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File handling in Node.js

September 17, 2020

Node.js is a server-side environment for executing JavaScript files. Off the browser, Node.js offers even more features to complement JavaScript functionalities. Node.js is built around adding functionalities that cannot be performed or utilized in a browser. In this article, we will cover file handling extensively, understand the various functionalities Node.js offers, and implement code to read and write to files.

Blocking and Non-Blocking I/O

Node.js is a non-blocking single-threaded server-side JavaScript environment. What do we mean by non-blocking? Non-blocking I/O ensures other processes don’t wait until I/O is completed.

Before we start coding, an overview of how non-blocking I/O works and where is being used.

File handling is completed in two ways: synchronous and asynchronous.

In synchronous file handling, all consecutive processes wait for the current I/O operation(read or write operation) to finish. Under non-blocking I/O, other processes take over the thread while the read or write operations finish. Upon execution, the callback function returns the data from the I/O operation.

The non-blocking nature of Node.js ensures non-blocking I/O operations and create an efficient execution of programs. Note that parallelism is not possible due to the single-threaded nature of Node.js. Given one single thread, non-blocking I/O is the most efficient way to go about optimizing the execution of your programs.

Applications of File Handling

For file handling, Node.js offers a module called fs (fileSystem). We must understand the significance of the fs module and some of the applications where the this module may be used. You might be wondering about the practical use of file handling apart from reading and writing files on a local machine. Let us look at some of use cases.

  1. Logging: While creating scalable systems, feedback is an important part of maintaining those systems(and sub-systems). Sub-systems generate log files that are read by other sub-systems and then take action based on the feedback given. All this is made possible by reading and writing into files.

  2. System maintenance: Lets consider an example of a Google servers system’s engineer. The current status of all the operations are communicated to the engineer via a detailed report given a suitable format.

  3. Temporary files: Many times when we run programs on servers, temporary files are created to store some temporary but important information. These temporary files can store data such as event IDs, cookies, etc. This is done to reduce the load on the main memory and balance the load between the main and secondary memory devices.

With a better understanding of how File Handling works lets dive into some coding.

File System Module

As we mentioned above for file handling, Node.js offers a module called fs (fileSystem). To use the features of this module, we need to begin by importing it. We import it by using the require keyword.

const fileSystem = require('fs')

The module offers functions to perform basic CRUD (Create, Read, Update, Delete) operations. Let us look at some of the prominent functions used regularly during development. readFile, readFileSync, writeFile, writeFileSync are the main functions that we will be looking at for this article.

Some of the functionalities the fs module provides are given below.

  1. unlinkSync: unlinkSync is used to delete files synchronously.

     const fs = require('fs');
     try {
         console.log('successfully deleted fileToBeDelted.txt');
     } catch (error) {
     // handle the error

    unlinkSync takes in the path of the file to be deleted as its parameter.

  2. unlink: unlink is used to delete files asynchronously using callbacks. Asynchronous execution refers to the execution of the next line of code while waiting for the function to execute. The order is not specific and therefore, the callback function returns the value upon successful execution of the function unlink.

     const fs = require('fs');
     fs.unlink('./fileToBeDeleted', (error) => {
     if (error) throw error;
     console.log('successfully deleted');

    unlink takes the path of the file to be deleted and a callback function as its parameters. The callback function returns the error upon unsuccessful deletion. If deleted, it displays a successfully deleted message.

  3. readFile: This function is used for reading files asynchronously. js fs.readFile('<directory>', (error, data) => { if (error) throw error; console.log(data'); }); readFile takes in the path and a callback function as its parameters.

  4. readFileSync: Used to read files synchronously in order.


    readFileSync takes in the path of the file as its parameter.

  5. writeFile: writeFile is a function used to write into files asynchronously. The function takes in four parameters: fileName, textToBeWritten, FileEncodingType, Callback function.

     const textToBeWritten = 'Sample Text'
     fs.writeFile('message.txt', textToBeWritten, 'utf8', (error, data)=> {
  6. writeFileSync: writeFileSync is used to write into files synchronously. The function takes three parameters, fileName, inputText and fileEncoding as its inputs.

    const textToBeWritten = 'Sample Text'
    fs.writeFileSync('message.txt', textToBeWritten, 'utf8')
  7. stats.isDirectory: To check if a file exists, we use the function stats.isDirectory. It returns a boolean value.

     const fs = require('fs');
     const pathsToCheck = ['./fileCheckForExistence'];
     for (let i = 0; i < pathsToCheck.length; i++) {
     fs.stat(pathsToCheck[i], function(err, stats) {

    The resulting output should look like this (Assuming the file exists).


    If the file does not exist, the output will be false

Reading From Files

We will begin by creating a JavaScript file called fileHandling.js. The fs module gives us access to the file system of the local machine. Before we begin reading the file, we will create a sample file that we will read and write to. We will name it sampleText.txt.

This is a sample text for a blog on file handling in Node.js. We will be covering the topic of file handling extensively.

We now have a file available to read. Let us look at the code and read the file.


const fileSystem = require('fs')
const textFromFile = fileSystem.readFileSync('./sampleText.txt','utf-8')

The contents of the .txt file are displayed on the console/terminal. require('fs') creates an object called fileSystem which has all the functionalities required to interact with local files. The readFileSync function returns the text from the file and it is stored in the textFromFile variable.


Consider the asynchronous counterpart for reading files. As discussed earlier, asynchronous execution does not guarantee the order of execution of code. Let us look at the example code to better understand:

const fileSystem = require('fs')
console.log("Before Reading")
fileSystem.readFile('sampleText.txt', 'utf-8', (err,data)=> {
    console.log("Read File")
console.log('Reading file')

Before moving onto the next section, try to guess the output.

I am sure you guessed it right. Upon executing the command node fileHandling.js, we obtain the following output.

Before Reading
Reading file
This is a sample text for a blog on file handling in Node.js. We will be covering the topic extensively.

Read File

Let us analyze the output. The order of code executed is not the same as the order in which it has been written. This is the beauty of asynchronous code. The callback function returns data upon reading the file.

Write To Files

Let us now consider writing into the files. The given snippet of code writes into the file synchronously.

const fileSystem = require('fs')
const textToBeWritten = 'Hopefully, you enjoyed going through the article'
fileSystem.writeFileSync('./newFileCreated.txt', textToBeWritten)
console.log("Creating and Writing to File DONE! :)")

The writeFilesync is executed before the console.log statement reasserting the fact that the order of execution is in-order. The asynchronous counterpart is similar to readFile. Since the execution order is not fixed, we cannot say which file will be written first. Although, in the example that we looked at, since the files were small, we might not find a change in the order of execution.

At the end of the execution of the code, we will have a new file created called newFileCreated.txt.

const fileSystem = require('fs')
const textToBeWritten = 'Hopefully, you enjoyed going through the article'
fileSystem.writeFile('./newFileCreated1.txt', textToBeWritten, (err,data)=>{
    console.log("inside file 1")
console.log("Creating and Writing to File DONE! :)111111")
const textToBeWritten2 = 'Hopefully, you enjoyed going through the blog'
fileSystem.writeFile('./newFileCreated2.txt', textToBeWritten2, (err,data)=>{
    console.log("inside file 2")
console.log("Creating and Writing to File DONE! :)2222222")

The output will resemble the text snippet given below:

Creating and Writing to File DONE! :)111111
Creating and Writing to File DONE! :)2222222
inside file 1
inside file 2

Upon analysis of the code snippet, we understand that the creation of the two files takes place after the console.log statements have been executed. This demonstrated the asynchronous nature of the code.


In this article, we have covered the various functionalities of the fs module. We have reviewed various applications of file handling, understood the non-blocking feature of Node.js, and how it makes use of it for quicker file handling. Next time you think of logging some data in your systems, make sure to think of file handling as a go-to solution.