I am trying to copy the value of a hidden text field from another button. At the moment I'm trying to simply show it with an alert when I click on the button but it doesn't work. What am I doing wrong ?

php:

echo "<p id='campoCopiar' class='btn btn-warning btn-lg paginasTotales'>Páginas totales: <b>" . $paginasTotales . "</b></p>";
echo "<input id='oculto' name='oculto' type='hidden' value='" . $paginasTotales . "'>";

js:

document.querySelector("#campoCopiar").addEventListener("click", function(){
    var oculto = document.querySelector("#oculto").value;
    alert(oculto);
});

The variable is defined because one line above I have another code where it shows it and it works correctly.

The call to the js file I have it at the bottom just before the closing tag of the body