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JSONC is a human-editable configuration dialect built on JSON. It adds JavaScript-style comments, and some JSONC parsers also allow a comma after the final array item or object member. RFC 8259 JSON defines neither extension, so a strict JSON parser is entitled to reject both.
The reason is a division of purpose: JSON keeps a small, predictable grammar for interoperable data exchange, while JSONC makes source files easier for people to explain and maintain. Trailing-comma support is optional rather than a universal JSONC rule, so the parser and its settings determine what is accepted.
The standards boundary: what strict JSON actually permits
RFC 8259, published in December 2017, describes JSON as “a lightweight, text-based, language-independent data interchange format.” Its design goals include minimal syntax, portability and interoperability. The grammar defines arrays and objects as comma-separated sequences, but it has no production for a comma after the final value or member. It also has no comment token.
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In strict JSON, an array such as [1, 2, 3] has two separators: one between 1 and 2 and one between 2 and 3. An object such as {"a": 1, "b": 2} follows the same rule for members. A comma immediately before ] or } is therefore outside the grammar, not merely a stylistic variation.
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Comments are not insignificant JSON whitespace
Whitespace may appear between JSON tokens, but a comment is a different kind of lexical construct. Because RFC 8259 does not define // or /* ... */ comments, a conforming parser can report an error as soon as it encounters one. A file can look like JavaScript and still be invalid JSON.
What JSONC adds
The JSONC specification draft defines JSONC as JSON plus two JavaScript-style comment forms. It also discusses trailing commas as an optional parser feature, not a required part of every JSONC implementation.
Line and block comments
A line comment begins with // and runs to the end of the line. A block comment begins with /* and ends with */; block comments do not nest. Comments can occur where insignificant whitespace is allowed, such as between object members or array values, but they are not allowed inside a quoted string. The JSONC draft says comments are ignored during parsing and “MUST NOT affect consumption,” so they do not become properties, array elements or other values in the resulting data representation.
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"port": 8080, // Public listener
/* Keep this low in development. */
"debug": true
}After parsing, the represented data is equivalent to the same object without the comments. A parser that preserves source text for an editor may retain comments separately, but they are not part of the JSON value delivered to application code.
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Trailing commas are deliberately optional
JSONC implementations differ on the second extension. The reference jsonc-parser described by the draft sets allowTrailingComma to false by default. A trailing comma is an error unless the caller enables that option. The draft also notes that Visual Studio Code’s JSON-with-comments mode accepts trailing commas, while warning about them outside certain official configuration files. Those editor behaviors are release-sensitive; check the current documentation for the tool you use.
{
"hosts": [
"api.example.test",
"worker.example.test",
],
}The comments and final commas in this example are independent features. A parser may accept comments while rejecting the commas, or accept both when configured to do so.
There is no single, universal JSONC mode
The Common JSONC Specification project describes a commonly implemented configuration dialect, but its repository labels the document as a draft as of 2025-09-01. It records ongoing discussion about trailing commas and notes that variants differ: some disallow them, while other spin-offs add syntax beyond the common variant. “JSONC” therefore identifies a family of practices rather than one immutable parser contract.
Why configuration files benefit from the extensions
Comments keep explanation next to the setting
Configuration often needs operational context: why a value has a non-obvious limit, which environment uses it, or when a temporary switch can be removed. A comment places that explanation beside the setting without inventing a data property that an application must ignore. This is the human-maintenance case the JSONC draft targets.
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Trailing commas reduce punctuation edits
When a parser permits a trailing comma, adding, deleting or reordering the last entry does not require changing the previous line’s punctuation. That is a practical editing convenience inferred from the syntax, not a measured performance or reliability result. It applies only when the selected parser and project style permit the extension.
Human-authored source and machine interchange have different constraints
A configuration file is read and edited by people, so annotations and forgiving punctuation can be useful. Data sent between independent systems has a different priority: every implementation needs to agree on one grammar. Keeping comments and trailing commas out of RFC 8259 JSON reduces the chance that one receiver silently accepts a construct another receiver rejects.
JSON versus JSONC at a glance
| Feature | RFC 8259 JSON | JSONC draft and common practice |
|---|---|---|
Line comments (//) |
Not defined; strict parsers may reject them. | Supported by the JSONC draft and many configuration parsers. |
Block comments (/* ... */) |
Not defined. | Supported; block comments do not nest. |
| Trailing commas | Not valid. | Optional and implementation-dependent; reference jsonc-parser disables them by default. |
| Comments in parsed data | Not applicable. | Ignored and must not change the resulting JSON data representation. |
| Recommended file extension | .json |
The JSONC draft recommends .jsonc. |
| Recommended media type | application/json |
The JSONC draft recommends application/jsonc; labeling JSONC as application/json can mislead strict consumers. |
| Interoperability | Defined by the RFC grammar. | Depends on the exact parser, options and dialect. |
Why a file named settings.json may still accept comments
Editors frequently provide a language mode for “JSON with comments” so that a well-known configuration filename can offer diagnostics, completion and annotations. The filename alone does not change RFC 8259; the editor or consuming program is choosing a JSONC-like parser. That explains the common question, “Why does settings.json support comments?” The answer is that the host application has assigned the file a permissive configuration grammar, even though the suffix says .json.
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How to use JSONC without breaking strict consumers
- Identify the actual dialect. Read the consuming tool’s documentation and record whether line comments, block comments and trailing commas are accepted. Do not infer behavior from the file extension or editor highlighting.
- Check trailing-comma defaults. If you use
jsonc-parser, the documented default isallowTrailingComma: false. Enable the option only when the project deliberately allows it, and make the setting explicit in code or configuration. - Keep JSONC at the authoring boundary. Store comments and human-oriented formatting in the configuration source that people edit. Treat that source as distinct from payloads sent to systems that advertise RFC 8259 JSON.
- Parse, then serialize for interchange. Read the JSONC source with a parser configured for the chosen dialect, obtain its data value, and serialize that value with a standards-compliant JSON serializer. Serialization removes comments and emits commas only where RFC 8259 permits them.
- Validate the serialized output. Run the resulting text through a strict JSON parser before publishing it, embedding it in another document or sending it over an API. This catches accidental comments, trailing commas or other extensions that may have entered during editing.
- Label the formats honestly. Use
.jsoncand, when transmitting JSONC itself, the draft’s recommendedapplication/jsoncmedia type. Useapplication/jsononly for output that is valid strict JSON.
Parsing and serializing normally discards comments because comments are not part of the data model. If preserving annotations matters, use a source-preserving editor or syntax-tree tool and keep that annotated source separate from the strict JSON artifact.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Diagnosing common failures
“Unexpected token /”
The consumer is probably using a strict JSON lexer and encountered a line or block comment. Remove comments for that payload, or parse the file as JSONC before converting it to strict JSON.
“Unexpected token ]” or “Unexpected token }” after a comma
The parser reached an array or object close after a trailing comma. Either remove the final comma or enable the parser’s documented trailing-comma option if the file is intentionally JSONC and the application supports that setting.
The editor accepts the file but a deployment step fails
The editor and deployment step are using different grammars. Make the parser choice explicit in the build process, generate a strict JSON artifact, and validate that artifact with the same class of parser used by the deployment target.
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Comments disappear after formatting
A formatter that parses to ordinary JSON data cannot retain comments that are outside that data model. Configure a comment-preserving formatter for the source file, or accept that comments belong only in the human-maintained JSONC version.
What to compare when choosing a parser
“Supports JSONC” is not enough information to predict compatibility. Compare these five behaviors in the implementation’s current documentation:
- Whether
//line comments are accepted. - Whether
/* ... */block comments are accepted and whether nesting is rejected. - Whether trailing commas are accepted by default, rejected, or enabled only through an option.
- Whether parsing discards comments and formatting or exposes a source-preserving representation.
- Whether the serializer’s output is valid RFC 8259 JSON for interchange.
Deno’s documentation, for example, presents JSONC as JSON extended with comments and trailing commas for friendlier configuration editing and demonstrates parsing it with a standard-library parser: Deno: Parsing JSONC. That example documents Deno’s behavior; it should not be treated as proof that every JSONC-labeled parser accepts the same syntax.
The practical rule
Use strict JSON when bytes must be portable between independent systems. Use JSONC when people maintain configuration and the selected tool documents the extensions you need. Keep the boundary visible: comments and optional trailing commas belong to the authoring dialect, while data exchanged as application/json must be serialized as RFC 8259 JSON.
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