Sending and receiving data in JSON format using POST method

// Sending and receiving data in JSON format using POST method
//
var xhr = new XMLHttpRequest();
var url = "url";
xhr.open("POST", url, true);
xhr.setRequestHeader("Content-Type", "application/json");
xhr.onreadystatechange = function () {
    if (xhr.readyState === 4 && xhr.status === 200) {
        var json = JSON.parse(xhr.responseText);
        console.log(json.email + ", " + json.password);
    }
};
var data = JSON.stringify({"email": "[email protected]", "password": "101010"});
xhr.send(data);

Sending and receiving data in JSON format using GET method

// Sending a receiving data in JSON format using GET method
//      
var xhr = new XMLHttpRequest();
var url = "url?data=" + encodeURIComponent(JSON.stringify({"email": "[email protected]", "password": "101010"}));
xhr.open("GET", url, true);
xhr.setRequestHeader("Content-Type", "application/json");
xhr.onreadystatechange = function () {
    if (xhr.readyState === 4 && xhr.status === 200) {
        var json = JSON.parse(xhr.responseText);
        console.log(json.email + ", " + json.password);
    }
};
xhr.send();

Handling data in JSON format on the server-side using PHP

<?php
// Handling data in JSON format on the server-side using PHP
//
header("Content-Type: application/json");
// build a PHP variable from JSON sent using POST method
$v = json_decode(stripslashes(file_get_contents("php://input")));
// build a PHP variable from JSON sent using GET method
$v = json_decode(stripslashes($_GET["data"]));
// encode the PHP variable to JSON and send it back on client-side
echo json_encode($v);
?>

The limit of the length of an HTTP Get request is dependent on both the server and the client (browser) used, from 2kB - 8kB. The server should return 414 (Request-URI Too Long) status if an URI is longer than the server can handle.

Note Someone said that I could use state names instead of state values; in other words I could use xhr.readyState === xhr.DONE instead of xhr.readyState === 4 The problem is that Internet Explorer uses different state names so it's better to use state values.

Not sure if you want jQuery.

var form;

form.onsubmit = function (e) {
  // stop the regular form submission
  e.preventDefault();

  // collect the form data while iterating over the inputs
  var data = {};
  for (var i = 0, ii = form.length; i < ii; ++i) {
    var input = form[i];
    if (input.name) {
      data[input.name] = input.value;
    }
  }

  // construct an HTTP request
  var xhr = new XMLHttpRequest();
  xhr.open(form.method, form.action, true);
  xhr.setRequestHeader('Content-Type', 'application/json; charset=UTF-8');

  // send the collected data as JSON
  xhr.send(JSON.stringify(data));

  xhr.onloadend = function () {
    // done
  };
};

In your example, the webhook.site service you're attempting to connect to with your JavaScript isn't enabled (by default) with the proper CORS headers that modern browsers respect & enforce to improve user security. The developer console in your browser of choice should point this out to you; the error mine gave back was:

Access to XMLHttpRequest at 'https://webhook.site/5c7a5049-9c5e-4bf7-b1cf-0e05f6503bfa' from origin 'null' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource.

As the error states, the requested resource doesn't respond with any valid Access-Control-Allow-Origin header, which will prevent the POST itself from being fully executed. In webhook.site, you can select the CORS Headers tickbox from the top of the user interface to enable the service to send the proper CORS headers to get this working.

Copylet xhr = new XMLHttpRequest();

xhr.open("POST","https://webhook.site/5c7a5049-9c5e-4bf7-b1cf-0e05f6503bfa",true); // configuration interface at https://webhook.site/#!/5c7a5049-9c5e-4bf7-b1cf-0e05f6503bfa/e14fc471-4bc4-410f-b16a-0755a231fb12/1

xhr.setRequestHeader("Content-Type", "application/json");

let data = JSON.stringify({'eventType' : 'test'});

xhr.send(data);