Node.js — Write a JSON Object to a File

When working on a new feature or app idea, storing data on the file system can be a good solution. You can skip the database setup and save JSON to a file instead.

Node.js Series Overview

Write JSON to File

JavaScript comes with the JSON class that lets you serialize an object to JSON with JSON.stringify. The file system fs module then writes data to the disk.

In the following, you’ll see examples for writing JSON to a file using callbacks and promises. Be careful with synchronous file operations in Node.js. The synchronous methods block the Node.js event loop and everything else has to wait for the file read/write.

With Callbacks

The first thing is to serialize data to JSON. Notice that JSON.stringify(data, null, 2) will add line breaks and indentation keeping the JSON human readable:

const Fs = require('fs')

function writeToFile (data, path) {  
  const json = JSON.stringify(data, null, 2)

  Fs.writeFile(path, json, (err) => {
    if (err) {
      console.error(err)
      throw err
    }

    console.log('Saved data to file.')
  })
}

The callback gives you a single error parameter. Add error handling for cases where the file writing process went wrong.

With Promises

You have multiple ways to use promises file system methods in Node.js:

The following example uses the fs-extra package providing the Node.js file system API with full promise support and methods like copy(), remove(), mkdirs().

const Fs = require('fs-extra')

async function writeToFile (path, data) {  
  const json = JSON.stringify(data, null, 2)

  try {
    await Fs.writeFile(path, json)
    console.log('Saved data to file.')
  } catch (error) {
    console.error(error)
  }
}

That’s it!

Read JSON From File

Retrieve data from a file and parse it to the JavaScript representation. Again, the examples will show you asynchronous methods using callbacks and promises and not the synchronous operations.

With Callbacks

Use fs.readFile to receive a file’s content. If you don’t specify the encoding, Node.js provides the data as a buffer.

The following example uses the utf8 encoding to retrieve the JSON string. Deserialize the JSON content to JavaScript with JSON.parse:

const Fs = require('fs')

function readFromFile (path) {  
  Fs.readFile(path, 'utf8', (err, json) => {
    if (err) {
      console.error(err)
      throw err
    }

    const data = JSON.parse(json)
    console.log(data)
  })
}

Remember to convert the JSON string to JavaScript. The formatted JSON strings and JavaScript objects look similar. Ensure you’re working with JavaScript and not JSON.

With Promises

Here’s the file read using promises with the help of fs-extra:

const Fs = require('fs-extra')

async function dataFromFile (path) {  
  try {
    const json = await Fs.readFile(path, 'utf8')
    const content = JSON.parse(json)
    console.log(content)
  } catch (error) {
    console.log(error)
  }
}

Again, parse the JSON content to JavaScript before moving on with other operations.

Enjoy coding & make it rock!


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