Bump gulp from 3.9.1 to 4.0.2 (#2200)

Bumps [gulp](https://github.com/gulpjs/gulp) from 3.9.1 to 4.0.2.
- [Release notes](https://github.com/gulpjs/gulp/releases)
- [Changelog](https://github.com/gulpjs/gulp/blob/master/CHANGELOG.md)
- [Commits](gulpjs/gulp@v3.9.1...v4.0.2)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>
Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>
Co-authored-by: Areeb Jamal <jamal.areeb@gmail.com>

package.json

@@ -34,9 +34,10 @@
     "body-parser": "^1.15.2",
     "chai": "^4.0.1",
     "compression": "^1.6.2",
-    "cookie-parser": "^1.4.4",
     "connect-domain": "^0.5.0",
+    "cookie-parser": "^1.4.4",
     "decompress-zip": "^0.3.0",
+    "del": "^5.1.0",
     "dotenv": "^8.2.0",
     "express": "^4.13.4",
     "express-session": "^1.15.0",
@@ -45,7 +46,7 @@
     "fs-extra": "^3.0.1",
     "ftp-deploy": "^2.3.6",
     "github": "^9.2.0",
-    "gulp": "^3.9.1",
+    "gulp": "^4.0.2",
     "gulp-babel": "^6.1.2",
     "gulp-clean": "^0.4.0",
     "gulp-concat": "^2.6.1",

src/backend/gulpfile.js

@@ -8,15 +8,15 @@ const concat = require('gulp-concat');
 const minify = require('gulp-minify-css');
 const htmlmin = require('gulp-htmlmin');
 const iife = require('gulp-iife');
-const clean = require('gulp-clean');
+const del = require('del');
 // eslint-disable-next-line no-var
 var exports = module.exports = {};
 
 exports.minifyJs = function(path, cb) {
   const dir = path + '/js/';
 
   gulp.task('scheduleJs', function() {
-    return gulp.src([dir + 'FileSaver.js', dir + 'social.js', dir + 'scroll.js', dir + 'navbar.js', dir + 'calendar.js', dir + 'popover.js', dir + 'html2canvas.js', dir + 'jquery.lazyload.js', dir + 'icsGen.js'])
+    return gulp.src([dir + 'FileSaver.js', dir + 'social.js', dir + 'scroll.js', dir + 'navbar.js', dir + 'calendar.js', dir + 'popover.js', dir + 'html2canvas.js', dir + 'jquery.lazyload.js', dir + 'icsGen.js'], {allowEmpty: true})
       .pipe(iife({useStrict: false}))
       .pipe(concat('schedule.min.js'))
       .pipe(babel({presets: ['es2015']}))
@@ -60,6 +60,7 @@ exports.minifyJs = function(path, cb) {
   });
 
   gulp.task('speakersJs', function() {
+    console.log('>>> Generating speakers.js')
     return gulp.src([dir + 'social.js', dir + 'scroll.js', dir + 'navbar.js', dir + 'popover.js', dir + 'jquery.lazyload.js'])
       .pipe(iife({useStrict: false}))
       .pipe(concat('speakers.min.js'))
@@ -91,12 +92,12 @@ exports.minifyJs = function(path, cb) {
       .pipe(gulp.dest(path + '/js/'));
   });
 
-  gulp.task('minifyJs', ['speakersJs', 'roomsJs', 'scheduleJs', 'eventJs', 'tracksJs', 'sessionJs', 'mainJs'], function() {
-    gulp.src(['!' + dir + '*.min.js', dir + '*.js']).pipe(clean());
+  gulp.task('minifyJs', gulp.series('speakersJs', 'roomsJs', 'scheduleJs', 'eventJs', 'tracksJs', 'sessionJs', 'mainJs', async function() {
+    await del([dir + '*.js', '!' + dir + '*.min.js']);
     cb();
-  });
+  }));
 
-  gulp.start('minifyJs');
+  gulp.series('minifyJs')();
 };
 
 exports.minifyCss = function(path, cb) {
@@ -109,7 +110,7 @@ exports.minifyCss = function(path, cb) {
       });
   });
 
-  gulp.start('minifyCss');
+  gulp.series('minifyCss')();
 };
 
 exports.minifyHtml = function(path, cb) {
@@ -122,5 +123,5 @@ exports.minifyHtml = function(path, cb) {
       });
   });
 
-  gulp.start('minifyHtml');
+  gulp.series('minifyHtml')();
 };