old_contents: 
  const { src, dest, task, series } = require('gulp');
  const fs = require('fs');
  const del = require('del');
  const gulpif = require('gulp-if');
  const replace = require('gulp-replace');
  const postcss = require('gulp-postcss');
  const autoprefixer = require('autoprefixer');
  const uglify = require('gulp-uglify');
  const cleanCSS = require('gulp-clean-css');
  const rename = require('gulp-rename');
  const header = require("gulp-header");
  const eslint = require('gulp-eslint');
  
  const pack = () => JSON.parse(fs.readFileSync("./package.json", "utf8"));
  const pkg = pack();
  const year = new Date().getFullYear();
  
  const banner = `/*!
  * round-slider v${pkg.version}
  *
  * @website ${pkg.homepage}
  * @copyright (c) 2015-${year} Soundar
  * @license MIT
  */\n\n`;
  
  const isJavaScript = file => file.extname === '.js';
  const isCSS = file => file.extname === '.css';
  
  function buildFiles(ext) {
      return src([`src/*.${ext || '*'}`])
          // replace the variables from the source files
          .pipe(replace('{VERSION}', pkg.version))
          .pipe(replace('{YEAR}', year))
  
          // add the CSS vendor prefixes (Browserslist config will be loaded from '.browserslistrc' file)
          .pipe(gulpif(isCSS, postcss([autoprefixer()])))
  
          // move the unminified version of source files to dist folder, for development purpose
          .pipe(dest('dist/'))
  
          // do the minification for JS and CSS files
          .pipe(gulpif(isJavaScript, uglify()))
          .pipe(gulpif(isCSS, cleanCSS({ level: { 1: { specialComments: 0 }} })))
  
          // rename the minified files with '.min' suffix
          .pipe(rename({ suffix: '.min' }))
  
          // add the banner content, since it should be removed from source files during minification
          .pipe(header(banner))
  
          // move the minified files also to the dist folder
          .pipe(dest('dist/'));
  }
  
  task('checkLint', () => {
      return src(['src/*.js'])
          .pipe(eslint())
          .pipe(eslint.formatEach('compact', process.stderr));
  });
  task('deleteFiles', () => del(['dist/*']));
  task('build', series('deleteFiles', 'checkLint', () => buildFiles()));
  task('build_js', series(() => buildFiles('js')));
  task('build_css', series(() => buildFiles('css')));
  
new_contents: 
  const { src, dest, task, series } = require('gulp');
  const fs = require('fs');
  const del = require('del');
  const gulpif = require('gulp-if');
  const replace = require('gulp-replace');
  const postcss = require('gulp-postcss');
  const autoprefixer = require('autoprefixer');
  const uglify = require('gulp-uglify');
  const cleanCSS = require('gulp-clean-css');
  const rename = require('gulp-rename');
  const header = require("gulp-header");
  const eslint = require('gulp-eslint');
  
  const pack = () => JSON.parse(fs.readFileSync("./package.json", "utf8"));
  const pkg = pack();
  const year = new Date().getFullYear();
  
  const banner = `/*!
  * round-slider v${pkg.version}
  *
  * @website ${pkg.homepage}
  * @copyright (c) 2015-${year} Soundar
  * @license MIT
  */\n\n`;
  
  const isJavaScript = file => file.extname === '.js';
  const isCSS = file => file.extname === '.css';
  
  function buildFiles(ext) {
      return src([`src/*.${ext}`])
          // replace the variables from the source files
          .pipe(replace('{VERSION}', pkg.version))
          .pipe(replace('{YEAR}', year))
  
          // add the CSS vendor prefixes (Browserslist config will be loaded from '.browserslistrc' file)
          .pipe(gulpif(isCSS, postcss([autoprefixer()])))
  
          // move the unminified version of source files to dist folder, for development purpose
          .pipe(dest('dist/'))
  
          // do the minification for JS and CSS files
          .pipe(gulpif(isJavaScript, uglify()))
          .pipe(gulpif(isCSS, cleanCSS({ level: { 1: { specialComments: 0 }} })))
  
          // rename the minified files with '.min' suffix
          .pipe(rename({ suffix: '.min' }))
  
          // add the banner content, since it should be removed from source files during minification
          .pipe(header(banner))
  
          // move the minified files also to the dist folder
          .pipe(dest('dist/'));
  }
  
  task('lint', () => {
      return src(['src/*.js'])
          .pipe(eslint())
          .pipe(eslint.formatEach('compact', process.stderr));
  });
  task('deleteFiles', () => del(['dist/*']));
  task('build_js', series('lint', () => buildFiles('js')));
  task('build_css', series(() => buildFiles('css')));
  task('build', series('deleteFiles', 'build_js', 'build_css'));
  
subject: Update gulpfile.js