Gulpfile with gulp 4

gulpfile.js

const gulp = require('gulp');
const sass = require('gulp-sass');
const postcss = require("gulp-postcss");
const sortmediaqueries = require('postcss-sort-media-queries');
const autoprefixer = require("autoprefixer");
const cssnano = require("cssnano");
const browserSync = require('browser-sync').create();
const sourcemaps = require('gulp-sourcemaps');

//compile scss into css
function style() {
    //1.where is my scss
    return gulp
    //gets all files ending with .scss in src/assets/scss
    .src('src/assets/scss/**/*.scss')
    // init sourcemaps
    .pipe(sourcemaps.init())
    //pass that file through sass compiler
    .pipe(sass({outputStyle: 'expanded'}).on('error', sass.logError))
    // Use postcss with autoprefixer
    .pipe(postcss([
        sortmediaqueries(),
        autoprefixer(),
        cssnano()
    ]))
    // sourcemaps write
    .pipe(sourcemaps.write('./sourcemap'))
    //where do I save the compiled css file
    .pipe(gulp.dest('src/assets/css'))
    //stream change to all browsers
    .pipe(browserSync.stream());
}

function watch() {
    browserSync.init({
        server: {
            baseDir: "./src",
            index: "/index.html"
        }
    });
    gulp.watch('src/assets/scss/**/*.scss', style);
    gulp.watch('src/*.html').on('change', browserSync.reload);
    gulp.watch('src/assets/js/**/*.js').on('change', browserSync.reload);
}

exports.style = style;
exports.watch = watch;

/*	
References:	
https://levelup.gitconnected.com/how-to-setup-your-workflow-using-gulp-v4-0-0-5450e3d7c512

Resolver error de node-sass en mac:
https://stackoverflow.com/questions/49348482/npm-install-with-error-gyp-failed-with-exit-code-1

Si al instalar no reconoce Gulp se debe instalar, de forma global, el Gulp CLI con el comando:
npm install --global gulp-cli
*/