<form method="post" action="process.php" >

    Firstname: <input type="text" name="firstname" />
    <br />

    Lastname: <input type="text" name="lastname" />
    <br />

    <hr />

    <input type="submit" value="Submit" />

</form>

<div id="output"></div>

<script>

    /* submit form */
    document.querySelector( "form" ).addEventListener( "submit", function (event) {

        event.preventDefault(); // return false


        var formHTML = event.target; // this

        console.log( formHTML ); // formHTML element


        // https://developer.mozilla.org/en-US/docs/Web/API/FormData/Using_FormData_Objects
        var formData = new FormData( formHTML );

        console.log( formData );


        // https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest
        // https://www.w3schools.com/xml/xml_http.asp
        // https://www.w3schools.com/js/js_ajax_http.asp

        /* AJAX request */
        ajax( formHTML, formData, "#output" ); // ajax( form, data destination )

    });

</script>

<script>

    function ajax( form, data, destination )
    {

        if (window.XMLHttpRequest)
        {
            var xhr = new XMLHttpRequest(); /* code for modern browsers */
        }
        else
        {
            var xhr = new ActiveXObject("Microsoft.XMLHTTP"); /* code for old IE browsers */
        }

        xhr.open( form.method, form.action, true ); // ( "post", "process.php", true )
        xhr.send( data );

        xhr.onreadystatechange = function () {
            if ( this.readyState == 4 && this.status == 200 )
            {
                document.querySelector( destination ).innerHTML = this.responseText;
            }
        };

    }

</script>

echo "<pre>";

var_dump($_POST);

echo "</pre>";