Developer Guide
JSON vs XML
JSON vs XML for APIs: syntax, size, validation, namespaces, and when XML still wins. A practical comparison, not a slogan.
JSON vs XML is the wrong question if you treat it as a loyalty test. Both are text formats for structured data. JSON won the web API default because it maps to objects and arrays with almost no ceremony. XML still wins when you need documents, namespaces, or a published schema that many vendors implement. This comparison covers syntax, size, validation, and the cases where you should still pick XML.
Syntax and size
JSON uses braces, brackets, and quoted keys. XML uses matching tags and optional attributes. The same record — id, name, tags — is usually smaller in JSON because you do not repeat the element name on close. Verbose XML is not always worse: attributes can store metadata without a nested element, and mixed content (text plus child elements) is something JSON cannot express without a convention.
Parsing and language fit
In the browser, JSON.parse is native and fast. XML needs DOMParser or a library, and you still walk nodes. In Java and .NET, XML tooling is mature (JAXB, XSD). In JavaScript-heavy stacks, JSON is the path of least resistance. Performance differences for typical API payloads (a few kilobytes) are rarely the deciding factor; schema and ecosystem are.
Validation and schemas
XML has XSD and RELAX NG: you can require elements, types, and enumerations and reject a document before business logic runs. JSON has JSON Schema, which is widely supported but not as uniformly implemented as XSD in enterprise stacks. If your integration is a legal or industry document (invoices, healthcare, publishing), XML plus XSD is still common. If your integration is a REST JSON API, JSON Schema or OpenAPI is the usual pair.
Namespaces, attributes, and documents
XML namespaces let two vocabularies share a document without key collisions. JSON has no built-in namespace mechanism; you invent prefixes or nest objects. XML also distinguishes attributes from child elements and allows mixed content. Those features matter for documents (DocBook, office formats, sitemaps) and matter less for a list of users from an API.
When to choose JSON
Choose JSON for public REST or GraphQL APIs, mobile and SPA clients, config files, and logs (or JSONL). It is easier to generate, smaller on the wire, and first-class in JavaScript. Pair it with a JSON Formatter during development so invalid trailing commas never ship.
When to choose XML
Choose XML for SOAP, RSS/Atom, sitemaps, Maven/POM-style build files, and any partner who already publishes an XSD. Use an XML Viewer to inspect payloads and an XML Validator to catch broken nesting before you debug business rules. Converting XML to JSON is fine for display; do not assume a round-trip will preserve attributes and namespaces unless you define the mapping.
Summary
Default to JSON for new HTTP APIs. Keep XML when the document model or an existing schema requires it. Format JSON in the JSON Formatter and inspect XML in the XML Viewer instead of arguing about the format in the abstract.