JSON Formatter and Validator

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json trailing comma comments

Trailing commas and comments in JSON: allowed or not?

Paste the file with comments or extra commas and turn repair on: every comment and trailing comma removed appears in a list with line and column, and you decide whether to apply the result in the editor.

What the specification says and what tools tolerate

The JSON grammar, in RFC 8259 and ECMA-404, defines a comma only between two items and defines no form of comment. A text with a comma after the last item, with // or with /* */ is not valid JSON, and JSON.parse throws a SyntaxError in both cases. There is no flag or option to relax this in the native parser. It was a deliberate choice for simplicity: without comments the format stays small, unambiguous and the same in every language.
The confusion comes from neighboring formats. JSONC, "JSON with comments", is used by files such as tsconfig.json and by VS Code settings, and accepts comments and, in those programs, a trailing comma. JSON5 is even more permissive: single quotes, unquoted keys, hexadecimal numbers and more. Each of these formats needs its own reader, so a JSONC file handed to JSON.parse fails. The safe workflow is to keep JSONC in local configuration and, when sending to an API or another system, convert to strict JSON, which is exactly what repair does.
Repair removes line comments, block comments and those starting with #, and removes every trailing or repeated comma, at any nesting level. The comment text is lost, because JSON has nowhere to keep it; if it matters, copy it first. If you need to keep notes, a common workaround is a regular key such as "_comment" or "__note", which is valid JSON, although the consumer will now receive it as data. The order of items and the numbers stay as they were: 1.50 remains 1.50 and does not become 1.5.

Removal examples

Array with trailing comma

Input: [1, 2, 3,]
Expected output: [1,2,3]

Trailing comma flagged at column 9.

Line comment

Input: {"a": 1, // nota "b": 2}
Expected output: {"a":1,"b":2}

Comment flagged at line 1, column 10.

Block comment

Input: /* cfg */ {"debug": true}
Expected output: {"debug":true}

Block comment at column 1.

Three nested trailing commas

Input: {"a": [1, 2,], "b": {"c": 3,},}
Expected output: {"a":[1,2],"b":{"c":3}}

Columns 12, 28 and 30.

tsconfig.json style

Input: {"compilerOptions": {"strict": true,},}
Expected output: {"compilerOptions":{"strict":true}}

Two trailing commas, columns 36 and 38.

Full tool FAQ

Why does my JSON give "Unexpected token" and where is the error?

The most common causes are a comma after the last item, single quotes, an unquoted key, a comment, and a missing comma between two values. JSON.parse reports only the first problem and, in some browsers, only a character position. This formatter lists every error in the text, each with line and column, shows the snippet with an arrow under the exact spot, marks the line in the editor margin, and has a Go to the error button that selects the spot.

Can JSON have comments or a trailing comma?

Do single quotes work in JSON? What about a Python dict?

What does Repair mode do and what are its limits?

What happens when the JSON has the same key twice?

Why does a big number change after going through JavaScript?

Does sorting keys or minifying change what the JSON means?

How do I validate a JSON Lines or NDJSON file?

Which query syntax is supported?

Is my JSON sent to any server? What about large files?

Frequently asked questions

Can I use comments in a .json file?

By the standard, no. Some programs read .json as JSONC and tolerate comments, but that is an extension of theirs. If the file goes to JSON.parse, an API or another language, remove the comments first.

Is a trailing comma accepted in JavaScript but not in JSON?

Exactly. In JavaScript object and array literals a trailing comma has been allowed for years, and many people copy a literal into a data file. JSON.parse still demands the strict grammar.

Does repair delete the comment contents?

It does, because JSON has no place for them. The list of changes shows the line and column of every comment removed, and your original text stays in the editor until you apply the result.

Is there a way to keep a note inside valid JSON?

The most common option is a regular string key, such as "_comment". It is valid JSON, but the system that reads the file now sees that key as data, so confirm that it ignores it.

Next steps

What the specification says and what tools tolerate

The JSON grammar, in RFC 8259 and ECMA-404, defines a comma only between two items and defines no form of comment. A text with a comma after the last item, with // or with /* */ is not valid JSON, and JSON.parse throws a SyntaxError in both cases. There is no flag or option to relax this in the native parser. It was a deliberate choice for simplicity: without comments the format stays small, unambiguous and the same in every language.

The confusion comes from neighboring formats. JSONC, "JSON with comments", is used by files such as tsconfig.json and by VS Code settings, and accepts comments and, in those programs, a trailing comma. JSON5 is even more permissive: single quotes, unquoted keys, hexadecimal numbers and more. Each of these formats needs its own reader, so a JSONC file handed to JSON.parse fails. The safe workflow is to keep JSONC in local configuration and, when sending to an API or another system, convert to strict JSON, which is exactly what repair does.

Repair removes line comments, block comments and those starting with #, and removes every trailing or repeated comma, at any nesting level. The comment text is lost, because JSON has nowhere to keep it; if it matters, copy it first. If you need to keep notes, a common workaround is a regular key such as "_comment" or "__note", which is valid JSON, although the consumer will now receive it as data. The order of items and the numbers stay as they were: 1.50 remains 1.50 and does not become 1.5.

Removal examples

Array with trailing comma

Input: [1, 2, 3,]
Expected output: [1,2,3]

Trailing comma flagged at column 9.

Line comment

Input: {"a": 1, // nota "b": 2}
Expected output: {"a":1,"b":2}

Comment flagged at line 1, column 10.

Block comment

Input: /* cfg */ {"debug": true}
Expected output: {"debug":true}

Block comment at column 1.

Three nested trailing commas

Input: {"a": [1, 2,], "b": {"c": 3,},}
Expected output: {"a":[1,2],"b":{"c":3}}

Columns 12, 28 and 30.

tsconfig.json style

Input: {"compilerOptions": {"strict": true,},}
Expected output: {"compilerOptions":{"strict":true}}

Two trailing commas, columns 36 and 38.

Full tool FAQ

Why does my JSON give "Unexpected token" and where is the error?

Can JSON have comments or a trailing comma?

Do single quotes work in JSON? What about a Python dict?

What does Repair mode do and what are its limits?

What happens when the JSON has the same key twice?

Why does a big number change after going through JavaScript?

Does sorting keys or minifying change what the JSON means?

How do I validate a JSON Lines or NDJSON file?

Which query syntax is supported?

Is my JSON sent to any server? What about large files?

Frequently asked questions

Can I use comments in a .json file?

Is a trailing comma accepted in JavaScript but not in JSON?

Exactly. In JavaScript object and array literals a trailing comma has been allowed for years, and many people copy a literal into a data file. JSON.parse still demands the strict grammar.

Does repair delete the comment contents?

It does, because JSON has no place for them. The list of changes shows the line and column of every comment removed, and your original text stays in the editor until you apply the result.

Is there a way to keep a note inside valid JSON?

The most common option is a regular string key, such as "_comment". It is valid JSON, but the system that reads the file now sees that key as data, so confirm that it ignores it.