JSON vs XML in REST API Design
Why REST APIs use JSON — verbosity, browser native support, and when XML is still appropriate.
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Tags: JSON vs XML REST API, REST API format choice, JSON API design
JSON vs XML in REST API Design The REST architecture says nothing about data format — Roy Fielding's original dissertation mentions neither JSON nor XML. The industry settled on JSON for REST not by mandate but by convergence around JavaScript-friendly tooling, smaller payloads, and simpler parsing. --- Why JSON became the REST default? Native JavaScript support When REST APIs started replacing SOAP in the browser era (2005–2012), reducing client-side parsing complexity was a major adoption driver. being built into every browser and Node.js runtime was decisive. Verbosity and bandwidth Same data, different sizes: JSON (87 bytes): XML (130 bytes): At millions of API calls per day, a 30-50% payload size difference has real bandwidth and CDN cost implications. Parse speed JSON parsing is…
Frequently Asked Questions
Why do most REST APIs use JSON?
JSON maps naturally to JavaScript objects, making browser clients trivial to write. It's also smaller than XML (30-50% less verbose for typical API payloads), parsed faster, and requires no library in modern JavaScript — JSON.parse() is built in. The REST ecosystem standardized on JSON around 2010 and client tooling followed.
When should a REST API use XML?
Use XML when: your consumers are enterprise systems that require SOAP or expect XML for compliance reasons; you need mixed content (text with embedded markup); you need namespace-qualified types across multiple schemas; or you are building on a platform (Android Manifest, Maven, Spring) where XML is the established convention.
What is content negotiation in HTTP?
Content negotiation lets a client tell the server what format it prefers via the Accept header. The server responds with that format (or the closest available), and signals the format used in Content-Type. For example, Accept: application/json requests JSON; Accept: application/xml requests XML. Servers that support both check the Accept header and respond accordingly.
How do I support both JSON and XML in an API?
Check the Accept header in your request handler. If the client sends Accept: application/xml, serialize the response as XML; otherwise default to JSON. In Express, use res.format({ 'application/json': () => res.json(data), 'application/xml': () => res.send(toXml(data)) }). Document both media types in your OpenAPI spec.
What is JSON:API specification?
JSON:API (jsonapi.org) is a specification for building APIs in JSON that standardizes request/response structure — resource objects, relationships, pagination, filtering, and error formats. It reduces API design bikeshedding by providing a comprehensive convention. Ember Data and other frameworks have built-in JSON:API support.
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