HTML

send html file express

res.sendFile(path.join(__dirname + '/index.html'));

express return html file

//res.sendFile(path [, options] [, fn])
app.get('/test', function(req, res) {
    res.sendFile('test.html')
});

express send html file

app.get('/test', function(req, res) {
    res.sendFile('test.html', {root: __dirname })
});

express send html from string

// Express.JS: Send HTML from string
res.send(`
  <h1>Welcome</h1>
  <h2>Scroll to learn more</h2>
`);