The Node.js fs module enables us to work with the file system, which is one of the most fundamental tasks when writing programs.
However, there are several ways in which we can interact with the file system using the fs module, and each method has its trade-offs.
This post will explain how to use the file system module to read and write files and directories using sync, async, and streaming APIs.
Reading Files
To get started, consider a directory with two files present inside it:
.
├── README.md
└── index.js
README.mdis a markdown file that contains the text:Hello world.index.jsis the file that contains the Node.js code that we will execute.
We can read the contents of README.md using the fs module:
const fs = require('fs')
// fs.readFile takes the file path and the callback
fs.readFile('README.md', (err, data) => {
// if there's an error, log it and return
if (err) {
console.error(err)
return
}
// Print the string representation of the data
console.log(data.toString())
})
This will give the output:
Hello World
Note that this is an async API, and we get the result in the form of a callback passed to the readFile function.
See the full example here
Writing to a File
We can write to a file in a similar fashion:
const fs = require('fs')
fs.writeFile('README.md', 'Hello World', (err) => {
// If there is any error in writing to the file, return
if (err) {
console.error(err)
return
}
// Log this message if the file was written to successfully
console.log('wrote to file successfully')
})
In this case, the program will create a new file README.md and write Hello World to it. If the file already exists, then it will be overwritten.
See the full example here
Asynchronous and Synchronous File System APIs
The APIs that we used in the previous example were asynchronous, which means they have a callback.
This is actually the recommended way to use the fs module, since almost all operations related to working with the file system are blocking. Using an asynchronous model will make our code run much faster since we don’t need to wait for the underlying OS to complete its operations:
The fs sync API, on the other hand, blocks the NodeJS process until the OS completes its task:
The fs module still provides synchronous APIs, which you can use as follows:
const fs = require('fs')
// The writeFileSync API takes the location of the file
// and the contents to be written to it
fs.writeFileSync('README.md', 'Hello Sync API!')
// The readFileSync API reads the file and returns a
// Buffer, whose `toString` method gives the string
// representation of the file
console.log(fs.readFileSync('README.md').toString())
Although they may look simpler to use, it’s generally recommended to use the async APIs for their better performance thanks to non-blocking I/O.
Changing File Permissions
Every file has permissions associated with them. These permissions determine who can read, write and execute a particular file on your system.
On linux systems, running ls -l will print information about the files and their permissions:
-rw-r--r-- 1 soham staff 11 Nov 16 13:46 README.md
-rw-r--r-- 1 soham staff 290 Nov 16 23:21 index.js
The last six characters of the first column give the permissions for the user, group, and public respectively:
These permissions can be represented by a six digit binary number, or a three digit octal number. For example, the permissions of the files I listed previously are rw-r--r-- which in binary is 110 100 100, and in octal is 644.
We can use the fs module to change the permission of README.md to add write access for the user group (664) :
const fs = require('fs')
fs.chmod('README.md', 0o644, (err) => {
if (err) {
console.error(err)
}
console.log('Permissions changed successfully');
})
If we list the file now, we can see the modified permissions:
↓write permission added
-rw-rw-r-- 1 soham staff 0B 17 Nov 23:33 README.md
-rw-r--r-- 1 soham staff 155B 17 Nov 23:40 index.js
See the full example here
Streaming Files
When we use the readFile and writeFile methods of the fs module, we treated the file as one chunk of data that we can read from or write to.
While this approach works for small files, it won’t scale for larger files. In this case, we need to think of each file as a stream of data, rather than a single large chunk.
Data streams allow us to work with large data without compromising the limited memory or CPU of our system. The fs module allows us to make use of streams for this purpose.
Read Streams
To illustrate how read streams work, let’s read a large text file (named words.txt) and count the total number of words in the file, using file streams:
const fs = require('fs')
// Initialize the time at which the program started
const startTime = new Date()
// create a read stream from the `words.txt` file
const rStream = fs.createReadStream('words.txt')
// initialize total word count
let total = 0
// the `on data` method registers a handler for everytime we
// receive new data from the file stream
rStream.on('data', b => {
// `b` here is the chunk of data received from the
// file stream
const bStr = b.toString()
// We split the string by spaces and new lines and add it to the
// total -- we subtract one because of the extra space/newline/broken word
// at the end of the chunk
// we shouldn't do this for the last chunk of data, which we handle later
total += bStr.split(/[\s\n]+/).length - 1
})
rStream.on('end', () => {
// Finally, the `on end` handler is called once the data stream completes
// we add one to the total, because we shouldn't subtract 1 from the last
// chunk of data in the `data` handler, for which we're compensating here
console.log('total words:', total + 1)
// Print the total time taken, as well as the total used program memory
console.log('total time:', (new Date()) - startTime)
const memoryUsedMb = process.memoryUsage().heapUsed / 1024 / 1024
console.log('the program used', memoryUsedMb, 'MB')
})
You can see the full example here
Running this code gives the following output:
total words: 1280004
total time: 126
the program used 10.192085266113281 MB
Let’s compare this to the naive version of the same problem, where we read and split the entire file contents all at once using fs.readFile method:
const fs = require('fs')
// Initialize the time at which the program started
const startTime = new Date()
fs.readFile('words.txt', (err, data) => {
if (err) {
console.error(err)
return
}
// Split the words based on spaces and newlines and print the length
const nWords = data.toString().split(/[\s\n]+/).length
console.log('total words:', nWords)
// print the total time taken and total program memory used
console.log('total time:', (new Date()) - startTime)
const memoryUsedMb = process.memoryUsage().heapUsed / 1024 / 1024
console.log('the program used', memoryUsedMb, 'MB')
})
Running this on my system showed me the stark difference in performance:
total words: 1280004
total time: 326
the program used 84.68199920654297 MB
The naive version took almost 3x as long, and more than 8x the memory as compared to using file streams.
See the full example here
Write Streams
Write streams are like read streams but in the other directions. Similar to how read streams work, we open a write stream to a file, and write to it in chunks, ending the stream once we’re done.
Here’s an example of how we can use write streams to store the first thousand numbers in the Fibonacci sequence:
const fs = require('fs')
class Fibonacci {
// The Fibonacci class has the previous number and current
// number as its instance attributes
constructor() {
this.prev = 0
this.current = 1
}
// the next method returns the current value, and
// increments the current value by adding the past value to it
next() {
const current = this.current
this.prev = current
this.current = current + this.prev
return current
}
}
// Iniitalize a writeStream to a a new file "fibonacci.txt"
const writeStream = fs.createWriteStream('fibonacci.txt')
// the on ready callback gets called once the file is available to write
writeStream.on('ready', () => {
// initialize a new object of the Fibonacci class
const f = new Fibonacci()
// For each iteration, obtain the next number in the sequence
// and write to the file, adding a newline each time
for (let i = 0; i < 1000; i++) {
const n = f.next()
writeStream.write(String(n) + '\n', err => {
// if there is any error in writing, log it
if (err) {
console.error('error writing:', err)
}
})
}
// The `end` method closes the write stream, once we're done
writeStream.end()
})
Similar to read streams, we gain immense performance benefits for cases where we need to write a large amount of information, or information which we do not always receive all at once, like logs.
See the full example here
Working with Directories
Consider a directory which has a file and a folder (with another file) inside it:
.
├── index.js
└── tmp
└── tmp.txt
Reading Directories
We can use the fs.readdir method to list all the files and directories within a specified path:
const fs = require('fs')
fs.readdir('./', (err, files) => {
if (err) {
console.error(err)
return
}
console.log('files: ', files)
})
This will give the output:
files: [ 'index.js', 'tmp' ]
While this gives us information about the names of the contents, it doesn’t tell us whether an entry is a file, or another directory. We can set the withFileType option to true to give us more information about each entry:
const fs = require('fs')
fs.readdir('./', { withFileTypes: true }, (err, files) => {
if (err) {
console.error(err)
return
}
console.log('files: ')
files.forEach(file => {
// the `isDirectory` method returns true if the entry is a directory
const type = file.isDirectory() ? '📂' : '📄'
console.log(type, file.name)
})
})
Which will give us:
files:
📄 index.js
📂 tmp
Creating and Deleting Directories
Directories can be created and removed with the fs.mkdir and fs.rmdir methods respectively:
Creating a new directory:
const fs = require('fs')
fs.mkdir('./newdir', err => {
if (err) {
console.error(err)
return
}
console.log('directory created')
})
Removing a directory:
const fs = require('fs')
fs.rmdir('./newdir', err => {
if (err) {
console.error(err)
return
}
console.log('directory deleted')
})
Directory Streams
Directory streams are used to walk through a directory entry-by-entry, rather than list all the entries at once.
A directory stream can be opened using the fs.opendir method. The directory stream is provided in the callback argument, and we can use the dir.read method to read the next file in the directory:
Similar to file streams, this is useful when a directory has a large number of files, or when you want to go through the files in a directory, and its subdirectories recursively.
const fs = require('fs')
// The async `opendir` method creates a stream from the directory
// passed as its first argument. The stream is present in the callback
fs.opendir('./', (err, dir) => {
if (err) {
// log and return if theres any error
console.error(err)
return
}
// A scoped function is defined that reads the next
// file in the directory and calls itself recursively
const readNext = (dir) => {
// The `read` method gives us information on the
// next file in the directory. If there are no
// more files left, the value of `file` is null
dir.read((err, file) => {
if (err) {
// log and return error
console.error(err)
return
}
// If file is null, we are done.
if (file === null) {
return
}
// If the file exists, log the name, along with
// the icon for its type
const type = file.isDirectory() ? '📂' : '📄'
console.log(type, file.name)
// Recursively call `readNext` for the next directory entry
readNext(dir)
})
}
// Call the `readNext` function with the first directory entry
readNext(dir)
})
This give me the output:
📄 delete.js
📄 index.js
📄 create.js
📄 streams.js
📂 tmp
See the full code for all examples here
Common Errors when Using the File System Module
Let’s talk more about the err argument that we keep seeing in all the callbacks of the file system API.
Under normal conditions, we expect err to be null, but there are some common errors that you should watch out for.
Let’s create a directory with two files and one folder:
-rw-r--r-- 1 soham staff 400B 26 Nov 10:06 index.js
-r--r--r-- 1 soham staff 0B 26 Nov 10:03 restricted.txt
drwxr-xr-x 3 soham staff 96B 26 Nov 10:04 tmp
Now, lets run some code in index.js to demonstrate some common errors:
ENOENT
Let’s try to read from a file that doesn’t exist:
fs.readFile('./does-not-exist.txt', (err, data) => {
// the error code is present in the errors `code` attribute
console.error('./does-not-exist.txt: ', err.code)
})
This gives us the output:
./does-not-exist.txt: ENOENT
ENOENT here means that the files does not exist (it literally expands to “__E__rror: NO __ENT__ry”)
EACCES
The restricted.txt file has only read permissions for the user as well as the group. This means a user running a program to write to this file will receive an error:
fs.writeFile('restricted.txt', 'sample data', (err) => {
console.error('restricted.txt: ', err.code)
})
This will output:
restricted.txt: EACCES
EISDIR
What do you think happens if we call the readFile method on a directory?
fs.readFile('./tmp', (err, data) => {
console.error('./tmp: ', err.code)
})
Well, of course, this will give us an error:
./tmp: EISDIR
ENOTDIR
As a corollary, if we try to run the opendir method on an entry that’s not a directory:
fs.opendir('./index.js', (err, dir) => {
console.error('index.js :', err.code)
})
This will give us the ENOTDIR error:
index.js : ENOTDIR
Note: These error codes are actually the same error codes returned by the OS. The codes discussed above are for Unix systems, and may differ if you’re on Windows.
See the full example here
Ok, so that’s about it for this post. Do you think there’s anything important I missed? Let me know in the comments!
