gulpfile.js

// https://github.com/gulpjs/gulp/blob/master/docs%2Frecipes%2Ffast-browserify-builds-with-watchify.md
'use strict';

var watchify = require('watchify');
var browserify = require('browserify');
var gulp = require('gulp');
var source = require('vinyl-source-stream');
var buffer = require('vinyl-buffer');
var gutil = require('gulp-util');
var sourcemaps = require('gulp-sourcemaps');
var babelify = require('babelify');
var assign = require('lodash').assign;
var browserSync = require('browser-sync');
var sass = require('gulp-sass');
var sourcemaps = require('gulp-sourcemaps');


// ============================================================
// Setup
// ============================================================
// gulp will look for an index.js within projectDir, and render it to bundle.js

var projectDir = 'js';

// add custom browserify options here
var customOpts = {
  entries: ['./'+projectDir+'/index.js'],
  extensions: ['.js', '.json', '.es6'],
  debug: true
};
var opts = assign({}, watchify.args, customOpts);
var b = watchify(browserify(opts));

// add transformations here
b.transform(babelify.configure({
  // https://github.com/babel/babelify
  // stage: 0,
  // compact: false
}));

gulp.task('default', ['js', 'sass:watch']);
gulp.task('js', bundle); // so you can run `gulp js` to build the file
b.on('update', bundle); // on any dep update, runs the bundler
b.on('log', gutil.log); // output build logs to terminal

gulp.task('sass', function () {
  gulp.src('./sass/**/*.scss')
    .pipe(sourcemaps.init())
      .pipe(sass().on('error', sass.logError))
    .pipe(sourcemaps.write())
    .pipe(gulp.dest('./css'));
});

gulp.task('sass:watch', function () {
  gulp.watch('./sass/**/*.scss', ['sass']);
});

function bundle() {
  return b.bundle()
    // log errors if they happen
    .on('error', gutil.log.bind(gutil, 'Browserify Error'))
    .pipe(source('bundle.js'))
    // optional, remove if you don't need to buffer file contents
    .pipe(buffer())
    // optional, remove if you dont want sourcemaps
    .pipe(sourcemaps.init({loadMaps: true})) // loads map from browserify file
       // Add transformation tasks to the pipeline here.
    .pipe(sourcemaps.write('./')) // writes .map file
    .pipe(gulp.dest('./'+projectDir));
}