Starting server by listen() function in app object

const express = require("express");
const app = express(); // The express() function returns an object (server-side application) that contains various functions used for building a server
console.dir(app); // this is object
const port = 3000; // Port 3000 is the endpoint for the connection between the client and server

app.listen(port); // This starts the server and makes it listen for client requests, although no response is given yet

Responding to the request by the res object

app.use((req,res)=>{ 
  console.log("request received");
  let code = "<h1>Fruits</h1> <ul><li>apple</li><li>mango</li>"
  res.send(code);
})

The above code displays a html output on the browser

Express 4.x - API Reference

The documentation of response object

Using get() which takes path and callback function as argument

app.get("/",(req,res) => {
  res.send("this response is from home page");
});
app.get("/apple",(req,res) => {
  res.send("this response is from apple");
});
app.get("/mango",(req,res) => {
  res.send("this response is from mango");
});

app.get("*",(req,res) => {
  res.send("path not found");
})

The above response is sent when a get request is sent.

There can be any number of paths like ‘/’ is root, /help, /home, /search are some of the paths

The below response is for post request

app.post("/",(req,res)=>{
  res.send("this is a post response");
})

Path parameters

app.get("/:username/:password",(req,res) => {
 let {username, password} = req.params;
 res.send(`this is the username ${username} and your password ${password}`);
});

the url which we send our request contains the username through which the respective account is opened like instagram/apple etc

Query Strings

app.get("/search",(req,res) => {
 let {q,color} = req.query; // accessing query strings through req obj
 if(!q){
  res.send(`<h1>not found</h1>`)
 }
  res.send(`<h1>the query is ${q} and the color is ${color}</h1>`);
});