app.post('/check', function (req, res) {

        console.log("Client checking for service...");

        res.writeHead(200, {
            'Access-Control-Allow-Origin': 'http://localhost',
            'Content-Length': 0,
            'Content-Type': 'text/plain'
        });
        res.end();
        console.log("Client checked in!");

    });

request = $.ajax({
    url: "http://127.0.0.1:8080/check",
    type: "POST",
    crossDomain: true,
    data: '',
    dataType: "json",

    error: function () {
        alert("Could not connect to the server! Returning to login screen.");
        signOut();
    },
    success: function () {

        DoStuff();

    }
});

'Content-Type': 'text/plain'

dataType: "json"

<p id="results"></p>
<button>localpost</button>

$(function() {

      var localpost = function() {

        var callback = function(name) {
                         $("#results").html(name)
                       };

        var request = $.ajax({
            type:"POST",
            dataType: "json",
            data: "",
            statusCode : {
            200: function(data, textStatus, jqxhr) {
                    if (textStatus === "success" && jqxhr.responseJSON) {
              alert(textStatus+"n  Could not connect to the server! Returning to login screen.");
               alert(JSON.stringify(jqxhr.responseJSON)+"n"
                    +String(jqxhr.getResponseHeader("Content-Type")));
               // signOut();
                    };
            }
          }
        });
                // `success` callback
        request.done(function(data, textStatus, jqxhr) {
            if (textStatus === "success") {
                // DoStuff()
              callback("textStatus: "
                       +textStatus+"<br />jqxhr status: "
                       +jqxhr.state()+"<br />content-type: "
                       +jqxhr.getResponseHeader("Content-Type")+"<br /><br />"
                       +jqxhr.getResponseHeader("Content-Length")+"<br /><br />"
                       +jqxhr.getResponseHeader("Access-Control-Allow-Origin")+"<br /><br />"
                       +JSON.stringify(data))
            };
        });
                // `error` callback
        request.fail(function(data, textStatus, jqxhr) {
               console.log(textStatus);
        })
      };

            $("button").one("click", localpost);
            // localpost()

        });