8.6 TRUjQ - A Simple Express-like Library

Learning outcomes
- Learn and practice JavaScript more through the designing and implementation if a simple Express-like library.

Observations on Express
- Let's check again the typical coding style to use Express.
```
const express = require("express");
const app = express();  // what is express?
                        // what is app?
app.listen(portnumber);
app.get(path, function(req,res) {  
});
app.post(..., ...);
app.put(..., ...);
app.delete(..., ...);
...
```
- We can see that express is a function and it returns an object. The return object has multiple methods.
- app.listen() is used to listen to a HTTP port number. app.method() is used to register a callback function for a path. It can be used multiple times for differnt paths.
- An http server is created when express() is envoked, so that when an http packet arrives, the path in URL is checked, a corresponding callback function is searched and invoked with the request and the response objects.

Requirements for a simple Express-like library, called TRUExpress
- ```
const truexpress = require("tru-express.js");
const app = truexpress();
```
- app.listen()
- app.get()

How to implement TRUExpress

tru-express.js

const truexpress = function() {
    // initialization
    ...
    
    // return an object of methods
    return {
        listen: function(port) {
            ...
        },
        get: function(path, callback) {  // It can be invoked multiple times for different paths.
            ...
        },
    }
}
module.exports = truexpress;

Initialization in tru-express.js
- Note that TRUExpress supports an app web server.
- An http server must be implemented. For example,
```
const http = require("node:http");

const server = http.createServer((request, response) => {  // request: from the client; response: to the client
    ...
});

server.listen(PORT_NO);
```
- What must be included in the initialization? Definitely not listening to a port number yet because ...

listen() in tru-express.js
- What must be included? Listening to the given port number.

get() in tru-express.js

Note that the role of this method is to register a callback function for a given path, which will be invoked when an HTTP GET request that includes the given path comes.

How to register? (Note that listeners for multiple GET routes can be registered.)

let getCallbacks = [];
...
// the get() method in the return object
get: function (path, callbackk) {
    getCallbacks.push({path:path, callback:callback});
}

How to use? When an HTTP request comes,

const url = require("node:url");
...
// In server.createServer((request, response) => { ... });
if (request.method.toLowerCase() == "get") {
    const path = decodeURI(url.parse(request.url).pathname);
    ...  // search getCallbasks to find the above path and get callback
    callback(request, response);
}

Now you can combine them all.

Trial 1: Let's try ...

8.6 TRUExpress - A Simple Express-like Library