Q:

'res.location(path)' does not do anything? (Express)

I have been searching around for answers on Stack Overflow; non have worked... Why does res.location(path) not work? My Location-header does not change. Normal rendering works fine.
I could add, that in the final code, I want to render the page. So, I want to replace res.end() with res.render('app', {...}) and use handlebars.js for rendering.
Code that does not work as expected:
app.get('/sub-link/:wildcard', (req, res) => {
    res.location('/new-header');
    res.end();
});

I have been reading the documentation; could not find the reason. The only thing I found that could be the problem, is the browser:

After encoding the URL, if not encoded already, Express passes the
specified URL to the browser in the Location header, without any
validation.
Browsers take the responsibility of deriving the intended URL from the
current URL or the referring URL, and the URL specified in the
Location header; and redirect the user accordingly.

I have tried this with the latest Node.js compatible with Firebase SDK 2018 (server)
I have tried this in Google Chrome and IE Edge (client)

A:

res.location doesn't set the response status code to 3xx or 201. It only set the Location header.
You can use res.redirect instead which set the status code to 302 and will make the browser change the URL.

The Location response header indicates the URL to redirect a page to.
  It only provides a meaning when served with a 3xx (redirection) or 201
  (created) status response.

This will work:
app.get('/sub-link', (req, res) => {
    res.location('/new-header');
    res.send(302);
});

or
app.get('/sub-link', (req, res) => {
    res.redirect('/new-header');
});

You can check res.location & res.redirect code to see the differences between them.

