Node.js – util.promisify() Method

The util.promisify() method basically takes a function as an input that follows the common Node.js callback style, i.e., with a (err, value) and returns a version of the same that returns a promise instead of a callback.

Syntax

util.promisify(original)

Parameters

It takes only one parameter −

• original −This parameter takes an input for the function and returns them as a promise.

Example 1

Create a file with the name "promisify.js" and copy the following code snippet. After creating the file, use the command "node promisify.js" to run this code.

// util.promisify() Demo example

// Importing the fs and util modules
const fs = require('fs');
const util = require('util');

// Reading the file using a promise & printing its text
const readFile = util.promisify(fs.readFile);
readFile('./promisify.js', 'utf8') // Reading the same file
   .then((text) => {
      console.log(text);
   })

// Printing error if any
   .catch((err) => {
      console.log('Error', err);
});

In the above example, we have used the fs module to read the files. We have used util.promisify() method to convert the fs.readFile to a promise-based method. Now,the above method gives us a promise instead of a callback. Promise further has two methods "then" and "catch" that are used to get the function result.

Output

C:\home\node>> node promisify.js
// util.promisify() Demo example

// Importing the fs and util modules
const fs = require('fs');
const util = require('util');

// Reading the file using a promise & printing its text
const readFile = util.promisify(fs.readFile);
readFile('./util.js', 'utf8')
   .then((text) => {
      console.log(text);
   })

   // Printing error if any
   .catch((err) => {
      console.log('Error', err);
});

Example 2

Let’s have a look at one more example

// util.promisify() Demo example

// Importing the fs and util modules
const util = require('util')
const fs = require('fs')

// Changing from callback to promise based
const readdir = util.promisify(fs.readdir)

// Reading files
const readFiles = async (path) => {
   const files = await readdir(path)
   // Printing current working directory
   console.log(files) //Print all files on current working directory
   console.log(process.cwd())
}

readFiles(process.cwd()).catch(err => {
      console.log(err)
})

Output

C:\home\node>> node promisify.js
[    'app.js',
   'index.html',
   'node_modules',
   'package-lock.json',
   'package.json',
   'promisify.js',
C:\home\node