Whenever an Express application server receives an HTTP request, it will provide the developer with an object, commonly referred to as res. For example,
Example
app.get('/test', (req, res) => {
// use req and res here
})The res object basically refers to the response that'll be sent out as part of this API call.
The res.send function sets the content type to text/Html which means that the client will now treat it as text. It then returns the response to the client.
The res.json function on the other handsets the content-type header to application/JSON so that the client treats the response string as a valid JSON object. It also then returns the response to the client.