Pondering how the best way to selectivity include CSS and related JS

all.js

/*
	If a user chooses to not include the partial for modal styles,
	how can we automatically not include the modal function and its init?
	
	Ideally, carousel and modal would be in their own file. It's like we need Sass for JS
*/

(function() {
	carousel.init();
	modal.init();
	
	var carousel = {
		init: function() {
			// setup carousels
		}
	};
	
	var modal = {
		init: function() {
			// setup modals
		}
	};
})();

base.js

(function() {
	// include carousel.init
	// include modal.init
	
	//include carousel.js
	//include modal.js
})();

base.scss

@include "partials/carousel"
@include "partials/modal"

// A user can easily choose to exclude these modules if they don't want them in the CSS

carousel.js

var carousel = {
	init: function() {
		// setup carousels
		// could be 1000 lines long
	}
};

carousel.scss

/* Here are my styles for carousels */

modal.js

var modal = {
	init: function() {
		// setup modals
		// could be 1000 lines long
	}
};

modal.scss

/* Here are my styles for modal dialogs */