When would I use TOML over JSON for configuration?

Use TOML over JSON for configuration files that humans write and maintain. TOML supports comments (# syntax), which let you document why a setting has a specific value. TOML does not require commas after each value, and trailing commas in arrays are allowed, which makes diffs cleaner. TOML tables ([section]) are more readable than deeply nested JSON objects for typical app config. If the config file will be committed to a repository, reviewed in code review, and edited by developers, TOML is easier to read and less error-prone than JSON. JSON is better when the config is generated programmatically or consumed by a browser.

TOML vs JSON: Config Format Differences and Examples

Q: Can TOML represent the same data as JSON?

TOML can represent most of the same data as JSON, with a few important differences. Both support strings, integers, floats, booleans, arrays, and nested objects (called tables in TOML). TOML adds native datetime types that JSON lacks. The key limitation is that TOML cannot represent null values — JSON null has no TOML equivalent. TOML also requires the root value to be a table (object), whereas JSON allows any value at the root including a number, string, or array. For data that avoids null and starts with an object, TOML and JSON are largely interchangeable in terms of the data they can express.

Q: Does TOML have a formal specification?

Yes. TOML has a formal specification maintained at toml.io. The current version is TOML v1.0.0, which was finalized and has been stable since 2021. The specification defines all value types, table syntax, array of tables, key naming rules, string escapes, and datetime formats precisely. JSON also has a formal specification in RFC 8259. Both formats are well-specified, which means well-implemented parsers should produce consistent results. TOML has been more cautious about versioning than YAML, which has incompatible 1.1 and 1.2 versions that different parsers implement differently.

Q: What TOML types does JSON not support natively?

TOML has four native datetime types that JSON does not support: offset datetime (with timezone, e.g. 1979-05-27T07:32:00Z), local datetime (without timezone), local date (date only), and local time (time only). In JSON, dates are represented as strings and parsed by convention — there is no standard for how a date string is formatted or typed. TOML also distinguishes between integers and floats more explicitly, and TOML 1.0 supports special float values like inf and nan. JSON does not allow non-finite numbers like Infinity or NaN.

Q: How do I convert TOML to JSON programmatically?

In Python, use the tomllib standard library (Python 3.11+) to parse TOML and then json.dumps to serialize the result: import tomllib, json; data = tomllib.loads(toml_string); print(json.dumps(data, indent=2)). For older Python, use the tomli third-party package instead. In JavaScript and Node.js, use the @iarna/toml or smol-toml package to parse, then JSON.stringify. In Rust, use the toml crate. Note that TOML datetimes become strings in JSON since JSON has no datetime type, and any null values must be removed before converting since TOML has no null.

Q: Does TOML allow comments?

Yes. TOML supports single-line comments using the # character. A comment can appear on its own line or after a value on the same line. For example: timeout = 30 # seconds. There is no multi-line comment syntax — each commented line must begin with or include a # after the value. This is a major practical advantage over JSON, which has no comment syntax at all. The ability to add explanatory comments is one of the main reasons TOML is preferred for configuration files that developers write and read by hand, such as Cargo.toml for Rust packages and pyproject.toml for Python projects.

Last updated: May 11, 2026

TOML is the right format for human-written config files; JSON is the right format for machine-to-machine data interchange — the 2 formats solve different problems and choosing the wrong one adds friction. TOML supports comments, 4 native datetime types, and trailing commas in arrays; JSON supports none of those. JSON has 1 hard conversion blocker when moving to TOML: null has no TOML equivalent and must be removed or replaced before conversion. This guide covers 5 key topics: syntax side-by-side, the full type mapping table, comment handling, the null problem, and when to pick each format for package metadata, CI config, REST APIs, and browser storage.

Same config in TOML and JSON

name = "jsonic"
debug = false

[server]
host = "localhost"
port = 3000

{
  "name": "jsonic",
  "debug": false,
  "server": {
    "host": "localhost",
    "port": 3000
  }
}

Quick comparison

Feature	TOML	JSON
Best for	Human-edited config	APIs and machine interchange
Comments	Supported	Not allowed
Trailing commas	Allowed in arrays	Not allowed
Nesting	Tables and dotted keys	Objects and arrays
Browser support	Parser library needed	Built in

Choose TOML when humans edit the file

TOML is a strong fit for package metadata, build settings, service config, and CLI tools. Comments let teams explain values inline, and tables avoid deeply nested braces.

# Production server
[server]
host = "0.0.0.0"
port = 8080
workers = 4

Choose JSON when data crosses system boundaries

JSON is the safer default for web APIs, browser storage, fixtures, and integration tests. It is strict, widely supported, and maps directly to JavaScript objects.

If strict syntax is causing errors, the fix invalid JSON guide covers common issues like trailing commas, comments, and single quotes.

Migration tips

Convert comments into documentation because JSON cannot preserve them.
Turn TOML tables into nested JSON objects.
Check dates explicitly because TOML has date/time types and JSON does not.
Validate the result before using it in a build or deployment pipeline.

Convert JSON to TOML

Use Jsonic's JSON to TOML converter to turn JSON objects into TOML tables and compare the output with your existing config style.

Open JSON to TOML

Frequently asked questions

Can TOML represent the same data as JSON?

Mostly yes. Both support strings, numbers, booleans, arrays, and nested objects. TOML adds native datetime types. The key difference: TOML cannot represent null, and the root value must be a table (object), not an array.

Does TOML have a formal specification?

Yes. TOML v1.0.0 is the stable spec at toml.io, finalized in 2021. Unlike YAML, which has incompatible 1.1 and 1.2 versions, TOML has a single stable specification that well-implemented parsers follow consistently.

What TOML types does JSON not support natively?

TOML has four native datetime types (offset datetime, local datetime, local date, local time) and distinguishes integers from floats explicitly. JSON represents dates as strings and has no standard datetime type. TOML also supports inf and nan float values; JSON does not.

When would I use TOML over JSON?

Use TOML for human-written config files: it supports comments, avoids trailing comma errors, and uses readable section headers instead of nested braces. Use JSON when config is generated programmatically, consumed by a browser, or shared across systems that expect JSON.

How do I convert TOML to JSON programmatically?

Python 3.11+: use tomllib to parse and json.dumps to serialize. For older Python, use the tomli package. In Node.js, use smol-toml or @iarna/toml. Note that TOML datetimes become strings in JSON and TOML null does not exist, so remove null values before converting.

Does TOML allow comments?

Yes. TOML uses # for single-line comments, same as YAML. Comments can appear on their own line or after a value. There is no multi-line comment syntax. JSON has no comment support at all, which is one reason TOML is preferred for hand-edited config files like Cargo.toml and pyproject.toml.