Readability and size
Compare the same data. JSON:
{"user": {"name": "Ada", "roles": ["admin", "editor"]}}XML:
<user><name>Ada</name><roles><role>admin</role><role>editor</role></roles></user>JSON is roughly half the size and maps directly onto the data structures every language already has (objects/dicts, arrays/lists). XML's opening and closing tags double the markup, and its text-node model fits documents better than data. For APIs and config, JSON's brevity means faster transfers and less visual noise.
Tooling and ecosystems
Every language ships a JSON parser in its standard library, and every HTTP client speaks it natively. XML parsing requires choosing between DOM, SAX, and pull parsers, each with sharp edges (entity expansion attacks, namespace handling). JSON won the tooling war so completely that new formats are usually described as "like JSON, but…".
XML's counterweight is maturity in specific domains: XSLT transforms, XPath queries, and decades of enterprise integration. If your pipeline already speaks XML fluently, that institutional knowledge has real value.
Schemas and validation
XML's strongest card is XSD: a rigorous, standardized schema language with decades of tooling. JSON Schema exists and works well, but adoption is patchier and validators vary. If you need contract-first design with strict validation — common in finance, healthcare, and government — XML's schema story is still more battle-tested.
XML also handles mixed content (text interleaved with markup, like <p>Hello <b>world</b></p>) naturally. JSON can represent documents, but it is awkward — which is why document formats (DOCX, SVG, HTML-adjacent tooling) remain XML-based.
The decision rule
- Choose JSON for web APIs, mobile apps, config files, and anything new. It is smaller, simpler, universally supported, and what developers expect. Validate with our JSON formatter.
- Choose XML when integrating with systems that demand it (SOAP services, Office documents, SVG, RSS/Atom feeds), when you need XSD-grade schema enforcement, or when documents mix text and markup.
- Never invent a third option without a strong reason — custom formats inherit none of either ecosystem's tooling.
Migrating from XML to JSON: practical tips
Moving an existing XML integration to JSON is usually worth it, but the translation has traps. Attributes vs. elements is the first: <user id="7"> has no direct JSON equivalent — conventions like {"user": {"@id": "7"}} or {"user": {"id": "7"}} both exist, so document your mapping. Repeated elements (<role> appearing three times) become arrays, but a single occurrence must still become a one-element array or consumers break.
Namespaces (soap:Envelope) have no JSON counterpart — drop them or encode the prefix into key names. Mixed content (text with inline markup) is the hardest: if your XML is really documents, JSON may be the wrong target and you should reconsider migrating at all.
Process advice: convert with a script, not by hand; validate output against a JSON Schema; run old and new formats in parallel during transition; and version the change — consumers need a migration window. For greenfield work the decision is already made: start with JSON, validate with our JSON formatter, and never look back.
One more migration trap: ordering assumptions. XML document order is significant and preserved; JSON object key order is technically insignificant, and some parsers reorder keys. If consumers depend on order, use JSON arrays of key/value pairs or document the contract explicitly. Arrays, unlike objects, guarantee order in every JSON implementation. Getting these mappings right up front saves painful debugging later, when subtle data-shape differences surface in production.
New to JSON itself? Start with JSON for beginners, then learn to squash the inevitable mistakes with common JSON errors, fixed.