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Working with JSON Files

JSON (JavaScript Object Notation) is the most common format for storing and exchanging data in Node.js applications. Configuration files, database exports, API responses – they all use JSON. Let's learn how to work with JSON files effectively.

What is JSON?

JSON is a lightweight data format that's easy for humans to read and write, and easy for machines to parse and generate. It looks exactly like JavaScript objects.
{
  "name": "John Doe",
  "age": 30,
  "email": "john@example.com",
  "hobbies": ["reading", "coding", "gaming"],
  "address": {
    "city": "New York",
    "zip": 10001
  }
}

Think of JSON as a universal language that JavaScript (and many other languages) understand perfectly. It's like having a conversation between your code and files.

Reading JSON Files

Method 1: Using fs.readFile + JSON.parse
const fs = require('fs');

fs.readFile('config.json', 'utf8', (err, data) => {
  if (err) {
    console.error('Error reading file:', err);
    return;
  }
  
  try {
    const config = JSON.parse(data);
    console.log('Server port:', config.port);
    console.log('Database:', config.database);
  } catch (parseErr) {
    console.error('Invalid JSON format:', parseErr);
  }
});

Method 2: Using require() (Simplest for JSON)

Node.js can directly require JSON files – it automatically parses them!
<!-- This works for JSON files! -->
const config = require('./config.json');

console.log('Server port:', config.port);

<!-- Note: require() caches the file, so changes won't be seen until restart -->

Method 3: Using fs.promises with async/await
const fs = require('fs').promises;

async function loadConfig() {
  try {
    const data = await fs.readFile('config.json', 'utf8');
    const config = JSON.parse(data);
    console.log('Config loaded', config);
  } catch (err) {
    console.error('Failed to load config:', err);
  }
}

loadConfig();

Writing JSON Files

When writing JSON, you need to convert JavaScript objects to JSON strings using `JSON.stringify()`.
const fs = require('fs').promises;

const user = {
  name: 'Alice',
  age: 25,
  email: 'alice@example.com',
  skills: ['JavaScript', 'Node.js', 'React']
};

async function saveUser() {
  try {
    <!-- Convert to JSON string (pretty printed with 2 spaces) -->
    const jsonData = JSON.stringify(user, null, 2);
    
    await fs.writeFile('user.json', jsonData);
    console.log('User data saved!');
  } catch (err) {
    console.error('Error saving user:', err);
  }
}

saveUser();

JSON.stringify() Options

ParameterDescription
`value`The object to convert to JSON
`replacer`Function or array to filter properties (optional)
`space`Number of spaces for indentation (pretty print)

Using replacer to filter properties:
<!-- Only include specific properties -->
const json = JSON.stringify(user, ['name', 'email']);
<!-- {"name":"Alice","email":"alice@example.com"} -->

<!-- Using a function to transform values -->
const json2 = JSON.stringify(user, (key, value) => {
  if (key === 'password') return undefined; <!-- Exclude password -->
  return value;
});

Updating JSON Files (Read-Modify-Write Pattern)

Often you need to update existing JSON files – read, modify, write back.
const fs = require('fs').promises;

async function updateCounter() {
  try {
    <!-- 1. Read existing data -->
    const data = await fs.readFile('counter.json', 'utf8');
    const counter = JSON.parse(data);
    
    <!-- 2. Modify the data -->
    counter.visits = (counter.visits || 0) + 1;
    counter.lastVisit = new Date().toISOString();
    
    <!-- 3. Write back -->
    await fs.writeFile('counter.json', JSON.stringify(counter, null, 2));
    
    console.log('Visits updated:', counter.visits);
  } catch (err) {
    console.error('Error:', err);
  }
}

updateCounter();

Practical Example: Simple Database with JSON

Create a simple key-value store using a JSON file.
const fs = require('fs').promises;

class JsonDB {
  constructor(filePath) {
    this.filePath = filePath;
  }
  
  async init() {
    try {
      await fs.access(this.filePath);
    } catch {
      <!-- File doesn't exist, create with empty object -->
      await fs.writeFile(this.filePath, '{}');
    }
  }
  
  async get(key) {
    const data = await fs.readFile(this.filePath, 'utf8');
    const obj = JSON.parse(data);
    return obj[key];
  }
  
  async set(key, value) {
    const data = await fs.readFile(this.filePath, 'utf8');
    const obj = JSON.parse(data);
    obj[key] = value;
    await fs.writeFile(this.filePath, JSON.stringify(obj, null, 2));
    return true;
  }
  
  async delete(key) {
    const data = await fs.readFile(this.filePath, 'utf8');
    const obj = JSON.parse(data);
    delete obj[key];
    await fs.writeFile(this.filePath, JSON.stringify(obj, null, 2));
    return true;
  }
}

async function example() {
  const db = new JsonDB('mydb.json');
  await db.init();
  
  await db.set('user1', { name: 'John', age: 30 });
  await db.set('user2', { name: 'Jane', age: 25 });
  
  const user1 = await db.get('user1');
  console.log('User1:', user1);
}

example();

Two Minute Drill

  • JSON is the standard data format for Node.js applications.
  • Use `JSON.parse()` to convert JSON string to JavaScript object.
  • Use `JSON.stringify()` to convert object to JSON string.
  • You can `require()` JSON files directly – Node.js parses them automatically.
  • For updates, use read-modify-write pattern.
  • Always handle JSON parsing errors – malformed JSON will throw an exception.
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