Trailing comma in a JSON array
A comma after the last element of an array. Usually produced by a loop that appends `item + ','` without special-casing the final iteration.
The input
[1, 2, 3,][1, 2, 3]How to fix it
Delete the comma before the closing bracket, or build the array with a join instead of manual concatenation.
Serialise with your language's JSON library rather than string concatenation and this class of bug disappears entirely.
What each parser reports
Every message below was captured by running this exact input through the parser named. 15 of 21 parsers rejected it; 6 accepted it.
| Parser | Exact message |
|---|---|
| CPython json.loads CPython 3.12.3 | Expecting value: line 1 column 10 (char 9)what this message means → |
| encoding/json Unmarshal Go go1.22.2 | invalid character ']' looking for beginning of valuewhat 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 93)): expected a valid value (JSON String, Number, Array, Object or token 'null', 'true' or 'false')what this message means → |
| JavaScriptCore JSON.parse JavaScriptCore (Bun 1.4.1) | JSON Parse error: Unexpected comma at the end of array expressionwhat this message means → |
| json-bigint Node.js 22.22.2 | Unexpected ']' |
| JSON5 Node.js 22.22.2 | accepted — no error |
| JSON::PP Perl 5.038002 | malformed JSON string, neither array, object, number, string or atom, at character offset 9 (before "]")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 | trailing comma is not allowed: line 1 column 9 (char 8)what this message means → |
| Ruby JSON.parse Ruby 3.2.3 | unexpected token at ']' |
| serde_json::from_str Rust (serde_json 1.x) | trailing comma at line 1 column 10what this message means → |
| SpiderMonkey JSON.parse SpiderMonkey 115 | JSON.parse: unexpected character at line 1 column 10 of the JSON datawhat this message means → |
| System.Text.Json Deserialize .NET 8.0.30 | The JSON array contains a trailing comma at the end which is not supported in this mode. Change the reader options. Path: $ | LineNumber: 0 | BytePositionInLine: 9.what this message means → |
| System.Text.Json JsonDocument .NET 8.0.30 | The JSON array contains a trailing comma at the end which is not supported in this mode. Change the reader options. LineNumber: 0 | BytePositionInLine: 9.what this message means → |
| ujson CPython 3.12.3 | Unexpected character found when decoding array value (1)what this message means → |
| V8 JSON.parse Node.js 22.22.2 | Unexpected token ']', "[1, 2, 3,]" is not valid JSONwhat 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 | An array is square brackets around zero or more values, with elements separated by commas. |
| Ecma International | Gives the array grammar; values are separated by commas. |
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