how to chnage object object format in expres code example

Example 1: node js return json

var http = require('http');

var app = http.createServer(function(req,res){
    res.setHeader('Content-Type', 'application/json');
    res.end(JSON.stringify({ a: 1 }));
});
app.listen(3000);

// > {"a":1}

Example 2: http header express

app.get('/', (req, res) => {
  req.header('User-Agent')
})
// Use the Request.header() method to access
//one individual request header’s value