keep-focus.js

var tabbableElements = 'a[href], area[href], input:not([disabled]),' +
	'select:not([disabled]), textarea:not([disabled]),' +
	'button:not([disabled]), iframe, object, embed, *[tabindex],' +
	'*[contenteditable]';

var keepFocus = function (context) {
	var allTabbableElements = context.querySelectorAll(tabbableElements);
	var firstTabbableElement = allTabbableElements[0];
	var lastTabbableElement = allTabbableElements[allTabbableElements.length - 1];

	var keyListener = function (event) {
		var keyCode = event.which || event.keyCode; // Get the current keycode

		// Polyfill to prevent the default behavior of events
		event.preventDefault = event.preventDefault || function () {
			event.returnValue = false;
		};

		// If it is TAB
		if (keyCode === 9) {

			// Move focus to first element that can be tabbed if Shift isn't used
			if (event.target === lastTabbableElement && !event.shiftKey) {
				event.preventDefault();
				firstTabbableElement.focus();

			// Move focus to last element that can be tabbed if Shift is used
			} else if (event.target === firstTabbableElement && event.shiftKey) {
				event.preventDefault();
				lastTabbableElement.focus();
			}
		}
	};

	context.addEventListener('keydown', keyListener, false);
};

// Call the function when the part of the page gets focus
var modal = document.querySelector('.modal');
keepFocus(modal);

modal.focus();