gulpfile.js

var gulp = require('gulp');
var sass = require('gulp-sass');
var rename = require('gulp-rename');
var cleanCSS = require('gulp-clean-css');
var autoprefixer = require('gulp-autoprefixer');
var del = require('del');
var header = require('gulp-header');
var pkg = require('./package.json');

var banner = ['/*!',
 ' * <%= pkg.name %> <%= pkg.version %> (http://javatmp.com)',
 ' * <%= pkg.description %>',
 ' *',
 ' * @link <%= pkg.homepage %>',
 ' * @copyright 2018 JavaTMP',
 ' * @license <%= pkg.license %>',
 ' */',
 ''].join('\n');

gulp.task('delete-css', function (cb) {
 return del.sync(['./dist/**/*'], cb());
});

gulp.task('sass', gulp.series("delete-css", function (cb) {
 return gulp.src([
 './src/bootstrap-foundation.scss',
 './src/bootstrap-reverse.scss',
 './src/bootstrap-ltr-extender.scss',
 './src/bootstrap-extender.scss'
 ])
 .pipe(sass().on('error', sass.logError))
 .pipe(autoprefixer({
 browsers: ['last 2 versions'],
 cascade: false
 }))
 .pipe(header(banner, {pkg: pkg}))
 .pipe(cleanCSS())
 .pipe(rename({suffix: '.min'}))
 .pipe(gulp.dest('./dist/'));
}));

gulp.task('default', gulp.series("sass", function (cb) {
 // place code for your default task here
 cb();
}));