how to select input base on name:

HTML

<form onsubmit="checkForm()">
    Email : <input type="email" name="billContact.email" placeholder="billContact.email">
    <input type="submit" value="Submit">
</form>

JavaScript

var form = document.forms[0];
    var selectElement = form.querySelector('input[name="billContact.email"]');
    var selectedValue = selectElement.value;

    alert(selectedValue);