JSON vs YAML vs XML: Which Format Should You Use?

Every few months, someone on my team starts a debate about data formats. "Why are we using JSON here? YAML is so much cleaner." Or, "XML has schemas, we should use that instead."

The truth is, there's no universally best format. Each one exists because it solves a specific set of problems really well. Let me break down when each format actually makes sense.

The Quick Decision Matrix

| Feature | JSON | YAML | XML | |---------|------|------|-----| | Human readability | Good | Excellent | Poor | | Machine parsing speed | Fast | Moderate | Slow | | Comments support | No | Yes | Yes | | Schema validation | JSON Schema | Limited | XSD (powerful) | | Data types | Basic | Rich | String-based | | Whitespace sensitivity | No | Yes (careful!) | No | | File size | Small | Smallest | Large | | Browser native support | Yes | No | Partial | | Best for | APIs, configs | Configs, DevOps | Documents, enterprise |

JSON: The Web's Lingua Franca

JSON won the web. Every API speaks it, every browser parses it natively, and every language has built-in support. If you're building a REST API, the decision is already made for you.

Where JSON shines:

• API responses and requests
• Package manifests (package.json, composer.json)
• Data interchange between services
• Anywhere you need fast parsing

Where it hurts: no comments. You can't annotate your JSON config file to explain why a value is set the way it is. People work around this with "_comment" keys, but that's a hack.

YAML: The Human-Friendly Option

YAML was designed to be read by humans first. No curly braces, no quotation marks on most strings, and you can add comments anywhere. DevOps teams love it — Kubernetes, Docker Compose, GitHub Actions, Ansible — they all chose YAML.

But YAML has a trap: whitespace matters. One wrong indent and your entire config breaks. And the error messages? Often unhelpful. "Mapping values are not allowed here" doesn't tell you much when you have 200 lines of config.

Also, YAML's type coercion can surprise you. yes becomes true. 3.10 becomes 3.1. Norway's country code NO becomes false. These aren't bugs — they're features that bite you when you least expect it.

XML: The Enterprise Workhorse

XML gets a lot of hate these days, and some of it is deserved. It's verbose. A simple key-value pair in JSON is one line; in XML, it's three.

But XML does things the others can't. XSD schemas provide powerful validation — you can enforce data types, patterns, required fields, even value ranges. XSLT lets you transform documents. Namespaces prevent naming collisions when combining documents from different sources.

If you're in healthcare (HL7), finance (FIXML), or publishing (DocBook), XML isn't going anywhere. These industries chose XML for its strictness, and that strictness is exactly what they need.

When to Use What

Choose JSON when:

• Building web APIs
• Storing simple configuration
• Exchanging data between frontend and backend
• You need fast parsing and broad language support

Choose YAML when:

• Writing CI/CD pipelines
• Kubernetes or Docker configuration
• Human-edited config files where comments help
• You need multi-line strings without escape characters

Choose XML when:

• Working with enterprise systems that require it
• You need powerful schema validation
• Documents with mixed content (text with embedded data)
• You need namespaces for combining document types

Converting Between Formats

Here's the practical reality: you'll often need to convert between formats. Maybe you're migrating an old XML config to YAML, or you need to transform YAML to JSON for an API.

The structure maps pretty cleanly between all three. Objects become maps become elements. Arrays become sequences become repeated elements. The tricky parts are attributes in XML (no direct equivalent in JSON or YAML) and comments (lost when converting to JSON).

You can also convert between XML and JSON or CSV and JSON when working with tabular data.

The Real Answer

Use what your ecosystem expects. If you're writing Kubernetes configs, use YAML. If you're building a web API, use JSON. If you're in enterprise healthcare, use XML.

Don't fight the conventions of your tools and team. The best data format is the one everyone on your project already understands.

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Data format debates are fun, but they rarely matter as much as we think. Pick the one that fits your use case, be consistent about it, and move on to solving actual problems. Your users don't care whether your config file uses curly braces or indentation.