Node Js REPL, Core Modules , File System using synchronous and Asynchronous method

Node JS REPL

    The REPL feature of NodeJS is very useful in experimenting with Node.js codes and to debug JavaScript codes.

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Read	Reads user's input, parse the input into JavaScript data-structure
Eval	Takes and evaluates the data structure
Print	Prints the result
Loop	Loops the above command until the user presses ctrl+c twice

Core Module

• Node.js has a set of built-in modules which you can use without any further installation. like fs, http, https, os, path, util.... etc.

using synchronous method: 

        synchronous means to be in a sequence, i.e. every statement of the code gets executed one by one. So, basically a statement has to wait for the earlier statement to get executed.

• fs is a build in module it help to store, access, and manage data on our operating system.
• writeFileSync - it help to write data in synchronous manner. if 'index.txt' is not found or nothing, so it created automatically. 
• readFileSync - it help to read file data. 

const fs = require("fs");

// write data 
fs.writeFileSync("index.txt","hey i'm shreyash");

// read file data 
const data = fs.readFileSync("index.txt")
console.log(data)

const newData = data.toString();
console.log(newData)

PS F:\nodejs> node app.js
<Buffer 68 65 79 20 69 27 6d 20 73 68 72 65 79 61 73 68>
PS F:\nodejs> node app.js
<Buffer 68 65 79 20 69 27 6d 20 73 68 72 65 79 61 73 68>
hey i'm shreyash

NodeJs include an additional data type called as buffer

Buffer is  mainly used to store binary data, while remaining from a file or receiving packets over the network. 

for avoiding buffer data so used 'utf-8' it help to Encodes any given JavaScript string as UTF-8, and returns the UTF-8-encoded version of the string

// rename file 
fs.renameSync("index.txt", "read.txt");
fs.appendFileSync("index.txt", "today is awesome day");

using asynchronous method:

        Asynchronous code allows the program to be executed immediately

const fs = require("fs");
// create and write file 
fs.writeFile("read.txt", "today is awesome day",(err)=>{
    console.log("file is created")
    // console.log(err)
})
// read file 
fs.readFile("read.txt","utf-8", (err, data)=>{
    console.log(data);
})

PS F:\nodejs> node app.js
file is created
today is awesome day

append file

//append file
fs.appendFile("read.txt","\n hey i'm shreyash",(err)=>{
    console.log("task completed")
})