javascriptで生Ajax

HTTP GET/POST だけのためにライブラリ使うのはオーバーという場合に。
（古いブラウザまでは対応していない）

// Usage:
// httpRequest( url, {
//     type: method name,
//     success: callback (optional),
//     data: map of params (optional),
// } );

function httpRequest (url, opt) {
    function throwExeption () {
        throw new Error('Usage: httpRequest(url, { type: method [, success: cb, data: params ] })');
    }
    if (!opt) throwExeption();
    var params = opt.data;
    var cb     = opt.success;
    var method = opt.type;
    if (!method) throwExeption();

    var req = new XMLHttpRequest();
    req.open(method, url, true);
    req.onreadystatechange = function() {
        if (req.readyState === 4) {
            if (req.status === 200) {
                if (cb) cb(req.responseText);
            } else {
                throw new Error("fetch failed: (" + req.status + ") " + url);
            }
        }
    };
    if (/post/i.test(method)) {
        req.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
        var data = '';
        for (var name in params) if (params.hasOwnProperty(name)) {
            data += encodeURIComponent(name) + "=" + encodeURIComponent(params[name]) + "&";
        }
        req.send(data);
    } else {
        req.send(null);
    }
}