// 为IE8/9添加XDomainRequest支持
if (window.XDomainRequest && !$.support.cors) {
    $.ajaxTransport(function(s) {
        if (s.crossDomain && s.async) {
            if (s.timeout) {
                s.xdrTimeout = s.timeout;
                delete s.timeout;
            }
            var xdr;
            return {
                send: function(_, complete) {
                    function callback(status, statusText, responses, responseHeaders) {
                        xdr.onload = xdr.onerror = xdr.ontimeout = null;
                        complete(status, statusText, responses, responseHeaders);
                    }
                    xdr = new XDomainRequest();
                    xdr.onload = function() {
                        callback(200, "OK", { text: xdr.responseText }, "");
                    };
                    xdr.onerror = function() {
                        callback(404, "Not Found", {}, "");
                    };
                    xdr.ontimeout = function() {
                        callback(0, "timeout", {}, "");
                    };
                    xdr.timeout = s.xdrTimeout || 0;
                    xdr.open(s.type, s.url);
                    xdr.send(s.hasContent && s.data || null);
                },
                abort: function() {
                    if (xdr) xdr.abort();
                }
            };
        }
    });
}

// 然后正常使用$.ajax
$.ajax({
    url: 'http://other-domain.com/api',
    type: 'GET',
    dataType: 'json',
    crossDomain: true,
    success: function(data) {
        console.log(data);
    },
    error: function(xhr, status, error) {
        console.error(error);
    }
});

Access-Control-Allow-Origin: *
Access-Control-Allow-Methods: GET, POST, OPTIONS
Access-Control-Allow-Headers: Content-Type

$.ajax({
    url: 'http://other-domain.com/api?callback=?',
    dataType: 'jsonp',
    success: function(data) {
        console.log(data);
    }
});