JSON, XML, and CSV all solve the same basic problem — representing structured data as text — but they make very different tradeoffs about structure, size, and readability. Picking the wrong one isn't usually fatal, but it does mean fighting the format for the rest of a project's life. Here's what each one is actually good at.
JSON: the default for APIs and config
JSON (JavaScript Object Notation) represents data as nested objects, arrays, strings, numbers, booleans, and null — a shape that maps directly onto how most programming languages already model data in memory. That's its biggest advantage: parsing JSON into a native object graph (or serializing one back to JSON) is close to a no-op in every mainstream language, with no schema or code generation step required.
JSON is the right choice when: the data is naturally hierarchical (nested objects, arrays of objects), it's going over an HTTP API, or it's a config file for a JavaScript/Node-based tool. It's the wrong choice when you need comments (JSON has none, by design), mixed content (text interspersed with markup, the way HTML mixes prose and tags), or a large, flat table of rows — representing a spreadsheet as JSON means either an array of objects with the same keys repeated on every row (verbose) or splitting into parallel arrays (awkward).
XML: verbose, but built for documents and validation
XML predates JSON and looks it — every value is wrapped in an opening and closing tag, <price currency="USD">19.99</price> instead of {"price": 19.99, "currency": "USD"} — which makes it noticeably more verbose. What XML has that JSON doesn't: attributes (metadata on an element, like currency above, that isn't quite "data" itself), a mature schema/validation ecosystem (XSD, DTD) for strictly enforcing document structure, namespaces for mixing vocabularies from different sources in one document, and native support for mixed content — a paragraph of text with inline markup, which is exactly what XML's close relative, HTML, is built for.
XML is the right choice when: you're integrating with an older enterprise system, a SOAP API, or a document format that already standardized on it (RSS feeds, SVG, many config formats in the Java/.NET world), or when strict schema validation genuinely matters. For a new project with no such constraint, JSON is almost always simpler.
CSV: flat tables, nothing else
CSV (comma-separated values) is the simplest of the three: one line per row, one column per field, separated by commas. There's no nesting, no types (everything is text until something else parses it), and — despite the name — no single agreed-upon standard for quoting fields that contain a comma, a newline, or a quote character themselves (RFC 4180 documents the most common convention, but real-world CSV varies).
CSV is the right choice when: the data is genuinely flat and tabular — a spreadsheet export, a bulk import into a database, a report a non-technical stakeholder will open in Excel. It's the wrong choice the moment the data has any real nesting (an order with a variable-length list of line items doesn't fit a single flat row without inventing a workaround), since forcing hierarchical data into CSV usually means denormalizing it into repeated, redundant rows.
Quick comparison
- Nested/hierarchical data: JSON or XML — never CSV.
- Flat tabular data, spreadsheet-friendly: CSV — JSON/XML work but add unneeded overhead.
- Needs schema validation or namespaces: XML.
- Needs comments in the file: XML (JSON and CSV have no comment syntax at all).
- Smallest size on the wire: JSON, usually — CSV can be smaller for pure tabular data, XML is almost always the largest due to closing tags.
- Easiest to hand-edit: JSON for structured data, CSV for simple tables.
Try it yourself
Validate and explore a JSON document with our JSON Formatter, or format and validate XML with the XML Formatter. Moving between formats? Use JSON ↔ XML Converter or JSON ↔ CSV Converter to convert a sample instantly — all client-side, nothing you paste in leaves your browser.