Respons.format() node js: How is it works?


Is anyone can explain how to works the response.format() method.
I followed the Express Api reference while I wrote an example code but the server send always “text/html” format. So cannot to understand any essential things. I did not find usable info on another site (stackoverflow)

My code is follow:

"use strict";
const express = require("express");
const app = express(); 

const port = 3000; 

// res.format(object)

// Performs content-negotiation on the Accept HTTP header on the request object, when present. 
// It uses req.accepts() to select a handler for the request, based on the acceptable types ordered by their quality values. 
// If the header is not specified, the first callback is invoked. 
// When no match is found, the server responds with 406 “Not Acceptable”, or invokes the default callback.

 // The Content-Type response header is set when a callback is selected. However, you may alter this within the callback using methods such as res.set() or res.type().

// The following example would respond with { "message": "hey" } when the Accept header field is set to “application/json” or “*/json” (however if it is “*/*”, then the response will be “hey”).

app.get("/", (request, response) => {
    response.set("Content-Type", "text/plain"); // expected output: "Hello text/plain" but I get "Hi text/html!"
        "text/plain": () => response.send("Hello  text/plain"),
        "text/html": () => response.send("<h1 style='color: blue'>Hi text/html!</h1>"),
        "application/json": () => response.send({message: "Aloha application/json"}),
        "default": () => response.status(406).send("Not acceptable")

app.listen(port, () => console.log("Node server running on port", port));


Your response is html so you wil get Hi text/html!