Gulp - build and watch

gulpfile.js

var watchify = require('watchify');
var browserify = require('browserify');

var paths = {
  lib: './www/lib/index.js',
  dist: './www/js/'
};


var bundler = browserify(paths.lib, browserify.args);
bundler.transform('brfs');
function watch(){
  var watcher= watchify(bundler);
  watcher.on('update', bundle); // on any dep update, runs the bundler
  watcher.on('log', gutil.log); // output build logs to terminal
  bundle(); 
}
function bundle() {
  // add any other browserify options or transforms here
  return bundler.bundle()
    // log errors if they happen
    .on('log', gutil.log)
    .on('error', gutil.log.bind(gutil, 'Browserify Error'))
    .pipe(source('bundle.js'))
    // optional, remove if you dont want sourcemaps
    .pipe(buffer())
    .pipe(sourcemaps.init({loadMaps: true})) // loads map from browserify file
    .pipe(sourcemaps.write('./')) // writes .map file
    //
    .pipe(gulp.dest(paths.dist));
}

gulp.task('build', bundle); 
gulp.task('watch', watch);