Very simple Gulp 4.0 file to watch and run SASS

gulpfile.js

var gulp = require('gulp');
var sass = require('gulp-sass');

var sassConfig = {
	inputDirectory: 'scss/**/*.scss',
	outputDirectory: 'css',
	options: {
		outputStyle: 'expanded'
	}
}

gulp.task('build-css', function() {
	return gulp
		.src(sassConfig.inputDirectory)
		.pipe(sass(sassConfig.options).on('error', sass.logError))
		.pipe(gulp.dest(sassConfig.outputDirectory));
});

gulp.task('watch', function() {
	gulp.watch('scss/**/*.scss', gulp.series('build-css'));
});

package.json

{
  "name": "gulp-example",
  "version": "1.0.0",
  "description": "Very Simple Gulp File",
  "main": "index.js",
  "repository": "github.com/kgoedecke",
  "author": "Kevin Goedecke <goedecke@net-studios.de>",
  "license": "MIT",
  "devDependencies": {
    "gulp": "^4.0.0"
  },
  "dependencies": {
    "gulp-sass": "^4.0.1"
  }
}