Uglify js, preprocess scss and watchers

gulpfile.js

const gulp = require('gulp');
var gutil = require('gulp-util');
const concat = require('gulp-concat');
const uglify = require('gulp-uglify-es').default;
const rename = require('gulp-rename');
var sass = require('gulp-sass');
let cleanCss = require('gulp-clean-css');
var sourcemaps = require('gulp-sourcemaps');

const vendorJs = [
    // vendor js files
]

const appJs = [
    // src js
]

// Tasks
gulp.task('uglify-app-js', () => {
    return gulp
        .src(appJs) 
        .pipe(sourcemaps.init())
            .pipe(concat('bundle.min.js'))
            .pipe(uglify())
        .pipe(sourcemaps.write('./maps'))
        .pipe(gulp.dest('./dist'));
});

gulp.task('uglify-vendor-js', () => {
    gulp
        .src(vendorJs)
        .pipe(concat('vendor.min.js'))
        .pipe(gulp.dest('./dist'))
        .pipe(uglify())
        .pipe(gulp.dest('./dist'))
});

gulp.task('process-scss', () => {
    gulp.src('./src/scss/main.scss')
        .pipe(sass().on('error', sass.logError))
        .pipe(cleanCss())
        .pipe(rename('main.min.css'))
        .pipe(gulp.dest('./dist'))
});

gulp.task('copy-assets', () => {
    gulp
        .src('./src/assets/**/*')
        .pipe(gulp.dest('./dist/assets'));
});

// Watchers
gulp.task('watch-app-js', function () {
    gulp
        .watch(appJs, ['uglify-app-js']);
});

gulp.task('watch-scss', function () {
    gulp
        .watch('./src/scss/**/*.scss', ['process-scss']);
});

gulp.task('watch-assets', function () {
    gulp
        .watch('./src/assets/**/*', ['copy-assets']);
});

gulp.task('build', ['uglify-app-js', 'uglify-vendor-js', 'copy-assets', 'process-scss']);
gulp.task('watch', ['build', 'watch-app-js', 'watch-scss', 'watch-assets']);