[Node.js] Inner workings of Node.js

I. V8 Engine

• V8 engine is JavaScript engine embedded in Node.js, it takes JS (computers cannot understand JavaScript) code and converts it into machine code.

II. Common used global objects

• __dirname: returns the current directory's name you are in.
• __filename: returns the absolute path to the current file you are in.
• setTimeout
• setInterval

III. Function expressions

• Using function keyword to define a function inside an expression.

let greeting = function(){
    console.log("Hello world!");
}
greeting();

IV. Module & require

1. module

• Node.js treats all JS files as a separate module.
• The code in each module is private to the module unless it is explicitly exported.

// greeting.js aka greeting module
let greeting = function(){
    console.log("Hello world!");
}
let greeting2 = function(name){
    console.log(`Welcome ${name} to Node.js!`);
}
module.exports = greeting; (1)
// or multiple exports
module.exports.greeting = greeting; (2) 
module.exports.greeting2 = greeting2;

2. require('something')

• require keyword refers to a function used to import all objects exported from module.exports from other modules.
• Values return by require function are equal to the module.exports

// app.js
let greeting = require("path-to-greet-module/greeting"); (1)
greeting(); // Hello world!

let greetings = require("path-to-greet-module/greeting"); (2)
greetings.greeting(); // Hello world!
greetings.greeting2("Vivian"); // Welcome Vivian to Node.js!

V. Events module and event emitter

• events module is used to create and handle custom events, it includes EventEmitter class.
• EventEmitter is used to raise and handle custom events.

// gets ref of EventEmitter class
let events = require('events');

// creates an object of EventEmitter
let myEventEmitter = new events.EventEmitter();

// subscribes for slide using .on or .addListener
// callback fnc will be called when an event is raised
myEventEmitter.on("slide", (data) => {  
        console.log("Slide is fired: " + data);
});

// raises slide event
myEventEmitter.emit("slide", "Slide smt");

VI. File system

• fs module is used to create, read, update, delete and rename files asynchronously or synchronously.

1. Reading file

• Synchronous operation with fs.readFileSync(path, options).
• Options optional is an object containing the encoding for data specification and flag for operations allowed in the file. Default values are null and r respectedly.
• This version will block code before its finished.

let fs = require('fs');
let output = fs.readFileSync('filename',{encoding: 'utf8', flag: 'r'}); 
// or 
let output = fs.readFileSync('filename','utf8'); // file's content

• Asynchronous operation with fs.readFile(path, options,callbackFnc)
• This version will NOT block code while its reading. callbackFnc is called after reading operation is done.

let fs = require('fs');
fs.readFile('filename','utf8',(err, data) => {
    if(err) 
        console.log(err);
    else 
       console.log(data); // file's content
});

2. Writing file

• Synchronous operation with fs.writeFileSync(path,data, options).
• Data can be a string, buffer TypedArray or DataView which can be written to file.
• Options optional is an object containing the encoding for data specification, mode - an integer for file mode and flag for operations allowed in the file. Default values are utf8, 0o666 and w respectedly.
• This version will block code before its finished.
• NOTE: file will be created automatically if it does not exist.

let fs = require('fs');
let data = "Hello world";
fs.writeFileSync('filename',data); 

• Asynchronous operation with fs.writeFile(path, data, options)
• This version will NOT block code while its reading.

let fs = require('fs');
let data = "Hello world";
fs.writeFile('filename', data);

3. Creating & removing file/directory

• Removing file

// Synchronous methods
fs.unlinkSync(path)

// Asynchronous methods: delete file if it exists else throw error
fs.unlink(path, (err)=>{});

• Creating & removing directory

 // Synchronous methods
fs.mkdirSync("path", options); 
fs.rmdirSync("path");

// Asynchronous methods
fs.mkdir("path", () => {});
fs.rmdir("path", () => {}); // throws error if directory is not empty