Working with JSON in Node.js

JSON is the standard data format. Node.js makes it easy.

Parse JSON String

const jsonString = '{"name": "Alice", "age": 25}';
const obj = JSON.parse(jsonString);

console.log(obj.name);  // Alice
console.log(obj.age);   // 25

Convert to JSON String

const obj = { name: 'Alice', age: 25 };
const jsonString = JSON.stringify(obj);

console.log(jsonString);  // {"name":"Alice","age":25}

// Pretty print
const pretty = JSON.stringify(obj, null, 2);
console.log(pretty);
// {
//   "name": "Alice",
//   "age": 25
// }

Read JSON File

const fs = require('fs').promises;

async function readJSON(filepath) {
  const data = await fs.readFile(filepath, 'utf8');
  return JSON.parse(data);
}

const config = await readJSON('config.json');
console.log(config);

Simpler with require (for static files):

const config = require('./config.json');
// Works but caches the file

Write JSON File

const fs = require('fs').promises;

async function writeJSON(filepath, data) {
  const json = JSON.stringify(data, null, 2);
  await fs.writeFile(filepath, json);
}

const users = [
  { id: 1, name: 'Alice' },
  { id: 2, name: 'Bob' }
];

await writeJSON('users.json', users);

Update JSON File

async function updateJSON(filepath, updateFn) {
  const data = await readJSON(filepath);
  const updated = updateFn(data);
  await writeJSON(filepath, updated);
}

// Add a user
await updateJSON('users.json', users => {
  users.push({ id: 3, name: 'Charlie' });
  return users;
});

Handle Errors

function safeJsonParse(str) {
  try {
    return JSON.parse(str);
  } catch (err) {
    console.error('Invalid JSON:', err.message);
    return null;
  }
}

const data = safeJsonParse('invalid json');  // null
const valid = safeJsonParse('{"ok": true}'); // {ok: true}

Working with API Responses

const https = require('https');

function fetchJSON(url) {
  return new Promise((resolve, reject) => {
    https.get(url, (res) => {
      let data = '';
      res.on('data', chunk => data += chunk);
      res.on('end', () => resolve(JSON.parse(data)));
    }).on('error', reject);
  });
}

const user = await fetchJSON('https://api.github.com/users/octocat');
console.log(user.name);

JSON with Dates

const obj = {
  name: 'Event',
  date: new Date()
};

const json = JSON.stringify(obj);
// {"name":"Event","date":"2024-01-15T10:30:00.000Z"}

// Parse with date conversion
const parsed = JSON.parse(json, (key, value) => {
  if (key === 'date') return new Date(value);
  return value;
});

Practical: Simple JSON Database

class JsonDB {
  constructor(filepath) {
    this.filepath = filepath;
  }
  
  async read() {
    try {
      const data = await fs.readFile(this.filepath, 'utf8');
      return JSON.parse(data);
    } catch {
      return [];
    }
  }
  
  async write(data) {
    await fs.writeFile(this.filepath, JSON.stringify(data, null, 2));
  }
  
  async add(item) {
    const data = await this.read();
    data.push(item);
    await this.write(data);
  }
}

const db = new JsonDB('data.json');
await db.add({ id: 1, name: 'Item 1' });

Key Takeaway

Use JSON.parse() to read JSON strings, JSON.stringify() to create them. For files, combine with fs module. Always handle parse errors and use pretty printing for readable files.