Example Gulp 4 sass

gulpfile.js

const gulp = require("gulp");
const plumber = require("gulp-plumber");
const sass = require("gulp-sass");
const postcss = require("gulp-postcss");
const cssnano = require("cssnano");
const rename = require("gulp-rename");
const del = require("del");
const autoprefixer = require("autoprefixer");


// CSS task
const css = () => {
    return gulp
        .src("./sass/**/*.scss")
        .pipe(plumber())
        .pipe(sass({ outputStyle: "expanded" }))
        .pipe(rename({ suffix: ".min" }))
        .pipe(postcss([autoprefixer(), cssnano()]))
        .pipe(gulp.dest("./css/"))
}

// Clean assets
const clean = () => {
    return del(["./css/"]);
}

// Watch files
const watchFiles = () => {
    gulp.watch("./sass/**/*", css);
}

// define complex tasks
const build = gulp.series(clean, gulp.parallel(css));
const watch = gulp.parallel(watchFiles);

// export tasks
exports.default = watch;
exports.build = build;