JSONC accepts comments because it is an extension of JSON for human-edited files, not the JSON interchange format defined by RFC 8259. Its comment syntax adds JavaScript-style // and /* … */ annotations. Trailing commas are a separate, optional extension: some JSONC parsers allow them, while others reject them unless an option is enabled.
That distinction explains why a file such as settings.json can contain comments in an editor but fail in a strict JSON parser. The editor is applying a JSONC-like dialect; the receiving program may require standards-conforming JSON.
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JSON and JSONC are different language contracts
JSON is a standardized data-interchange format. JSONC is a family of JSON-based syntaxes used primarily for configuration files that people read and edit. JSONC keeps JSON’s objects, arrays, strings, numbers, booleans and null, then adds conveniences that are outside the strict JSON grammar.
| Feature | RFC 8259 JSON | JSONC implementations |
|---|---|---|
| Single-line comments | Not part of the grammar | Usually accepts // comments |
| Block comments | Not part of the grammar | Usually accepts /* ... */ comments |
| Trailing commas | Not allowed | Optional and parser-dependent |
| Primary use | Interchange between independent systems | Human-maintained configuration |
| Interoperability | Expected by strict JSON consumers | Requires agreement on the dialect and parser options |
The word “JSONC” therefore does not guarantee one universal behavior. The JSONC.org specification is a draft, and the Common JSONC Specification repository was still marked as a draft on September 1, 2025. Some tools use the label for comments only; others also permit trailing commas or add further syntax.
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What RFC 8259 actually permits
RFC 8259 describes JSON as “a lightweight, text-based, language-independent data interchange format.” Its stated goals include being minimal, portable and textual. The grammar represents arrays and objects as comma-separated sequences. A comma separates two values or two members; there is no grammar production for a comma after the final value or member.
Why a trailing comma is invalid JSON
This array is valid JSON because the comma appears only between elements:
["red", "green", "blue"]This one is not valid RFC 8259 JSON:
["red", "green", "blue",]The final comma tells a strict parser that another element should follow. Because the grammar ends the sequence after "blue", the parser encounters an unexpected token and reports a syntax error.
Why comments are invalid JSON
JSON permits insignificant whitespace between tokens, but it has no comment token. Inserting // explain this value or /* explain this value */ introduces characters that RFC 8259 does not define as part of a JSON value.
{
"timeout": 30 // seconds
}A strict parser must reject that document. The fact that JavaScript, a text editor or a particular configuration loader accepts it does not change its status under RFC 8259.
The RFC does not assign a single historical motive for omitting comments. What it establishes is a deliberately narrow interchange grammar and the boundary of that grammar. Claims that comments were excluded for one definitive reason go beyond what the standard itself says.
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What JSONC adds
JavaScript-style comments
The draft at JSONC.org defines JSONC as JSON plus two comment forms:
- Line comments:
//continues to the end of the line. - Block comments:
/*continues until the next*/.
Comments may appear anywhere insignificant whitespace is allowed. A block comment cannot nest inside another block comment. A conforming JSONC parser ignores comments when it constructs the data representation. The specification states: “Comments provide context alongside the data, but they MUST NOT affect consumption.” In other words, a comment can document a value, but it must not create a property, array element or other piece of data.
{
// Network connection limit
"maxConnections": 20,
"retry": /* temporary failures */ true
}The represented data is equivalent to the following comment-free object:
{
"maxConnections": 20,
"retry": true
}Whether a tool preserves the original comments and spacing for later editing is an implementation question. A parser that returns ordinary data can discard that authoring information, so check the chosen library if round-tripping comments matters.
Trailing commas are optional
Comments and trailing commas should not be treated as one inseparable feature. The JSONC.org draft says parsers may support trailing commas; it does not require them. Its reference implementation, jsonc-parser, sets allowTrailingComma to false by default. A document ending an array or object with a comma is therefore an error unless the caller enables that option.
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Some editor modes accept trailing commas. The draft notes that Visual Studio Code’s JSON-with-comments mode accepts them, while warning about them outside certain official configuration files. These details can change with editor and parser releases, so verify the current setting rather than inferring behavior from the file extension.
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{
"formats": [
"json",
"jsonc",
],
}This is JSONC only when the active dialect allows trailing commas. A comments-only JSONC parser can still reject it even though it accepts the comments.
Why configuration files use JSONC
Comments keep explanations next to settings
Configuration values often need context that should not become application data: units, safe ranges, migration notes, links to internal documentation or a warning about a setting. JSON has no comment construct, so authors who need strict JSON sometimes add artificial properties such as "_note". That changes the data model and may collide with a real property name. JSONC lets the explanation remain metadata for the human reader.
Trailing commas simplify manual edits
When trailing commas are supported, adding a new final item can be a smaller edit because the previous line already ends with a comma. Reordering or temporarily removing an item also requires less punctuation adjustment. This is a practical editing benefit, not a standards requirement or a measured performance claim.
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A build or deployment process can read JSONC, remove comments and permitted trailing commas, then serialize strict JSON for a service that speaks RFC 8259. The two stages serve different audiences: JSONC for maintainers and JSON for interchange.
Why a file named settings.json may allow comments
File extensions are conventions, not proof of the grammar accepted by a program. An editor may associate settings.json with a JSON-with-comments language mode for compatibility with its configuration ecosystem. The same text sent to an API, another programming language’s standard-library parser or a validator may be processed as strict JSON and fail.
The safest interpretation is: identify the component that reads the file, then consult that component’s documented dialect. Do not assume that every file ending in .json is interchangeable with every other JSON reader.
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Parser behavior to check before choosing JSONC
Implementations differ in more than the spelling of the format. Compare these behaviors for the exact parser and version used in your project:
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| Question | Why it matters |
|---|---|
Are // comments accepted? |
Line annotations may be rejected by a comments-only or strict parser. |
Are /* ... */ comments accepted? |
Some tools support one comment form but not the other; block comments do not nest. |
| Are trailing commas accepted by default? | Support may require an option such as allowTrailingComma. |
| Are comments and formatting preserved? | A plain data parse may lose authoring information needed for an editor or formatter. |
| Does the output serialize as RFC 8259 JSON? | Strict downstream consumers need comments and trailing commas removed. |
| What file extensions and media types are advertised? | Tooling, validators and HTTP clients may select a parser from those labels. |
The Deno documentation demonstrates parsing JSONC with a standard-library parser and presents the format as JSON extended with comments and trailing commas for friendlier configuration editing: Deno’s JSONC parsing example. That example illustrates one ecosystem; it is not evidence that every JSONC parser uses identical defaults.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to use JSONC without breaking a strict consumer
1. Identify the boundary
Write down where the file is edited, where it is parsed and where the resulting data is transmitted. A local editor, a build script and a remote API may each enforce different grammars.
2. Select the dialect explicitly
Confirm whether the parser supports line comments, block comments and trailing commas. If trailing commas are optional, decide whether to enable them rather than relying on an undocumented default. Pin or review the parser version when configuration files are shared across machines.
3. Use an unambiguous name and type
The JSONC draft recommends the .jsonc extension and the application/jsonc media type. It warns against labeling JSONC as application/json, because a strict consumer can reject or misinterpret it. Follow those recommendations when you control the file or HTTP metadata.
4. Validate with the same settings used in production
A formatter or editor preview is not a substitute for the production parser. Test a file containing each syntax feature you intend to permit, including a line comment, a block comment and a final comma. Then test malformed variants to ensure failures are reported before deployment.
5. Convert at the interchange boundary
When a downstream system requires JSON, parse the JSONC with the agreed options and serialize the resulting data as ordinary JSON. The serialized output must contain no comments and no trailing commas. Do not merely rename the file or change its media type; those actions do not change the bytes.
Common failures and their fixes
“Unexpected token /” or “comments are not allowed”
The reader is operating in strict JSON mode. Remove the comments, use a JSONC-aware parser, or add a preprocessing step that converts JSONC to strict JSON. Do not add comment text as fake application properties unless those properties are intentionally part of the schema.
“Trailing comma” errors even though comments work
The parser supports comments but has trailing-comma support disabled. Enable the documented option if the file is controlled and the dialect permits it, or remove the final commas. The JSONC.org reference parser’s default is to reject them.
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The editor is likely using a JSONC-like language mode while the service expects RFC 8259 JSON. Add an explicit conversion and validation step before upload, and send the service the serialized JSON with the correct application/json media type.
Different tools disagree about the same file
That is possible because “JSONC” is not one universal parser mode. Compare the tools’ comment rules, trailing-comma defaults and supported extensions. The Common JSONC Specification project tracks a commonly implemented dialect, but its document remains a draft and notes that variants differ: some disallow trailing commas, while others add syntax beyond the common form. See the Common JSONC Specification for the current project status and scope.
Choosing between strict JSON and JSONC
- Choose strict JSON when the text is an interchange payload, public API body, signed artifact or input consumed by unknown third-party implementations.
- Choose JSONC when people maintain the file and the complete toolchain explicitly supports the dialect.
- Use a conversion step when maintainers need comments but an external system requires RFC 8259 JSON.
- Document the parser options when trailing commas are enabled, because another tool may default to rejecting them.
- Use the recommended extension and media type when you can control them, so readers and software can distinguish JSONC from strict JSON.
The central reason JSONC can accept syntax that JSON cannot is scope: JSONC deliberately extends the JSON data model for human-edited configuration, while RFC 8259 defines a minimal grammar for interoperable data exchange. Comments are a defined JSONC extension; trailing commas are an optional implementation feature. Treating those facts separately prevents the most common portability mistakes.
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