How to handle a download HTTP response or text

Question

In my web app, the user can download file that are stored in the server. At the moment, when a file doesn't exist, another page open with just the response of the server. I would like now to have this response displayed in an alert box.

Server-side, this is the code that send the file for download, if it exists

router.get('/download/file', function(req, res){
    const file = `${__dirname}/uploads/Environnement/file.pdf`;
    try {
        if (fs.existsSync(file)) {
            res.download(file);
        } else {
            res.send("File does not exist!")
        }
    } catch (e) {
        console.error(e);
    }
});

For now, the code that allow to get the file from the server was like this :

<form action="/HSE/Environnement/download/file" method="get">
   <button type="submit" name="button">Download</button>
</form>

And I try to do like below :

function getFile(url) {
    fetch(url).then(function(response) {
        response.text().then((text) => {
            alert(text);
        })
    });  
}

The problem is that when the file exist, it doesn't download it.

I tried also to send different status, but on client-side I always have the same.

router.get('/download/file', function(req, res){
    const file = `${__dirname}/uploads/Environnement/file.pdf`;
    if (fs.existsSync(file)) {
        res.status(200).download(file);
    } else {
        res.status(404).send("File does not exist!")
    }
});

File exist

Response {type: "basic", url: http://localhost:8001/HSE/Environnement/download/file", redirected: false, status: 200, ok: true, …}

File doesn't exist

Response {type: "basic", url: "http://localhost:8001/HSE/Environnement/download/file", redirected: false, status: 200, ok: true, …}

Any idea ?