XML vs JSON: Complete Comparison

XML and JSON are the two most popular data exchange formats. Both can represent structured data, but they have different strengths, weaknesses, and ideal use cases. Choosing the wrong format can lead to bloated file sizes, poor performance, or unnecessarily complex code.

This guide provides a complete comparison of XML and JSON. You'll learn the technical differences, performance implications, when to use each format, and how to migrate between them. By the end, you'll know exactly which format is right for your project.

XML vs JSON: Quick Comparison

Syntax Comparison: Same Data, Different Formats

<?xml version="1.0" encoding="UTF-8"?>
<user>
  <id>123</id>
  <name>John Doe</name>
  <email>[email protected]</email>
  <age>30</age>
  <active>true</active>
  <roles>
    <role>admin</role>
    <role>editor</role>
  </roles>
  <address>
    <street>123 Main St</street>
    <city>New York</city>
    <zip>10001</zip>
  </address>
</user>

<!-- Characters: ~350 -->

{
  "id": 123,
  "name": "John Doe",
  "email": "[email protected]",
  "age": 30,
  "active": true,
  "roles": ["admin", "editor"],
  "address": {
    "street": "123 Main St",
    "city": "New York",
    "zip": "10001"
  }
}



// Characters: ~200

<book id="123" format="hardcover">
  <title>The Great Gatsby</title>
  <author>F. Scott Fitzgerald</author>
  <price currency="USD">15.99</price>
</book>

<!-- Attributes: id, format, currency -->
<!-- Elements: title, author, price -->

{
  "id": 123,
  "format": "hardcover",
  "title": "The Great Gatsby",
  "author": "F. Scott Fitzgerald",
  "price": {
    "amount": 15.99,
    "currency": "USD"
  }
}

// Everything is a property

Performance Comparison

// JavaScript example
const jsonData = '{"users": [...]}';  // 10,000 users

console.time('JSON');
const obj = JSON.parse(jsonData);     // Native, very fast
console.timeEnd('JSON');              // ~50ms

const xmlData = '<users>...</users>';

console.time('XML');
const parser = new DOMParser();
const doc = parser.parseFromString(xmlData, 'text/xml');
console.timeEnd('XML');               // ~250ms (5x slower)

When to Use XML vs JSON

Migration Strategies

// JavaScript - xml2js library
const xml2js = require('xml2js');

const xmlData = `
<user>
  <id>123</id>
  <name>John Doe</name>
  <active>true</active>
</user>
`;

xml2js.parseString(xmlData, (err, result) => {
  console.log(JSON.stringify(result, null, 2));
});

// Output:
{
  "user": {
    "id": ["123"],           // Arrays by default!
    "name": ["John Doe"],
    "active": ["true"]       // Still strings!
  }
}

// JavaScript - json2xml library
const json2xml = require('json2xml');

const jsonData = {
  user: {
    id: 123,
    name: "John Doe",
    active: true,
    roles: ["admin", "editor"]
  }
};

const xml = json2xml(jsonData, { header: true });
console.log(xml);

// Output:
<?xml version="1.0" encoding="UTF-8"?>
<user>
  <id>123</id>
  <name>John Doe</name>
  <active>true</active>
  <roles>admin</roles>
  <roles>editor</roles>
</user>

// Express.js - Support both formats
app.get('/api/users/:id', (req, res) => {
  const user = getUser(req.params.id);
  
  // Check Accept header
  const acceptsXML = req.accepts('application/xml');
  const acceptsJSON = req.accepts('application/json');
  
  if (acceptsXML && !acceptsJSON) {
    // Client explicitly wants XML
    res.type('application/xml');
    res.send(json2xml({ user }));
  } else {
    // Default to JSON (modern standard)
    res.json({ user });
  }
});

Advantages & Disadvantages Summary

Related Tools & Resources

External References

Conclusion

Both XML and JSON have their place in modern development. JSON dominates web APIs and modern applications due to its simplicity, performance, and perfect JavaScript integration. XML excels in document-centric data, enterprise systems, and scenarios requiring rich metadata and namespaces.

For new projects, JSON is usually the right choice unless you specifically need XML features like attributes, namespaces, or XSLT transformations. If you're maintaining legacy XML systems, consider gradual migration to JSON for better performance and developer experience. The best format is the one that fits your specific use case - now you have the knowledge to make that decision confidently.