gulpfile.js

var gulp = require('gulp');
var uglifyjs = require('uglify-es');
var composer = require('gulp-uglify/composer');
var pump = require('pump');
var del = require('del');
var concat = require('gulp-concat');
// var sourcemaps = require('gulp-sourcemaps');
var minify = composer(uglifyjs, console);

gulp.task('compressJS', function (cb) {
  // the same options as described above
  var options = {};
 
  return pump([
      gulp.src('src/script.js'),
      // sourcemaps.init(),
      minify(options),
      // sourcemaps.write('.'),
      gulp.dest('dist'),
    ],
    cb
  );
});

gulp.task('add-header', function() {
  return pump([
    gulp.src(['src/header.js', 'dist/script.js']),
    concat('script.user.js'),
    gulp.dest('dist')
  ]);
});

gulp.task('del-script', function() {
  del.sync('dist/script.js');
  return Promise.resolve('');
});

gulp.task('clean:dist', function() {
  return del.sync('dist');
});

gulp.task('watch', function(){
  gulp.watch('src/*.js', gulp.series('compressJS')); 
});

// To compile just run $ gulp
exports.default = gulp.series('compressJS', 'add-header', 'del-script');