Confine focus inside a limited set of elements

FocusConfiner.ts

/// <reference path="../../defs/jquery/jquery.d.ts" />

module fg.dom {

    // get the next element in a JQuery set (returns the first element if 'current' is the last in the set)
    var nextElement = (set: JQuery, current: Element) => {
        var index = set.index(current);

        if (index < set.length - 1)
            return set.get(index + 1);

        return set.first().get(0);
    };

    // get the previous element in a JQuery set (returns the last element if 'current' is the first in the set)
    var prevElement = (set: JQuery, current: Element) => {
        var index = set.index(current);

        if (index > 0)
            return set.get(index - 1);

        return set.last().get(0);
    };

    /** Confines focus to a limited set of DOM elements and their children. Requires JQuery UI. */
    export class FocusConfiner {

        private lastFocus: Element;

        private confinementEnabled = false;

        constructor(private confines: JQuery) {
            if (confines.find(confines).length) {
                //console.warn("FocusContainer confines cannot contain other confines.");
                confines = confines.not(confines.find(confines));
            }
        }

        /** Start confining focus. */
        public confine() {
            if (this.confinementEnabled)
                return;

            this.confinementEnabled = true;

            $(window).on('blur', this.instanceOnWindowBlur);
            $(document.body).delegate(':focusable', 'focus', this.instanceOnFocusChanged);

            this.instanceOnFocusChanged(); // ensure a valid state
        }

        /** Stop confining focus. */
        public suspend() {
            if (!this.confinementEnabled)
                return;

            this.confinementEnabled = false;

            $(window).off('blur', this.instanceOnWindowBlur);
            $(document.body).undelegate(':focusable', 'focus', this.instanceOnFocusChanged);
        }

        private instanceOnFocusChanged = () => this.onFocusChanged();
        private onFocusChanged() {
            if (!document.activeElement || document.activeElement === document.body) {
                this.lastFocus = null;
                return;  // nothing focused, this is a valid state
            }

            if (this.confines.has(document.activeElement).length) {
                this.lastFocus = document.activeElement;
                return; // we are already in a valid state
            }

            // this might be slow if we have a few thousand buttons or links on the page
            var globalFocusables = $(document.body).find(':focusable');
            var focusables = this.confines.find(':focusable');

            // focusing forwards
            if (nextElement(globalFocusables, this.lastFocus) === document.activeElement)
                this.lastFocus = nextElement(focusables, this.lastFocus);

            // focusing backwards
            else if (prevElement(globalFocusables, this.lastFocus) === document.activeElement)
                this.lastFocus = prevElement(focusables, this.lastFocus);

            // focus restored from outside window
            else if (!this.lastFocus)
                this.lastFocus = focusables.first().get(0);

            // the user mouse-jumped to a focusable element outside confines
            //else
            //    this.lastFocus = this.lastFocus;

            $(this.lastFocus).focus();
        }

        private instanceOnWindowBlur = () => this.onWindowBlur();
        private onWindowBlur(): void {
            this.lastFocus = null;
            $(document.activeElement).blur();
        }
    }
}