gistfile1.html

<form if="form_cadastro" method="post" action="functons.php">
    
    <input type="text" id="nome" name="nome"/>
    <button type="submit" id="gravar">Gravar Dados</button>

</form>

gistfile2.js

 var form = document.getElementById('form_cadastro'),
     nome = form.nome;
     gravar = form.gravar;