gulp-concat does not concat one of my files on heroku - angularjs

Question

I'm using gulp to generate a config.js file for angular, then use another gulp task to concat all .js files together, the gulp tasks look like this:
gulp.task('config', function() {
var environment = process.env.NODE_ENV || 'development';
gulp.src('public/config/' + environment + '.json')
.pipe(ngConstant({
name: 'app.config'
}))
.pipe(concat('public/js/config.js'))
.pipe(gulp.dest('.'));
});
gulp.task('js', ['config'], function() {
gulp.src('public/js/*.js')
.pipe(concat('app.js'))
.pipe(gulp.dest('public/dist'));
});
gulp.task('default', ['config', 'js']);
If I run gulp locally, everything works file.
But when I push to heroku, using this post install script:
"postinstall": "bower install && gulp"
I can see gulp run successfully, after adding some debug, I can see the config.js file is even created correctly, but the generated app.js does not include config.js. Can anyone suggest what might be wrong here?
UPDATE: I found it works if I do gulp config && gulp js but not gulp, is this because gulp is async and config.js wasn't created when js job started to run? but i thought I have already specified the task dependencies?

Answer 1

The problem is: I'm not returning a stream from tasks, for example the config task should be like this:
gulp.task('config', function() {
var environment = process.env.NODE_ENV || 'development';
var stream = gulp.src('public/config/' + environment + '.json')
.pipe(ngConstant({
name: 'app.config'
}))
.pipe(concat('public/js/config.js'))
.pipe(gulp.dest('.'));
return stream; // THIS IS IMPORTANT!
});