How to change the state of a variable using Express and EJS?

Question

I am trying to implement a Loader in my application as follows:

1 - seto a variável isLoading para true

2 - faço as requests

3 - seto a variável isLoading para false

With this variable I can make an if in front-end (EJS) to show or not my gif of loading.

The problem is this, so I saw the only way to send a variable to the EJS is by using the req.render and if I do this twice I need to change the state of the variable I end up taking an Exception:

(Ode:11752) Unhandledpromiserejectionwarning: Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client

How can I solve the problem? Follows code:

Express:

router.get('/backup', AuthCheck, async (req, res) => {
    res.render('my-page', {
        isLoading: true,
    });

    await saveProduct();

    res.render('my-page', {
        isLoading: false,
    });
});

EJS:

<%if (isLoading) { %>
    <div style="width: 100%;text-align: center">
        <img style="width: 120px; height: 120px;" src="/imgs/loading.gif" />
    </div>
<% } %>