If you want to set this up once and use it everywhere, just configure your own middleware. When you are setting up your app, use the following to define a new function on the response object:
app.use((req, res, next) => {
res.show = (name) => {
res.sendFile(`/public/${name}`, {root: __dirname});
};
next();
});
Then use it as follows:
app.get('/demo', (req, res) => {
res.show("index1.html");
});