var data = JSON.stringify({
  "number": 1,
  "string": "f\"oo",
  "arr": [
    1,
    2,
    3
  ],
  "nested": {
    "a": "b"
  },
  "arr_mix": [
    1,
    "a",
    {
      "arr_mix_nested": {}
    }
  ],
  "boolean": false
});

var xhr = new XMLHttpRequest();
xhr.withCredentials = true;

xhr.addEventListener("readystatechange", function () {
  if (this.readyState === this.DONE) {
    console.log(this.responseText);
  }
});

xhr.open("POST", "http://mockbin.com/har");
xhr.setRequestHeader("content-type", "application/json");

xhr.send(data);