gulpfile.js

const gulp    = require('gulp');
const changed = require('gulp-changed');
const babel   = require('gulp-babel');
const sass    = require('gulp-sass');

const logTaskError = function(error) {
  console.log(error.toString());
  this.emit('end');
};

gulp.task('es6', () => {
  return gulp.src('src/js/**/*.js')
    .pipe(changed('public/js'))
    .pipe(babel({
      presets: ['es2015'],
      plugins: ['transform-object-assign']
    }))
    .on('error', logTaskError)
    .pipe(gulp.dest('public/js'));
});

gulp.task('sass', () => {
  return gulp.src('src/scss/main.scss')
    .pipe(sass().on('error', sass.logError))
    .pipe(gulp.dest('public/css'));
});

gulp.task('default', ['es6', 'sass'], () => {
  gulp.watch('src/js/**/*.js', ['es6']);
  gulp.watch('src/scss/**/*.scss', ['sass']);
});