How can gulp monitor file changes automatically?

  node.js, question

My main purpose is to merge js
Now every js written has to be executed gulp.
Is there any way for gulp to monitor js folder, and when this folder is changed, merge will be performed automatically

gulp.task('watch',function(){
 gulp.watch('resource/css/**/*.css',['styles']);
 gulp.watch('resource/js/**/*.js',['scripts']);
 gulp.watch('resource/img/**/*.{png,jpg,gif}',['images']);
 });

Write a listening task like this, which is executed in the default task.
Gulp Hatsu Taiken, Front End Automation Tool