How to operate callback based fs.writeFile() method with promises in Node.js ?

Last Updated : 18 Jul, 2020

The fs.writeFile() is a method defined in the File System module of Node.js. The File System module is basically to interact with the hard disk of the user’s computer. The fs.writeFile() method asynchronously writes data to a file, replacing the file if it already exists.

The fs.writeFile() method is based on callback. Using callback methods leads to a great chance of callback nesting or callback hell problems. Thus to avoid it we almost always like to work with a promise-based method. Using some extra node.js methods we can operate a callback-based method in promise way.

Syntax:

fs.writeFile(path, data, options)

Note: Callback not required since we operate the method with promises.

Parameters: Accepts three parameter path, data and options. The options is an optional parameter.

path: It is a String, Buffer or URL that specifies the path to the file where write operation is to be done.
data: It is string a String, Buffer or Uint8Array instance. It is the data which will write to the file.
options: It is an optional parameter that affects the output in someway accordingly we provide it to the function call or not.
- encoding: It is a string that specifies the encoding technique, 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 that specifies the file system flags. Default value is ‘w’.

Approach: The fs.writeFile() method based on callback. To operate it with promises, first, we use promisify() method defined in the utilities module to convert it into a promise based method.

Example 1: Filename: index.js

   // Importing File System and Utilities module 
const fs = require('fs') 
const util = require('util') 
  
// Convert callback based methods to  
// promise based methods 
const writeFile = util.promisify(fs.writeFile) 
const readFile = util.promisify(fs.readFile) 
  
// The readFile method read the contents of 
// the file and returns the buffer form of 
// the data 
readFile('./testFile.txt') 
.then(buff => { 
  const oldContent = buff.toString() 
  console.log(`\nOld content of the file  
                     :\n${oldContent}`) 
  
  // The writeFile method write to the file 
  // If is already exist, replaces the file 
  return writeFile('./testFile.txt',  
        "Hey, I am newly added!") 
}) 
  
.then(() => { 
  
  // Fetching contents of the file 
  // after write operation 
  return readFile('./testFile.txt') 
}) 
  
.then(buff => { 
  const newContent = buff.toString() 
  console.log(`\nNew content of the  
            file :\n${newContent}`) 
}) 
  
// If promise get rejected 
.catch(err => { 
   console.log(`\nError Occurs,  
          Error code -> ${err.code},  
   Error NO -> ${err.errno}`) 
}) 
 
  

Implementing the same functionality using async-await.

   // Importing File System and Utilities module 
const fs = require('fs') 
const util = require('util') 
  
// Convert callback based methods to  
// promise based methods 
const writeFile = util.promisify(fs.writeFile) 
const readFile = util.promisify(fs.readFile) 
  
const writeFileContent = async (path, data) => { 
  
  // The readFile method read the contents 
  // of the file and returns the buffer  
  // form of the data 
  const oldBuff = await readFile(path) 
  const oldContent = oldBuff.toString() 
  console.log(`\nOld content of the  
             file :\n${oldContent}`) 
  
  // The writeFile method write to the file 
  // If is already exist, replaces the file 
  await writeFile(path, data) 
  
  // Fetching contentsof the file after  
  // write operation 
  const newBuff = await readFile(path) 
    
  const newContent = newBuff.toString() 
  console.log(`\nNew content of the  
             file :\n${newContent}`) 
} 
   
writeFileContent('./testFile.txt',  
          "Hey, I am newly added!") 
  
// If promise get rejected 
.catch(err => { 
  console.log(`\nError Occurs,  
    Error code -> ${err.code}, 
    Error NO -> ${err.errno}`) 
}) 
 
  

Run the index.js file using the following command:

node index.js

Output:

Example 2: When given path to the file does not exist.
Filename: index.js

   // Importing File System and Utilities module 
const fs = require('fs') 
const util = require('util') 
  
// convert callback based methods to  
// promise based methods 
const writeFile = util.promisify(fs.writeFile) 
const readFile = util.promisify(fs.readFile) 
  
// The writeFile method write to the file if 
// the file does not exist, creates the file 
// and then write to the file 
writeFile('./testFile.txt',  
"Hey there, I am newly added content of newly added file!") 
.then(() => { 
  
  // The readFile method read the contents of the file 
  // and returns the buffer form of the data 
  
  // Fetching contents of the file after write operation 
  return readFile('./testFile.txt') 
}) 
  
.then(buff => { 
  const content = buff.toString() 
  console.log(`\nContents of the file :\n${content}`) 
}) 
  
// If promise get rejected 
.catch(err => { 
  console.log(`\nError Occurs, Error code -> ${err.code}, 
  Error NO -> ${err.errno}`); 
}) 
 
  

Implementing the same functionality using async-await.

   // Importing File System and Utilities module 
const fs = require('fs') 
const util = require('util') 
  
// Convert callback based methods to  
// promise based methods 
const writeFile = util.promisify(fs.writeFile) 
const readFile = util.promisify(fs.readFile) 
  
const writeFileContent = async (path, data) => { 
  
  // The writeFile method write to the file 
  // If the file does not exist, creates the 
  // file and then write to the file 
  writeFile(path, data) 
  
  // The readFile method read the contents of 
  // the file and returns the buffer form of 
  // the data 
  const buff = await readFile(path) 
    
  const content = buff.toString() 
  console.log(`\nContents of the file :\n${content}`) 
} 
   
writeFileContent('./testFile.txt',  
"Hey there, I am newly added content of"
           + " newly added file!") 
  
// If promise get rejected 
.catch(err => { 
  console.log(`\nError Occurs,  
    Error code -> ${err.code}, 
    Error NO -> ${err.errno}`); 
}); 
 
  