change_pc.js

var movePCSiteBtn = document.querySelector(".btn-pcsite a");
function movePCSite(e) {
    var xhr = new XMLHttpRequest(),
        url = location.href,
        pcUrl = url.replace("/sp/", "/");
    e.preventDefault();
    xhr.open("HEAD", pcUrl, true);
    xhr.send();
    xhr.onreadystatechange = function () {
        if (xhr.status === 200) {
            location.href = pcUrl;
        } else {
            location.href = "/?viewmode=pc";
        }
    }
}
movePCSiteBtn.addEventListener("click", movePCSite, false);

change_sp.js

var SDJL;
if(!SDJL) {
    SDJL = {};
} else if(typeof SDJL !== "object") {
    throw new Error("Namespace 'SDJL' already exists.");
}

// ------------------------
// User-Agent簡易判定
// (Ver. 2.0)
// ------------------------
SDJL.mobile = (function() {
    var ua = navigator.userAgent;
    var ret = {
        ios        : false,
        android    : false,
        windows    : false,
        smartphone : false
    };
    
    if(ua.indexOf("iPhone") > 0 || ua.indexOf("iPad") > 0 || ua.indexOf("iPod") > 0) {
        // iOS搭載端末
        ret.ios = true;
        if(ua.indexOf("iPad") === -1) {
            ret.smartphone = true;
        }
    } else if(ua.indexOf("Android") > 0) {
        // Android搭載端末
        ret.android = true;
        if(ua.indexOf("Mobi") > 0) {
            ret.smartphone = true;
        }
    } else if(ua.indexOf("Windows Phone") > 0) {
        // Windows搭載端末
        ret.windows = true;
        ret.smartphone = true;
    }
    
    return ret;
}());

(function () {
    if (!SDJL.mobile.smartphone) {
        return;
    }

    if (sessionStorage.viewmode === undefined) {
        sessionStorage.viewmode = "";
    } else if (!!(location.search.indexOf("viewmode=pc") > -1)) {
        sessionStorage.viewmode = "pc";
    }

    if (sessionStorage.viewmode === "") {
        if( confirm( 'このサイトにはスマートフォン用のサイトがあります。\n表示しますか？' ) ) {
            sessionStorage.viewmode = "sp";
            location.href = '/sp/';
        } else {
            sessionStorage.viewmode = "pc";
        }
    }
}());