gulpfile.js

let gulp = require('gulp'),
    sass = require('gulp-sass'),
    rename = require('gulp-rename'),
    browserSync = require('browser-sync'),
    autoprefixer = require('gulp-autoprefixer'),
    concat = require('gulp-concat'),
    uglify = require('gulp-uglify'),
    cssmin = require('gulp-cssmin');

gulp.task('sass', function(){
    return gulp.src('app/scss/style.scss')
            .pipe(sass({outputStyle: 'expanded'}))
            .pipe(rename({suffix: '.min'}))
            .pipe(autoprefixer({
                overrideBrowserslist: ['last 8 versions']
            }))
            .pipe(gulp.dest('app/css'))
            .pipe(browserSync.reload({stream: true}))
});

gulp.task('style', function(){
    return gulp.src([
            'node_modules/normalize.css/normalize.css'
            // 'node_modules/slick-carousel/slick/slick.css'
            
        ])
        .pipe(concat('libs.min.css'))
        .pipe(cssmin())
        .pipe(gulp.dest('app/css'))
});

gulp.task('script', function(){
    return gulp.src([
            'node_modules/slick-carousel/slick/slick.js'
            // 'node_modules/magnific-popup/dist/jquery.magnific-popup.js'
        ])
        .pipe(concat('libs.min.js'))
        .pipe(uglify())
        .pipe(gulp.dest('app/js'))
});

gulp.task('html', function(){
    return gulp.src('app/*.html')
            .pipe(browserSync.reload({stream: true}))
});

gulp.task('js', function(){
    return gulp.src('app/js/*.js')
            .pipe(browserSync.reload({stream: true}))
});

gulp.task('browser-sync', function(){
    browserSync.init({
        server: {
            baseDir: 'app/'
        }
    });
});

gulp.task('watch', function(){
    gulp.watch('app/scss/**/*.scss', gulp.parallel('sass'));
    gulp.watch('app/*.html', gulp.parallel('html'));
    gulp.watch('app/js/*.js', gulp.parallel('js'));
});

gulp.task('default', gulp.parallel('style', 'script', 'sass', 'watch', 'browser-sync'));