<form action="" id="form_form">
<input type="text" name="login" id="login">
<button>отправить</button>
</form>
<script>
let formData = new FormData(document.querySelector('#form_form'));
document.querySelector('#form_form').addEventListener('submit', (e)=>{
e.preventDefault();
formData.forEach((element, value) => {
console.log(element, value);
});
});
</script>