Node.js fsPromises.appendFile() Function

The fsPromises.appendFile() method is used to asynchronously append the given data to a file. A new file is created if it does not exist. The options parameter can be used to modify the behavior of the operation. Syntax:

fsPromises.appendFile( path, data, options )

Parameters: This method accepts three 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 that 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 ‘0fso666’.
  • flag: It is a string which specifies the flag used while appending to the file. The default value is ‘a’.

Return Value: It returns the Promise.

Example: This example uses a sample txt file named "example_file" with Hello text. Filename: index.js javascript

// Node.js program to demonstrate the 
// fsPromises.appendFile() method 
   
// Import the filesystem module 
const fs = require('fs'); 
const fsPromises = fs.promises;
   
// Get the file contents before the append operation  
console.log("\nFile Contents of file before append:", 
fs.readFileSync("example_file.txt", "utf8")); 
   
fsPromises.appendFile("example_file.txt", "GeeksforGeeks")
.then(function(){
     console.log("\nFile Contents of file after append:", 
     fs.readFileSync("example_file.txt", "utf8"))
})
.catch( function (err) { console.log(err); });

Note: Run index.js file with below command:

node index.js

Output:

File Contents of file before append: Hello
File Contents of file after append: HelloGeeksforGeeks

Note: If options is a string, then it specifies the encoding. The path may be specified as a FileHandle that has been opened for appending (using fsPromises.open()).