XML vs JSON in modern APIs
If you've been writing REST APIs for the past decade, you might wonder why XML still exists. The short answer: it has a decade-long head start. The SOAP era (roughly 2000–2012) built enormous amounts of enterprise infrastructure on XML. Web services, ERP systems, banking APIs, healthcare data exchanges, and countless internal integrations were all built when XML was the only serious option for structured data interchange.
JSON emerged as the dominant API format in the REST era for good reasons: it's a strict subset of JavaScript object literal syntax (sort of — there are minor differences), it's more compact, and its type system maps directly to JavaScript primitives. When Node.js and browser-heavy applications became the dominant API consumers, JSON won by default.
But XML never disappeared from enterprise software. SAP, Oracle EBS, Salesforce's SOAP API, and most healthcare interoperability standards (HL7 FHIR allows both, but HL7 v2/v3 are XML-only) still use XML. If you're integrating with enterprise systems, you will encounter XML regardless of what your own stack uses.
When XML is still required
Several modern and widely-used formats are XML-based by definition:
RSS and Atom feeds — both are XML vocabularies. Every podcast feed is XML (the Podcasting 2.0 namespace adds new XML elements). If you're building a blog or podcast platform, you're generating XML.
SVG — Scalable Vector Graphics is XML. SVG files are XML documents and can be embedded directly in HTML. Manipulating SVG programmatically means working with XML elements and attributes.
XHTML — HTML5 has an XML serialization (XHTML5) and the older XHTML 1.0/1.1 specs are still encountered in legacy systems.
Office Open XML — .docx, .xlsx, and .pptx files are ZIP archives containing XML. If you're generating Office documents programmatically, you're generating XML.
SAML — the Security Assertion Markup Language used for enterprise single sign-on is XML-based. If you're building SAML SP or IdP support, XML is unavoidable.
Namespace handling gotchas
XML namespaces are the feature that trips up most developers coming from JSON. In JSON, every key in an object is just a string — there's no concept of namespacing. In XML, namespaces allow elements from different vocabularies to coexist in the same document without name collisions.
<?xml version="1.0" encoding="UTF-8"?>
<soap:Envelope
xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<soap:Body>
<GetUserRequest xmlns="https://example.com/api">
<UserId>12345</UserId>
</GetUserRequest>
</soap:Body>
</soap:Envelope>The soap: prefix is bound to the SOAP envelope namespace URI. The parser uses the URI (not the prefix) as the actual namespace identifier — two documents using different prefixes for the same URI refer to the same namespace. This is why you can't simply convert namespaced XML to JSON by treating prefixed element names as key names — the colon is not a valid JSON key character, and the prefix binding is context-dependent.
For simple XML-to-JSON conversion without namespaces, this tool works perfectly. For SOAP or heavily namespaced XML, use a library like fast-xml-parser (Node.js) or Python's lxml which understands namespace resolution.
The verbosity cost of XML
XML's requirement to close every element means that the same data takes significantly more bytes than JSON. A simple comparison: a JSON object {"name":"Alice","age":30} is 24 bytes. The equivalent XML is <root><name>Alice</name><age>30</age></root> — 47 bytes, nearly double.
For SOAP API payloads with extensive namespace declarations and envelope wrapping, the overhead is even larger. This is why REST/JSON displaced SOAP/XML for public APIs — lower bandwidth, faster parsing, easier debugging. However, XML's verbosity also makes it somewhat self-documenting: element names appear twice (open and close tag), making structure visible even without syntax highlighting.
For high-throughput internal APIs where bandwidth matters, neither XML nor JSON is the right choice — binary formats like Protocol Buffers, Avro, or MessagePack are 3–10x more compact and significantly faster to serialize and parse.
Parsing XML in production
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Malformed XML from a third-party API throws a parse error and silently swallows the request. A missing closing tag, a stray ampersand, an unexpected namespace declaration — these show up at 3am as "integration stopped working." Sentry captures the raw payload alongside the stack trace so you can reproduce the exact failure, not just the symptom.
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Frequently Asked Questions
How does the converter map JSON arrays to XML?
JSON arrays are converted by repeating the parent element tag for each item. For example, a JSON array like {"tags": ["admin", "editor"]} becomes two sibling <tags> elements. If the top-level input is a JSON array, each element becomes an <item> element nested inside an <items> root. This is the most common and widely-understood XML array convention.
What happens to JSON null values?
JSON null values are represented as self-closing XML elements: <fieldName />. This is equivalent to an empty element and is distinguishable from an element with an empty string value. When converting back from XML to JSON, self-closing elements without text content map back to empty strings rather than null — XML has no native null concept, so this round-trip is lossy for null values.
How are special characters like < > & handled?
The converter escapes all XML special characters in element text content and attribute values using the standard XML entity references: & becomes &, < becomes <, > becomes >, double-quote becomes ", and single-quote becomes '. This ensures the output is always well-formed XML that any parser will accept.
What if my JSON keys contain spaces or special characters?
XML element names cannot contain spaces or most special characters. This converter sanitises invalid characters by replacing them with underscores. Keys that start with a digit get a leading underscore prefix (XML names cannot start with digits either). For example, a JSON key '2fa-enabled' becomes '_2fa-enabled' in XML. If you need specific tag names, rename the JSON keys before converting.
Does this converter support XML namespaces?
No. Namespace handling (xmlns declarations, prefixed elements like soap:Body) requires understanding the intended namespace structure, which can't be inferred from plain JSON. If you need namespaced XML output — for example, for a SOAP endpoint or an RSS/Atom feed — you'll need to post-process the output or use a dedicated library that understands your target schema.