gulpfile.js

var gulp = require('gulp');
var clean = require('gulp-clean');
var util = require('gulp-util');
var through2 = require('through2');
var watch = require("gulp-watch");

var fs = require('fs');
var path = require('path');

function mkdir(dir) {
  if(!fs.existsSync(dir)) {
    var parent = path.dirname(dir);
    mkdir(parent);
    fs.mkdirSync(dir);
  }
}

gulp.task('clean-web', function() {
  return gulp.src('./web/*')
    .pipe(clean());
});

function cb(file, enc, cb) {
  var target = file.path.replace(path.sep + 'src' + path.sep,  path.sep + 'web' + path.sep);
  util.log(path.relative(file.cwd, file.path), '->', path.relative(file.cwd, target));
  var content = file.contents.toString('utf-8');
  content = "define(function(require, exports, module) {" + content + '});';
  file.contents = new Buffer(content);
  cb(null, file);
}

gulp.task('default', gulp.series('clean-web', function() {
  return gulp.src('./src/**/*.js')
    .pipe(through2.obj(cb))
    .pipe(gulp.dest('./web/'));
}));

gulp.task('watch', function() {
  return watch('./src/**/*.js', function(file) {
    var to = file.path.replace(path.sep + 'src' + path.sep,  path.sep + 'web' + path.sep);
    to = path.dirname(to);
    gulp.src(file.path)
      .pipe(through2.obj(cb))
      .pipe(gulp.dest(to));
  });
});