• Node.js is a runtime environment that allows Javascript files to run outside of browser runtime; Express.js is a server-side framework built to work with Node.js
• CRUD means the api allows clients to create, read, update and delete information.
• Require the express module and invoke an instance.

const express = require('express')
const app = express()

• app.listen(PORT, callback) can start a server. The first parameter is the port number argument tells the server where to listen; the callback function is executed when the server is listening and ready for response.
• route is a section of Express code that is associated with an HTTP verb, a url path, and a function that is called to handle that pattern.
• Express searches through the routes and responses with the first route matches the request url.
• path is part of the requested url after the hostname and port.

HTTP verb(GET,PUT,UPDATE,DElETE)

• app.get() is used to match the route with GET request. The first parameter is the route path(string), the second parameter is the callback function to handle the request and send a response. When a requested url arrives, req.params object will store the information from the requested url path. (query string is not necessary)

app.get('/monsters/:name', (req, res, next) => {
  res.send(...); // or res.json() to send json format response
});
//request url : 'monsters/rx'  req.params = {name: 'rx'}

• app.put() is used to update existing data from the database. It needs data from query string.
• app.post() is used to create a new resources. It needs data from query string.
• app.delete() is used to delete a resource. (query string is not necessary)

req.params and req.query

• route parameter is in route path and begins with ':' it. req.params stores route parameters in an object. It is like a unique identifier for the database, so the server can send the requested resource.
• query string is the part of url after '?'. Express parse the information into req.query object. The value of the key is a string. It stores information the client send to the server.

Router

• using router can make code cleaner. It adds the router path to the path in app.use() Eg:

// in router.js file:
const monserRouter = express.Router()
monsterRouter.get('/:name',(req,res,next)=>{....})
module.exports = monsterRouter;

// in main.js file
const monsterRouter = require('./router.js')
app.use('/monster',monsterRouter)

res.status