The short answer
Use TOML for app configuration, YAML when a tool demands it (Kubernetes, CI pipelines), and JSON for data interchange between systems. If you only remember one sentence from this article, that's the one. Now here's why.
JSON: the data format cosplaying as config
JSON's strengths are universality and predictability — every language parses it, and there's exactly one way to write anything. Its weaknesses as a human-written config format are real: no comments, strict quoting, trailing commas are fatal, and deeply nested braces get hard to scan. JSON is the right choice when machines write and read the file (API payloads, package.json, tsconfig). For files humans edit daily, it fights you. Validate yours with our JSON Validator before you blame your app.
YAML: powerful, popular, and quietly dangerous
YAML won DevOps because it's readable — compare a Kubernetes manifest in YAML vs JSON and there's no contest. But YAML's flexibility is its trap: significant whitespace, the infamous Norway problem (NO parsing as boolean false), and multiple ways to write the same thing mean subtle bugs. My rule: use YAML where the ecosystem requires it, keep files small, and always validate. Converting between the two? Our JSON to YAML and YAML to JSON converters handle it in one paste.
TOML: the config format done right
TOML was designed from scratch for configuration: obvious syntax, native dates, comments, no significant whitespace, and a spec small enough to implement correctly. It's why Cargo, Poetry, and Hugo adopted it. Its weakness is verbosity for deeply nested data and weaker ecosystem support in some languages — but for app settings, it's the best-designed option of the three.
Quick comparison
- Comments: YAML ✓, TOML ✓, JSON ✗
- Trailing commas: all ✗ (JSON strictest about it)
- Human readability: TOML ≥ YAML > JSON
- Machine universality: JSON > YAML > TOML
- Footguns: YAML (whitespace, type coercion) > JSON (syntax strictness) > TOML (fewest)
Frequently Asked Questions
- Can I convert my JSON config to YAML or TOML?
- JSON to YAML is straightforward with our converter — the data model maps cleanly. TOML conversion is trickier because TOML's table structure doesn't map 1:1 to arbitrary JSON nesting; for complex configs, a manual rewrite is safer.
- Why do so many tools still use YAML despite its problems?
- Momentum and readability. Kubernetes, Ansible, and GitHub Actions standardized on YAML early, and the readability advantage for large manifests is genuine. The ecosystem isn't switching, so the pragmatic move is to learn YAML's sharp edges rather than fight it.
- Is JSON5 a good middle ground?
- JSON5 (comments, trailing commas, unquoted keys) fixes JSON's biggest config annoyances and is worth considering for greenfield projects where you control the parser. Adoption is the limitation — most tools expect strict JSON.