GULP: Basic configuration for GULP 4

basic_gulp4.js

const {src, dest, watch, series, parallel} = require('gulp'); //ES destructuring assignment

    var sass = require("gulp-sass"),
        autoPrefixer = require("gulp-autoprefixer"),
        minifyCss = require("gulp-clean-css"),
        rename = require("gulp-rename"),
        concat = require("gulp-concat"), 
        uglify = require("gulp-uglify"),
        plumber = require("gulp-plumber"),
        util = require("gulp-util"),
        browserSync = require("browser-sync").create(),
        reload = browserSync.reload;

    var srcs = { 
        html: ["./**/*.html"],
        css: ["./assets/css/**/*.css"],
        sass: ["./assets/sass/**/*.scss"],
        js: ["./assets/js/*.js"],
    };



    function sass() {
        return src("./assets/sass/sys.scss")
            .pipe(plumber())
            .pipe(sass())
            .pipe(autoPrefixer({
                browsers: ["> 5%", "last 2 versions", "not ie <=8"],
                cascade: true,
                remove: true
            }))
            .pipe(rename({
                basename: "sys"
            }))
            .pipe(dest("./assets/css"))
            .pipe(reload({
                stream: true
            }))
            .pipe(minifyCss())
            .pipe(rename({
                suffix: ".m"
            }))
            .pipe(dest("./assets/css"))
            .pipe(reload({
                stream: true
            }));
        util.log(util.colors.yellow("website styles compied and minified"));
    }

    function js(){
        return src("./assets/js/sys.js")
            .pipe(reload({
                stream: true
            })); //not sure if you intentionally did not put dest() stream method
    }

    function browser_sync(cb) {
        browserSync.init({
            server: {
                baseDir: "./"
            },
            browser: ["google chrome"]
        });
        const watcher = watch(srcs.html);

        watcher.on('change',  reload);

        watcher.on("change", reload);
        watch(srcs.sass, series(sass));
        watch(srcs.js,  series(js));
        cb();
    }

    //this will first trigger sass() and js() functions parallel, then after executing these two, browser_sync will run
    exports.default = series(parallel(sass, js), browser_sync);