my gulpfile for front-end task

closer.js

(function ($window, $document, bs) {

    var socket = bs.socket;
    socket.on("disconnect", function (client) {
        window.close();
      window.log
    });

})(window, document, ___browserSync___);

gulpfile.js

'use strict';

const gulp = require('gulp'),
      sass = require('gulp-sass'),
      pug = require('gulp-pug'),
      browserSync = require('browser-sync').create(),
      del = require('del');
const pugIndex = './src/*.pug',
      pugDest = './dist/';
const sassIndex = './src/sass/app.scss',
      sassPath = './src/sass/**/*',
      sassDest = './dist/';

function serveFiles() {
  browserSync.use({
    plugin: function () { /* noop */},
    hooks: {
        'client:js': require("fs").readFileSync("./closer.js", "utf-8")
    }
  });
  browserSync.init({
    server: {
      baseDir : './dist',
      port    : 3000
    }
  });
  gulp.watch(sassPath, {
    interval: 500
  }, buildSass);
  gulp.watch(pugIndex, {
    interval: 500
  }, buildPug).on('change', browserSync.reload);
}

function clean() {
  return del('');
}

function buildPug() {
  return gulp.src(pugIndex)
    .pipe(pug({
      doctype : 'html',
      pretty  : true
    }))
    .pipe(gulp.dest(pugDest));
}

function buildSass() {
  return gulp.src(sassIndex)
    .pipe(sass())
    .pipe(gulp.dest(sassDest))
    .pipe(browserSync.stream());
}

exports.default = serveFiles;
exports.pug = buildPug;
exports.sass = buildSass;