What JSON and XML actually are
JSON (JavaScript Object Notation) is a lightweight, text-based format for representing structured data as key-value pairs and arrays, formalized in the early 2000s and now the default format for most modern web APIs. XML (eXtensible Markup Language) is an older, markup-based format standardized by the W3C in the late 1990s, representing data with nested opening and closing tags similar to HTML. XML came first and still underpins a huge amount of existing enterprise software, document formats, and legacy integrations; JSON came later and won out for most new web development because it maps more directly onto how JavaScript — and, by extension, most web applications — actually represents data in memory.
Syntax, side by side
The same piece of data looks noticeably different in each format. A person's name and age in JSON looks like {"name": "Ada", "age": 34} — compact key-value pairs inside curly braces. The same data in XML looks like <person><name>Ada</name><age>34</age></person> — each value wrapped in its own opening and closing tag. JSON's syntax is generally shorter and closer to how a programmer already thinks about an object; XML's tag-based syntax is more verbose but supports things JSON doesn't have a native equivalent for, like attributes on an element, inline comments, and mixed content that combines text and markup in the same node.
Readability and file size
Because JSON skips closing tags and extra markup, it's typically 30–50% smaller than an equivalent XML document representing the same data — a meaningful difference for anything sent repeatedly over a network, like API responses. JSON is also generally considered easier to read at a glance for typical key-value data, while XML's verbosity can make deeply nested documents harder to scan by eye, even though its explicit open/close tag structure makes the document's hierarchy unambiguous in a way that can help when the structure itself is complex or deeply nested.
Parsing and validation
JSON parses natively into a JavaScript object with a single built-in function call, which is a big part of why it became the default for web and mobile development — there's essentially no translation step between the data format and the code using it. XML requires a dedicated parser (typically a DOM or SAX parser) and is usually handled through APIs like XPath or XQuery, which is more overhead but also more mature — XML's validation tooling, particularly XML Schema (XSD) and DTDs, has been battle-tested for decades in industries like finance and healthcare that require strict, formally verifiable document structures. JSON has a comparable tool in JSON Schema, but it's a newer, less universally adopted standard by comparison.
When XML still wins
XML remains the right choice, or simply the required one, in several specific areas: document formats like Microsoft's .docx and .xlsx files and SVG images are XML under the hood, RSS and Atom feeds are XML-based, SOAP-based enterprise web services still rely on it, and industries with strict electronic data interchange (EDI) or regulatory schema requirements often mandate XML specifically for its mature validation tooling. XML's support for XSLT — a language for transforming one XML document into another format entirely — also has no direct JSON equivalent, which matters for document-processing pipelines that need that kind of transformation.
When JSON wins
JSON is the practical default for REST APIs, mobile app data exchange, and most modern configuration files — package.json and countless other tooling config files are JSON specifically because it's compact and parses natively in JavaScript environments. Most NoSQL databases, including MongoDB, store data internally in BSON, a binary variant of JSON, reinforcing just how central the format has become to modern application architecture. If you're starting a new web project today with no legacy constraints pulling you toward XML, JSON is very likely the right default.
Working with JSON day to day
Whichever format a project uses, working with real JSON payloads day to day usually means formatting a minified API response to actually read it, and validating a payload's syntax when something isn't parsing correctly. Both of those are common enough tasks that having a formatter and validator on hand saves real time over debugging JSON syntax by eye.
Format or validate your JSON
Migrating between the two
It's fairly common to inherit a system built on one format and need to work with the other, and the conversion is usually mechanical rather than conceptually hard. Converting XML to JSON typically maps each tag to a key and its text content to a value, though anything relying on XML-specific features — attributes, mixed text-and-markup content, or namespaces — needs a deliberate decision about how to represent it in JSON's simpler structure, since there's no exact one-to-one equivalent. Converting JSON to XML is usually more straightforward in the other direction, since JSON's simpler key-value model maps cleanly onto basic XML tags without needing to invent anything.
If you're maintaining an older system that speaks XML while building new features that expect JSON, it's worth deciding early on whether to convert at the boundary between the two systems just once, or to support both formats long-term — the former is usually simpler to maintain unless there's a specific reason both need to stay first-class.