gulpfile.js

var gulp = require('gulp');
var sass = require('gulp-sass');
var babel = require('gulp-babel');
var concat = require('gulp-concat');
var uglify = require('gulp-uglify');
var del = require('del');

var paths = {
  styles: {
    src: 'assets/css/**/*.scss',
    dest: 'build/css/'
  },
  scripts: {
    src: 'assets/js/*.js',
    dest: 'build/js/'
  }
};

/* Not all tasks need to use streams, a gulpfile is just another node program
 * and you can use all packages available on npm, but it must return either a
 * Promise, a Stream or take a callback and call it
 */
function clean() {
  // You can use multiple globbing patterns as you would with `gulp.src`,
  // for example if you are using del 2.0 or above, return its promise
  return del([ 'build' ]);
}

function styles() {
  return gulp.src(paths.styles.src)
    .pipe(sass())
    .pipe(gulp.dest(paths.styles.dest));
}

function scripts() {
  return gulp.src(paths.scripts.src, { sourcemaps: true })
    .pipe(babel())
    .pipe(uglify())
    .pipe(gulp.dest(paths.scripts.dest));
}

function watch() {
  gulp.watch(paths.scripts.src, scripts);
}

/*
 * Specify if tasks run in series or parallel using `gulp.series` and `gulp.parallel`
 */
var build = gulp.series(clean, gulp.parallel(styles, scripts));

/*
 * You can use CommonJS `exports` module notation to declare tasks
 */
exports.clean = clean;
exports.scripts = scripts;
exports.watch = watch;
exports.build = build;
/*
 * Define default task that can be called by just running `gulp` from cli
 */
exports.default = build;