CH 6: FILE SYSTEM
Prepared By: Bareen Shaikh

Common use for the File System
module
 Read files:
fs.readFile()
fs.readFileSync()
 Create files:
fs.open()
fs.writeFile()
 Update files:
fs.appendFile()
fs.writeFile()
 Delete files:
fs.unlink()
 Rename files:
fs.rename()

Creating a file: fs.open()
 fs.open() method does several operations on a file.
 Syntax:
fs.open( filename, flags, mode, callback )
 Parameter: This method accept four parameters as
below:
 filename: It holds the name of the file to read or the entire
path if stored at other location.
 flag: The operation in which file has to be opened.
 mode: Sets the mode of file i.e. r-read, w-write, r+ -
readwrite. It sets to default as readwrite.
 callback: It is a callback function that is called after reading
a file. It takes two parameters:
 err: If any error occurs.

Flags of fs.open()
 All the types of flags are described below:
 r: To open file to read and throws exception if file doesn’t
exists.
 r+: Open file to read and write. Throws exception if file
doesn’t exists
 rs+: Open file in synchronous mode to read and write.
 w: Open file for writing. File is created if it doesn’t exists.
 wx: It is same as ‘w’ but fails if path exists.
 w+: Open file to read and write. File is created if it doesn’t
exists.
 wx+: It is same as ‘w+’ but fails if path exists.
 a: Open file to append. File is created if it doesn’t exists.
 ax: It is same as ‘a’ but fails if path exists.
 a+: Open file for reading and appending. File is created if it
doesn’t exists.

Example of : fs.open()
var fs = require('fs');
// Open file a.txt in read mode
fs.open(‘a.txt', 'r', function (err, data) {
console.log(data);
console.log('Saved!');
});

Reading a file: fs.readFile()
 The fs.readFile() method is an inbuilt method which is
used to read the file. This method read the entire file
into buffer.
 Using fs.readFile() method, file can be read in a non-
blocking asynchronous way.
 Syntax:
fs.readFile( filename, encoding, callback_function )
Parameters: The method accept three parameters as
below:
 filename: It holds the name of the file to read or the entire
path if stored at other location.
 encoding: It holds the encoding of file. Its default value
is ‘utf8’.
 callback function: It is a callback function that is called
after reading of file. It takes two parameters:

Example
var http = require('http');
var fs = require('fs');
http.createServer(function (req, res) {
fs.readFile(‘app.js', function(err, data) {
if(err)
console.log(err);
else
{
res.writeHead(200, {'Content-Type': 'text/html'});
res.write(data);
res.end();
}
});
}).listen(8080);

Reading File: fs.readFileSync()
 The fs.readFileSync() method is an inbuilt application
programming interface of fs module which is used to
read the file and return its content.
 fs.readFileSync() method, read files in a synchronous
way, i.e. we are telling node.js to block other parallel
process and do the current file reading process.
 When the fs.readFileSync() method is called
 The original node program stops executing
 Executing node waits for the fs.readFileSync() function to
get executed
 After completion of the fs.readFileSync() method the
remaining node program is executed.

Syntax of fs.readFileSync()
 Syntax:
fs.readFileSync( path, options )
 Parameters:
 path: It takes the relative path of the text file. The path can
be of URL type.
 options: It is an optional parameter which contains the
encoding and flag, the encoding contains data specification.
It’s default value is null which returns raw buffer and the flag
contains indication of operations in the file. It’s default value
is ‘r’.
 Example:
const fs = require('fs');
const data = fs.readFileSync(‘app.js',
{encoding:'utf8',
flag:'r'});

Example of fs.readFile() &
fs.readFileSync()
const fs = require('fs');
// Calling the fs.readFile() method for reading file
fs.readFile(‘a.txt', {encoding:'utf8', flag:'r'}, function(err,
data) {
if(err)
console.log(err);
else
console.log(data);
});
// Calling the fs.readFileSync() method for reading file
‘b.txt'
const data = fs.readFileSync(‘b.txt', {encoding:'utf8',
flag:'r'});

Writing to a File: fs.writeFile()
 The fs.writeFile() method is used to asynchronously write the specified data to a file.
 The file would be replaced if it ex
 Syntax:
fs.writeFile( file, data, options, callback )
 Parameters: This method accept four parameters as mentioned above and described
below:
 file: It is a string denotes the path of the file where it has to be written.
 data: It is a string, Buffer, TypedArray or DataView that will be written to the file.
 options: It is an string or object that can be used to specify optional parameters
that will affect the output. It has three optional parameter:
encoding: It is a string value that specifies the encoding of the file. The default
value is ‘utf8’.
mode: It is an integer value that specifies the file mode. The default value is 0o666.
flag: It is a string value that specifies the flag used while writing to the file. The
default value is ‘w’.
 callback: It is the function that would be called when the method is executed.

Example of fs.writeFile()
const fs = require('fs');
let data =
"This is a file containing a a data n1111n 2222n 3333";
fs.writeFile(“a.txt", data, (err) => {
if (err)
console.log(err);
else {
console.log("File written successfullyn");
console.log("The written has the following contents:");
console.log(fs.readFileSync(“a.txt", "utf8"));
}
});

Writing to a File: fs.writeFileSync()
 The fs.writeFileSync() is a synchronous method.
 The fs.writeFileSync() creates a new file if the specified file does not
exist.
 Syntax:
fs.writeFileSync( file, data, options )
 Parameters: This method accept three parameters as follows:
file: It is a string, that denotes the path of the file where it has to be
written.
data: It is a string, Buffer, TypedArray or DataView that will be written
to the file.
options: It is an string or object that can be used to specify optional
parameters that will affect the output. It has three optional parameter:
 encoding: It is a string which specifies the encoding of the file. The
default value is ‘utf8’.
 mode: It is an integer which specifies the file mode. The default value is
0o666.
 flag: It is a string which specifies the flag used while writing to the file.

Example of fs.writeFileSync()
const fs = require('fs');
let data = "This is a file containing a collection"
+ " of programming languages.n"
+ "1. Cn2. C++n3. Python";
fs.writeFileSync(“programming.txt", data);
console.log("File written successfullyn");
console.log("The written has the following contents:");
console.log(fs.readFileSync("programming.txt",
"utf8"));

Appending to file fs.appendFile()
 fs.appendFile() method is used to asynchronously append the given data to a
file.
 A new file is created if it does not exist.
 Syntax
fs.appendFile( path, data[, options], callback)
 Parameters: This method accepts four parameters as mentioned above and
described below:
path: It is a String, Buffer, URL or number that denotes the source filename or file
descriptor that will be appended to.
data: It is a String or Buffer that denotes the data that has to be appended.
options: It is an string or an object that can be used to specify optional
parameters that will affect the output. It has three optional parameters:
 encoding: It is a string which specifies the encoding of the file. The default
value is ‘utf8’.
 mode: It is an integer which specifies the file mode. The default value is
‘0o666’.
 flag: It is a string which specifies the flag used while appending to the file. The
default value is ‘a’.
callback: It is a function that would be called when the method is executed.

Appending to file fs.appendFile()
const fs = require('fs');
// Get the file contents before the append operation
console.log("nFile Contents of file before append:",
var data=fs.readFileSync(“programming.txt”,{})
fs.readFileSync("example_file.txt", "utf8"));
fs.appendFile("example_file.txt", data, (err) => {
if (err) {
console.log(err);
}
else {
// Get the file contents after the append operation
console.log("nFile Contents of file after append:“),
console.log(fs.readFileSync("example_file.txt", "utf8"));
}
});

Var fs=require(‘fs’)
 Open
 fs.open()
 Write
 fs.writeFileSync()
 fs.writeFile()
 Reading
 fs.readFileSync()
 fs.readFile()
 Append
 fs.appenFile() //asyn