Detect user's platform type: smartphone, tablet or desktop computer.

detect.js

var Detect = function(options) {
    if (options === undefined) options = {};

    this.init = function() {
        this.mm = window.matchMedia;

        if (this.mm && !options.useUA) {
            this.method = 'media queries';
            this.type   = mq();
        }
        else {
            this.method = 'user agent strings';
            this.type   = ua();
        }

        return options.verbose ? [ this.type, this.method ] : this.type;
    };


    /**
     * Media queries
     */

    var mq = function() {
        if (minMatch(320) && maxMatch(480))
            return 'smartphone';

        if (minMatch(768) && maxMatch(1024))
            return 'tablet';

        return 'desktop';
    };

    var match = function(m, width) {
        return window.matchMedia(
                'screen and (' + m + '-device-width: '
                    + width + 'px)').matches;
    };

    var minMatch = function(width) {
        return match('min', width);
    };

    var maxMatch = function(width) {
        return match('max', width);
    };


    /**
     * User agent strings
     */

    var ua = function() {
        var uas = navigator.userAgent;

        tablets = [ /Android 3/i, /iPad/i ];
        for (i=0, l=tablets.length; i<l; i++)
            if (uas.match(tablets[i])) return 'tablet';

        smartphones = [
            /Mobile/i,
            /Android/i,
            /iPhone/i,
            /BlackBerry/i,
            /Windows Phone/i,
            /Windows Mobile/i,
            /Maemo/i,
            /PalmSource/i,
            /SymbianOS/i,
            /SymbOS/i,
            /Nokia/i,
            /MOT-/i,
            /JDME/i,
            /Series 60/i,
            /S60/i,
            /SonyEricsson/i,
        ];
        for (i=0, l=smartphones.length; i<l; i++)
            if (uas.match(smartphones[i])) return 'smartphone';

        return 'desktop';
    };

    return this.init();
};

/**
 * Usage examples:
 *    str1  = Detect(); // -> 'tablet'
 *    str2  = Detect({ useUA: true }); // -> 'tablet'
 *    array = Detect({ verbose: true }); // -> ['tablet', 'media queries']
 */