Modal Base: Two Options

export default class ModalTrap {
	constructor() {

		// select all modals with the dialog role and modal classe.
		// this ensures that only modals with the right attributes are selected
		this.modals = document.querySelectorAll("[role='dialog'].modal");

		// select all modal trigger buttons that don't have a data-dismiss attribute
		// buttons with data-dismiss and data-toggle are typically only used within modals to trigger another modal
		// we want to exclude these from the selection
		this.modalToggles = document.querySelectorAll("[data-toggle='modal']:not([data-dismiss='modal'])");

		// list of focusable elements inside a modal
		this.focusableElements = [
			"a[href]",
			"button:not([disabled])",
			"textarea:not([disabled])",
			"input:not([disabled])",
			"select:not([disabled])",
			"[tabindex]:not([tabindex='-1'])"
		];

		// this will ensure that any modal with the dialog-scrollable modifier gets the intended behaviour
		// all other modals will get ignored
		this.modalScrollBody = ".modal-dialog-scrollable .modal-body"

		// modal trigger element with same data-target value as id of triggered modal
		this.modalTriggerElement = null;

		// Used to store keydown handlers per modal
		this.focusHandlers = new WeakMap();
		this.init();

	}

	init() {

		// loop over all modal trigger buttons
		this.modalToggles.forEach(btn => {

			// if data target value is erroneous or doesn't match any modal ID, return false
			const targetSelector = btn.getAttribute('data-target');
			if ((targetSelector === "#") || (!targetSelector)) return;

			// on modal trigger interaction, set the modal trigger element's value to that of its DOM representation
			// and store it
			btn.addEventListener('click', (e) => {
				e.preventDefault();
				this.modalTriggerElement = e.target;
			});

		});

		// loop over all modals
		this.modals.forEach(modal => {

			const closeBtn = modal.querySelector('.sds-modal__close');

			// when a modal shown event is fired:
			$(modal).on('shown.bs.modal', (e) => {

				// check if a close button exists and focus it if so
				if (closeBtn) {
					closeBtn.focus();
				}
				else {

					// if no close is present focus first focusable element instead
					let focusableElements = this.getFocusableElements(modal);
					let first = focusableElements[0];

					first.focus()

				}

				// check for a scrollable body element inside the modal
				// set focus trap inside modal while open
				this.checkScrollabilityAndSetFocusTrap(modal);

				/*
				 * if the modal gets shown through the bootstrap method instead of
				 * through a trigger element interaction, store the value of the trigger element
				 * so it can be focused when closing the modal
				 *
				 * this will ensure that no matter how the modal is shown,
				 * there's always an element to put the focus back on when closed
				 *
				 * closing a modal after navigating from modal to modal will also put the focus back on
				 * the initial modal trigger element or corresponding modal's trigger element, if applicable
				 * for this purpose, modal trigger elements inside modals that have the data-dismiss="modal" attribute are ignored
				 */

				let lastFocusedElement = document.querySelector("[data-toggle='modal']:not([data-dismiss='modal'])[data-target='#"+ e.target.getAttribute("id") +"']");

				if (lastFocusedElement) {
					this.modalTriggerElement = lastFocusedElement;
				}

			});

			$(modal).on('hidden.bs.modal', (e) => {

				// if it exists focus the modal's trigger element upon closing the modal
				if (this.modalTriggerElement) {
					this.modalTriggerElement.focus();
				}
			});

		});
	}

	checkScrollabilityAndSetFocusTrap(modal) {

		const modalBodySelector = this.modalScrollBody;

		// check if the modal scrollable modifier is applied to the current modal
		// if not, the last element to receive focus will be the modal itself
		const modalBody = modal.querySelector(modalBodySelector) ? modal.querySelector(modalBodySelector) : modal;

		// add or remove tabindex depending on whether the modal body area is scrollable
		function setScrollability () {

			const isScrollable = modalBody.scrollHeight > modalBody.clientHeight;

			if (isScrollable) {
				// Ensure it's focusable
				if (!modalBody.hasAttribute('tabindex')) {
					modalBody.setAttribute('tabindex', '0');
				}
			} else {
				if (modalBody.hasAttribute('tabindex')) {
					modalBody.removeAttribute('tabindex');
				}
			}

		}

		// check if modal body is scriollable or not on page load
		setScrollability();
		// Initial trap
		this.trapFocus(modal);

		// Observe scrollability changes
		const observer = new ResizeObserver(() => {
			// check if modal body becomes scrollable on resize
			setScrollability();
			this.trapFocus(modal);
		});

		// Required to detect resizing/scrollability
		observer.observe(modalBody);
	}

	// trap the kesyboard focus navigfation inside the modal while it is shown
	trapFocus(modal) {

		const focusable = this.getFocusableElements(modal);

		// abort if no focusable elements are found inside modal
		if (!focusable.length) return;

		// define first and last focusable elements inside modal
		const first = focusable[0];
		const last = focusable[focusable.length - 1];

		// Remove old handler if exists
		if (this.focusHandlers.has(modal)) {
			modal.removeEventListener('keydown', this.focusHandlers.get(modal));
		}

		// move focus from first to last and vice-versa when using tab or shift+tab
		const handleKeydown = (e) => {
			if (e.key !== 'Tab') return;

			if (e.shiftKey && document.activeElement === first) {
				e.preventDefault();
				last.focus();
			} else if (!e.shiftKey && document.activeElement === last) {
				e.preventDefault();
				first.focus();
			}
		};

		modal.addEventListener('keydown', handleKeydown);
		this.focusHandlers.set(modal, handleKeydown);
	}

	// function to get list of focusable elements in modal
	getFocusableElements(container) {

		// stringify list of focusable element to pass it to querySelectorAll
		const selector = this.focusableElements.join();
		return container.querySelectorAll(selector);

	}
}