// comments are not valid JSON
JSON has no comment syntax. Comments are ubiquitous in config files, and several tools (VS Code settings, tsconfig) accept them via the JSONC dialect, which trains people to expect them everywhere.
The input
{
// note
"a": 1
}{
"_note": "note",
"a": 1
}How to fix it
Strip the comments, or parse with a JSONC-aware reader if you control the consumer.
A conventional workaround is a `_comment` key. It is ugly but it survives every parser.
What each parser reports
Every message below was captured by running this exact input through the parser named. 14 of 21 parsers rejected it; 7 accepted it.
| Parser | Exact message |
|---|---|
| CPython json.loads CPython 3.12.3 | Expecting property name enclosed in double quotes: line 2 column 3 (char 4)what this message means → |
| encoding/json Unmarshal Go go1.22.2 | invalid character '/' looking for beginning of object key stringwhat this message means → |
| Gson Java 21.0.12 | accepted — no error |
| Gson JsonParser (lenient off) Java 21.0.12 | accepted — no error |
| hjson Node.js 22.22.2 | accepted — no error |
| Jackson ObjectMapper Java 21.0.12 | Unexpected character ('/' (code 47)): maybe a (non-standard) comment? (not recognized as one since Feature 'ALLOW_COMMENTS' not enabled for parser)what this message means → |
| JavaScriptCore JSON.parse JavaScriptCore (Bun 1.4.1) | JSON Parse error: Unrecognized token '/'what this message means → |
| json-bigint Node.js 22.22.2 | Bad string |
| JSON5 Node.js 22.22.2 | accepted — no error |
| JSON::PP Perl 5.038002 | unexpected end of string while parsing JSON string, at character offset 5 (before "/ note\x{a} "a": 1\x{a}...")what this message means → |
| json_decode PHP 8.3.6 | Syntax error |
| Newtonsoft.Json JsonConvert Newtonsoft.Json 13.0.3 on .NET 8.0.30 | accepted — no error |
| Newtonsoft.Json JsonTextReader Newtonsoft.Json 13.0.3 on .NET 8.0.30 | accepted — no error |
| orjson CPython 3.12.3 | unexpected character, expected a string key: line 2 column 3 (char 4)what this message means → |
| Ruby JSON.parse Ruby 3.2.3 | accepted — no error |
| serde_json::from_str Rust (serde_json 1.x) | key must be a string at line 2 column 3what this message means → |
| SpiderMonkey JSON.parse SpiderMonkey 115 | JSON.parse: expected property name or '}' at line 2 column 3 of the JSON datawhat this message means → |
| System.Text.Json Deserialize .NET 8.0.30 | '/' is invalid after a value. Expected either ',', '}', or ']'. Path: $ | LineNumber: 1 | BytePositionInLine: 2.what this message means → |
| System.Text.Json JsonDocument .NET 8.0.30 | '/' is invalid after a value. Expected either ',', '}', or ']'. LineNumber: 1 | BytePositionInLine: 2.what this message means → |
| ujson CPython 3.12.3 | Expected object or value |
| V8 JSON.parse Node.js 22.22.2 | Expected property name or '}' in JSON at position 4 (line 2 column 3)what this message means → |
What the specification says
The exact sections that govern this error. Descriptions are our paraphrase; follow the links for the normative text.
| Body | Section |
|---|---|
| IETF | A JSON text is a single value with optional surrounding whitespace ( JSON-text = ws value ws). The token set is six structural characters, strings, numbers and three literal names — nothing else. |
| Ecma International | Lists the six structural tokens, the three literal name tokens, and the four whitespace code points. Whitespace is not allowed within a token. |
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