5.2 Express.js

1. Preparation
2. Indication of use of middlewares
3. Support of CORS
4. Route handlers

const express = require('express');
const app = express();
const PORT = 3000;
...
app.listen(PORT, function (err) {
    if (err) console.log(err);
    console.log("Server listening on PORT", PORT);
});

app.use(...);
// E.g.,
// app.use(express.urlencoded({ extended: false }));  // for the POST query data
//   From Github: The "extended" syntax allows for rich objects and arrays to be encoded into the URL-encoded format, allowing for a JSON-like experience with URL-encoded.
// app.use(express.json());

// Allow Cross-domain requests, i.e., CORS 
//   Why do we need this?
app.all('/*', function(req, res, next) {  // all(): any HTTP method; /*: any path
    res.header("Access-Control-Allow-Origin", "*");
    res.header("Access-Control-Allow-Headers", "X-Requested-With");
    res.header("Access-Control-Allow-Methods", "GET, POST, PUT, DELETE");  // Default: only GET and POST
    next();  // Express middleware function; To continue to next operations
});

// get, post, put, delete, ..., all
// E.g., app.post("/echo", function(req, res) { ... });
// E.g., app.get("/calculator/adder", function(req, res) { ... });
// req and res objects are different from those used in http.createServer().
app.methodname(path, (req, res) => {
    ...
    res.send(...);
})
// path could be "*"
get.all(path, (req, res) => {
    ...
    res.send(...);
})

app.get(path, (req, res) => {
    ...
    req.query.queryname
    ...
});

app.use(express.urlencoded({ extended: false }));
app.methodname(path, (req, res) => {
    ...
    req.body.queryname
    ...
});