Node.js File System (FS)

In Node.js, file I/O is provided by simple wrappers around standard POSIX functions. Node File System (fs) module can be imported using following syntax:

Syntax:

Node.js FS Reading File

Every method in fs module has synchronous and asynchronous forms.

Asynchronous methods take a last parameter as completion function callback. Asynchronous method is preferred over synchronous method because it never blocks the program execution where as the synchronous method blocks.

Let's take an example:

Create a text file named "input.txt" having the following content.

File: input.txt

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Let's take an example to create a JavaScript file named "main.js" having the following code:

File: main.js

var fs = require("fs");
// Asynchronous read
fs.readFile('input.txt', function (err, data) {
   if (err) {
       return console.error(err);
   }
   console.log("Asynchronous read: " + data.toString());
});
// Synchronous read
var data = fs.readFileSync('input.txt');
console.log("Synchronous read: " + data.toString());
console.log("Program Ended");

Open Node.js command prompt and run the main.js:

Node.js Open a file

Syntax:

Following is the syntax of the method to open a file in asynchronous mode:

Parameter explanation:

Following is the description of parameters used in the above syntax:

path: This is a string having file name including path.

flags: Flag specifies the behavior of the file to be opened. All possible values have been mentioned below.

mode: This sets the file mode (permission and sticky bits), but only if the file was created. It defaults to 0666, readable and writeable.

callback: This is the callback function which gets two arguments (err, fd).

Node.js Flags for Read/Write

Following is a list of flags for read/write operation:

Flag	Description
r	open file for reading. an exception occurs if the file does not exist.
r+	open file for reading and writing. an exception occurs if the file does not exist.
rs	open file for reading in synchronous mode.
rs+	open file for reading and writing, telling the os to open it synchronously. see notes for 'rs' about using this with caution.
w	open file for writing. the file is created (if it does not exist) or truncated (if it exists).
wx	like 'w' but fails if path exists.
w+	open file for reading and writing. the file is created (if it does not exist) or truncated (if it exists).
wx+	like 'w+' but fails if path exists.
a	open file for appending. the file is created if it does not exist.
ax	like 'a' but fails if path exists.
a+	open file for reading and appending. the file is created if it does not exist.
ax+	open file for reading and appending. the file is created if it does not exist.

Create a JavaScript file named "main.js" having the following code to open a file input.txt for reading and writing.

File: main.js

var fs = require("fs");
// Asynchronous - Opening File
console.log("Going to open file!");
fs.open('input.txt', 'r+', function(err, fd) {
   if (err) {
       return console.error(err);
   }
  console.log("File opened successfully!");     
});

Open Node.js command prompt and run the main.js:

Node.js File Information Method

Syntax:

Following is syntax of the method to get file information.

Parameter explanation:

Path: This is string having file name including path.

Callback: This is the callback function which gets two arguments (err, stats) where stats is an object of fs.Stats type.

Node.js fs.Stats class Methods

Method	Description
stats.isfile()	returns true if file type of a simple file.
stats.isdirectory()	returns true if file type of a directory.
stats.isblockdevice()	returns true if file type of a block device.
stats.ischaracterdevice()	returns true if file type of a character device.
stats.issymboliclink()	returns true if file type of a symbolic link.
stats.isfifo()	returns true if file type of a fifo.
stats.issocket()	returns true if file type of asocket.

Let's take an example to create a JavaScript file named main.js having the following code:

File: main.js

var fs = require("fs");
console.log("Going to get file info!");
fs.stat('input.txt', function (err, stats) {
   if (err) {
       return console.error(err);
   }
   console.log(stats);
   console.log("Got file info successfully!");
   // Check file type
   console.log("isFile ? " + stats.isFile());
   console.log("isDirectory ? " + stats.isDirectory());    
});

Now open the Node.js command prompt and run the main.js

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Interview Questions

Node.js File System (FS)

Node.js FS Reading File

Node.js Open a file

Node.js Flags for Read/Write

Node.js File Information Method

Node.js fs.Stats class Methods