Gulpfile.js

var gulp = require('gulp');
var browserify = require('browserify');
var babelify = require('babelify');
var source = require('vinyl-source-stream');

gulp.task('browserify', function() {
    return browserify('./js/app.js')
        .transform(babelify, { stage: 0 })
        .bundle()
        .on('error', function(e){
            console.log(e.message);

            this.emit('end');
        })
        .pipe(source('bundle.js'))
        .pipe(gulp.dest('js'));
});

gulp.task('watch', function() {
    gulp.watch('**/*.js', ['browserify']);
});

package.json

{
  "devDependencies": {
    "babelify": "^6.0.2",
    "browserify": "^9.0.7",
    "gulp": "^3.8.11",
    "gulp-plumber": "^1.0.0",
    "vinyl-source-stream": "^1.1.0"
  }
}