build with browserify and watchify

gulpfile.js

'use strict';

var gulp = require('gulp');
var gutil = require('gulp-util');
var del = require ('del');
var watchify = require('watchify');
var browserify = require('browserify');

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

var source = require('vinyl-source-stream');
var assign = require('lodash.assign');

var opts = {
  entries: ['./app/assets/javascripts/index.js'],
  debug: true
};
var opts = assign({}, watchify.args, opts);
var b = watchify(browserify(opts));

gulp.task('js', bundle);
b.on('update', bundle);
b.on('log', gutil.log);

gulp.task('build', ['js']);

function bundle() {
  return b.bundle()
    // log errors if they happen
    .on('error', gutil.log.bind(gutil, 'Browserify Error'))
    .pipe(source('index.js'))
    .pipe(gulp.dest('app/assets/javascripts/dist/'));
};

package.json

{
  "name": "name",
  "version": "1.0.0",
  "description": "desc",
  "private": true,
  "scripts": {
    "watch": "$(npm bin)/gulp js",
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "Takashi Nakagawa",
  "devDependencies": {
    "browser-sync": "^2.5.0",
    "browserify": "^9.0.3",
    "del": "^1.1.1",
    "gulp": "^3.8.11",
    "gulp-util": "^3.0.4",
    "lodash.assign": "^3.1.0",
    "vinyl-source-stream": "^1.1.0",
    "watchify": "^3.0.0"
  }
}