprefixer = require('autoprefixer');
var importCSS = require('postcss-import');
var simpleCSSVars = require('postcss-simple-vars');

var browserSync = require('browser-sync').create();

/** Convert our ES6 JS into a usable form for browsers. */
function buildJS(dev) {
  var bundler = browserify('./source/index.js', {debug: dev})
                .transform(babel, {presets: ['es2015'], global: true});

  return bundler.bundle()
    .on('error', function(error) {
      console.error(error); this.emit('end');
    })
    .pipe(source('index.js'))
    .pipe(buffer())
    .pipe(sourcemaps.init({loadMaps: true}))
    //.pipe(uglify({preserveComments: 'license'}))
    .pipe(sourcemaps.write())
    .pipe(gulp.dest('./build/'));
}

/** Use PostCSS to merge concatenate our CSS together. */
function css() {
  var processors = [
    importCSS(),
    autoprefixer(),
    simpleCSSVars()
  ];
  return gulp.src('./source/app.css')
    .pipe(postcss(processors))
    .pipe(gulp.dest('./build/'));
}

gulp.task('javascript', function() {
  return buildJS();
});

gulp.task('javascript_dev', function() {
  return buildJS(true);
});

gulp.task('javascriptsync', ['javascript_dev'], function() {
  browserSync.reload();
});

gulp.task('serve', function() {
  browserSync.init({
    server: {
      baseDir: './'
    },
    ghostMode: false
  });

  gulp.watch('./source/*.css', ['synccss']);
  gulp.watch('./source/*.js', ['javascriptsync']);
  gulp.watch('./index.html', browserSync.reload);
});

gulp.task('css', css);
gulp.task('synccss', function() {
  css().pipe(browserSync.stream());
});

gulp.task('lint', function() {
  return gulp.src('source/**/*.js').pipe(eslint())
    .pipe(eslint.format())
    .pipe(eslint.failOnError());
});

gulp.task('default', ['javascript', 'css']);