gulp build sass and js

gulpfile.js

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

gulp.task('build', function () {
    gulp.src('./prebuild/js/*.js')
        .pipe(uglify())
        .pipe(concat('application.min.js'))
        .pipe(gulp.dest('./static/js'));

    gulp.src('./prebuild/sass/*.scss')
        .pipe(sass().on('error', gutil.log))
        .pipe(concat('main.min.css'))
        .pipe(cssmin())        
        .pipe(gulp.dest('./static/css'));
});

gulp.task('watch', function() {
      gulp.watch(['./prebuild/js/*.js', './prebuild/sass/*.scss'], ['build']);
  });

gulp.task('default', ['watch']);