Summary

NodeJS has fs module that helps in managing files
'fs' module has Promise API, Callback API, and Synchronous API.
There are various methods in fs from creating, to deleting the files.
Always use Async methods over sync for improving performance unless and until you have requirements to use sync methods.

Welcome back to my 2nd blog from the series of NodeJS. If you haven't checked my 1st blog, please check here.

Today we will explore the 2nd module (in-built package) of NodeJS - fs (file system).

We have 3 ways to use fs module in NodeJS:

Promise API
callback API
Synchronous API

	import	mode	return
Promise	node:fs/promises	promise based (async)	promise
Callback	fs	async	error/success or data
Synchronous	fs	sync	error/success or data

In this tutorial we will go through the Callback.

What is fs?

NodeJS has native or built-in package fs. FS stands for filesystem. FS enables devs to create, open, read and write files.

This is an important feature. Developers can do a lot by using FS. Developers will have control over the files.

Where to Use?

fs in configurations.
Build Scripts
Temporary data storage
Processing of data before storing in database

How to create a file?

There are 3 ways (methods) to create a new file:

open: open an existing file.
writeFile: it will create a file if it's not exist and write content in it.
appendFile: it will create a file if it's not exist and append content in it.

How to write content in a file?

writeFile

file name
data
options: encoding, mode, flag
callback: error or success



// Example: Callback API
const fs = require('fs');

fs.writeFile('logs.txt','hello', 'utf8' (err) => {
    if(err) {
      console.log('error')
     }
    else{
      console.log('data saved')
     }
});

// Example: Promise API

import { writeFile } from 'node:fs/promises';

(async function main() {
    try {
        await writeFile(
                "logs.txt",'hello')

        console.log("The written file has"
            + " the following contents:");

    } catch (err) {
        console.error(err);
    }
})();

2 . open

file name
mode
callback: error or success



const fs = require('fs');

// Callback API
fs.open('logs.txt', 'w', (err) => {
    if(err) {
      console.log('error')
     }
    else{
      console.log('file open')
     }
});

// Promise API
import { open } from 'node:fs/promises';

(async function main() {
    try {
        await open(
                "logs.txt")

        console.log("The file has been open");

    } catch (err) {
        console.error(err);
    }
})();

3 . appendFile

file name
data
options: encoding, mode, flag
callback: error or success



// callback API
fs.appendFile('logs.txt', 'new text', 'utf8' , (err) => {
    if(err) {
      console.log('error')
     }
    else{
      console.log('data added')
     }
});

// Promise API
import { appendFile } from 'node:fs/promises';

(async function main() {
    try {
        await appendFile("logs.txt", 'hello');
        console.log("The file has been open");

    } catch (err) {
        console.error(err);
    }
})();

How to read content from a file?

So far we saw how to create and write content to a file. We can also read the file's content.

read

file name
encoding options: utf8, utf16, base ,etc.
callback: error or data



fs.readFile('logs.txt', (err,data) => {
    if(err) {
      console.log('error')
     }
    else{
      console.log(data); // this will return buffer
      console.log(data.toString()); // convert the buffer to string
     }
});

Code:



// import package

const fs = require('fs')

// read a file

const readfile = fs.readFile('logs.txt', 'utf-8');
console.log(readfile);

// write content of a file

const writefile = fs.writeFile('logs.txt', 'hello');
const readfile = writefile.readFile(writefile)
console.log(readfile);

// append content

const readfile = fs.appendFile('logs.txt', 'utf-8');
console.log(readfile);

// rename

const logs = fs.createFile('logs.txt');

fs.rename('logs.txt', 'userlogs.txt', () => {
    console.log('done')
});

// delete

const logs = fs.createFile('logs.txt');

fs.unlink('logs.txt', () => {
    console.log('deleted')
});

Code

Important

Content in the file would be either string, buffer, or text. Not, object
writeFile will overwrite the content. Hence, use appendFile

sync vs async

You will see that there are 2 methods in fs: readfile and readFileSync. readFile is async operation. It's a blocking operation and will impact the performance. Hence, prefer to use readFile over readFileSync unless and until you don't have any requirement.

Resources

NodeJS Documentation

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#2 NodeJS - What is fs module