How to Update Data in JSON File using JavaScript ?

JSON stands for JavaScript Object Notation. It is a text-based data exchange format that allows you to transfer data between different languages and platforms. JavaScript is commonly used to interact with JSON files. Updating data in a JSON file involves reading the file and then making the necessary changes to the JSON object and then writing the updated data back to the file.

Using require() Method

We are going to use a Direct Approach, In this approach we can directly import the json data file and update like we normally update the JavaScript object. In last to update the changes in a Json file that we have made, we need to use a writeFileSync() function of a File System Module to update the changes.

Syntax:

const fs = require('fs');

// Read the JSON file
const jsonData = require('./data.json');

// Update the data
jsonData.key = 'new value';

// Write the updated data back to the JSON file
fs.writeFileSync('./data.json', JSON.stringify(jsonData, null, 2));

console.log('Data updated successfully.');

Before Updating data.json file:

{
  "key": "old value"
}

After Updating data.json file:

{
  "key": "new value"
}

Steps to create Node.js Application:

Step 1: Create a a folder for your application and move to that folder using below command.

cd foldername

Step 2: Create a NodeJS application using the following command:

npm init

Example : The below example demonstrate updating data in json file using require() method

JavaScript

//File path: /index.js
const fs = require('fs');

// Read the JSON file
const jsonDataBefore = require('./data.json');

// Print JSON data before updating
console.log('Before updating JSON:');
console.log(JSON.stringify(jsonDataBefore, null, 2));

// Update the data
jsonDataBefore[0].programmingLanguage.push("JavaScript");
jsonDataBefore[1].programmingLanguage.push("JavaScript");

// Write the updated data back to the JSON file
fs.writeFileSync('./data.json', JSON.stringify(jsonDataBefore, null, 2));

// Print JSON data after updating
console.log('\nAfter updating JSON:');
console.log(JSON.stringify(jsonDataBefore));

console.log('\nData updated successfully.');

JavaScript

//File path: data.json (note: remove this comment line)

[
    { "name": "John", "programmingLanguage": ["Python"] },
    { "name": "Alice", "programmingLanguage": ["C++"] }
]

To run the application use the following command:

node index.js 

Output:

Before updating JSON:
[
  {
    "name": "John",
    "programmingLanguage": [
      "Python"
    ]
  },
  {
    "name": "Alice",
    "programmingLanguage": [
      "C++"
    ]
  }
]

After updating JSON:
[
  {
    "name": "John",
    "programmingLanguage": [
      "Python",
      "JavaScript"
    ]
  },
  {
    "name": "Alice",
    "programmingLanguage": [
      "C++",
      "JavaScript"
    ]
  }
]

Data updated successfully.

Using File System Module

We are going to use a File System Module ("fs"). File System Module provides a different different functions to read and write the data to the file. We can use fs.readFileSync(file_name) or fs.readFile(file_name) function to read the content of the data.json file.

Once the data is read from the file. we will parse it into a JavaScript object using JSON.parse() method. JSON.parse() method will converts the JSON string into a JavaScript object. After parsing the JSON data, we can manipulate the JavaScript object like any other object.

Once the data is updated, we will convert the JavaScript object back to a JSON string using JSON.stringify() method. After that we will use fs.writeFileSync() or fs.writeFile() function to write the updated JSON data back to the file.

Note: fs.readFile(file_name) and fs.writeFile() are asynchronous, so we have to handle it using callback functions, promises or the async/await syntax.

Syntax:

const fs = require('fs');

// Read the JSON file
let rawData = fs.readFileSync('data.json');
let jsonData = JSON.parse(rawData);

// Update the data
jsonData.key = 'new value';

// Write the updated data back to the file
fs.writeFileSync('data.json', JSON.stringify(jsonData));

Before Updating data.json file:

{
  "key": "old value"
}

After Updating data.json file:

{
  "key": "new value"
}

Example : The below example demonstrate updating data in json file using file system module

JavaScript

//File path: /index.js

const fs = require('fs');

// Define the file path
const filePath = 'data.json';

// Read the JSON file
fs.readFile(filePath, 'utf8', (err, data) => {
    if (err) {
        console.error('Error reading file:', err);
        return;
    }
    
    try {
        // Parse the JSON data
        const jsonData = JSON.parse(data);

        // Display the original data
        console.log('Before updating JSON:');
        console.log(jsonData);

        // Update the data
        jsonData.forEach(item => {
            item.programmingLanguage.push('JavaScript');
        });

        // Display the updated data
        console.log('\nAfter updating JSON:');
        console.log(jsonData);

        // Write the updated data back to the file
        fs.writeFile(filePath, JSON.stringify(jsonData), (err) => {
            if (err) {
                console.error('Error writing file:', err);
                return;
            }
            console.log('\nData updated successfully.');
        });
    } catch (error) {
        console.error('Error parsing JSON data:', error);
    }
});

JavaScript

//File path: data.json (note: remove this comment line)

[
    { "name": "John", "programmingLanguage": ["Python"] },
    { "name": "Alice", "programmingLanguage": ["C++"] }
]

To run the application use the following command:

node index.js 

Output:

Before updating JSON:
[
  { name: 'John', programmingLanguage: [ 'Python' ] },
  { name: 'Alice', programmingLanguage: [ 'C++' ] }
]

After updating JSON:
[
  { name: 'John', programmingLanguage: [ 'Python', 'JavaScript' ] },
  { name: 'Alice', programmingLanguage: [ 'C++', 'JavaScript' ] }
]

Data updated successfully.