gulpfile.js

var gulp = require('gulp'),
    uglify = require('gulp-uglify'),
    concat = require('gulp-concat'),
    sass = require('gulp-sass');

gulp.task('compress-js', function () {
  gulp.src('assets/js/*.js')
    .pipe(uglify())
    .pipe(gulp.dest('build/js'));
});

gulp.task('concat', function () {
  gulp.src(['assets/css/reset.css', 'assets/sass/_mixins.scss', 'assets/sass/_vars.scss', 'assets/sass/_custom-reset.scss', 'assets/sass/_custom-mixins.scss', 'assets/sass/dc.scss'])
    .pipe(concat('app.scss'))
    .pipe(gulp.dest('assets/concat'));
});

gulp.task('sass', function () {
  gulp.src('assets/concat/app.scss')
    .pipe(sass({
      outputStyle: 'compressed'
    }))
    .pipe(gulp.dest('build/css'));
});

gulp.task('watch', function () {
  gulp.watch('assets/sass/*.scss', ['concat', 'sass']);
  gulp.watch('assets/js/*.js', ['compress-js']);
});

gulp.task('default', ['compress-js', 'concat', 'sass', 'watch']);