How to submit this form using AJAX without jQuery but pure Javascript

formElem.onsubmit = async (e)  =>
{
    var url = document.getElementById("formElem").getAttribute("action");
    e.preventDefault();
    var data =  new FormData(formElem);
    data.append("submit", "Submit");
    let response = await fetch(url, 
    {
        method: 'POST',
          body: data
    });
    let result = await response.json();
    document.getElementById("field").value = "";
    document.getElementById('charNum').innerHTML="0";
    setTimeout(function autoscroll(){
    var div = document.getElementById('loadmsg');
    div.scrollTop = div.scrollHeight - div.clientHeight; 
    },1000);
  
  };

Here is how you can submit your form via Ajax:

function submitFormAjax() {
    let xmlhttp= window.XMLHttpRequest ?
        new XMLHttpRequest() : new ActiveXObject("Microsoft.XMLHTTP");

    xmlhttp.onreadystatechange = function() {
        if (this.readyState === 4 && this.status === 200)
            alert(this.responseText); // Here is the response
    }

    let name = document.getElementById('name').innerHTML;
    let email = document.getElementById('email').innerHTML;

    xmlhttp.open("GET","your_url.php?name=" + name + "&email=" + email, true);
    xmlhttp.send();
}

This example is using GET, but you could also use POST:

xmlhttp.open("POST","your_url.php",true);
xmlhttp.setRequestHeader("Content-type","application/x-www-form-urlencoded");
xmlhttp.send("name=" + name + "&email=" + email);

Note:

You must call submitFormAjax() after validateFormOnSubmit is done with no errors, here:

if (reason.length == 0) {
    // Show some loading image and submit form
    submitFormAjax();
} else {
    return false;
}

In modern browsers, you can use fetch in just a few lines:

fetch(form.action, {
  method: form.method,
  body: new URLSearchParams(new FormData(form)),
  headers: {
    'Content-Type': 'application/x-www-form-urlencoded'
  }
});

I also published a slightly more complex version of this as an npm package, if you don't like copy-pasting code from StackOverflow: push-form