Edit in JSFiddle

// классический подход
var request = new XMLHttpRequest();

request.open('POST', '/echo/html/', true);

request.setRequestHeader("Content-Type", "application/json;charset=UTF-8");

request.onload = function (e) {
    if (request.readyState === 4) {
      if (request.status === 200) {
            console.log(request.responseText);
        } else {
            console.error(request.statusText);
        }
    }
};

request.onerror = function (e) {
    console.error(request.statusText);
};

request.send('html=response+from+XMLHttpRequest&delay=1');


// использование библиотеки reqwest (https://github.com/ded/reqwest)
reqwest({
    url: '/echo/html/',
    method: 'post',
    data: {
        html: "response from reqwest",
        delay: 1
    },
    error: function (err) {
    },
    success: function (resp) {
        console.log(resp);
    }
});