index.js

function sendJSON(){
            let result = document.querySelector('.result');
            let pergunta = document.querySelector('#pergunta');
         let xhr = new XMLHttpRequest();
            let url = "submit.php";

            // open a connection
            xhr.open("POST", url, true);

            // Set the request header i.e. which type of content you are sending
            xhr.setRequestHeader("Content-Type", "application/json");

            // Create a state change callback
            xhr.onreadystatechange = function () {
                if (xhr.readyState === 4 && xhr.status === 200) {

                    // Print received data from server
                    result.innerHTML = this.responseText;

                }
            };

            // Converting JSON data to string
            var data = JSON.stringify({ "name": name.value, "email": email.value });

            // Sending data with the request
            xhr.send(data);
        }