gulpfile.js

var gulp = require('gulp');
var bro = require('gulp-bro');
var babelify = require('babelify')
var uglify = require('gulp-uglify');
var sourceFile = 'js/mithril-app/app.js',
  destFolder = './js/',
  destFile = 'app.js';

gulp.task('build-js', function () {
  gulp.src('./js/mithril-app/app.js')
    .pipe(bro({ transform: [babelify.configure({ presets: ['es2015'] })] }))
    .pipe(uglify())
    .pipe(gulp.dest('./js'))
})