The literal string 'undefined' as a JSON document
A JavaScript variable holding `undefined` was interpolated into a string and sent as the body. `String(undefined)` is `"undefined"`.
The input
undefinednullHow to fix it
Guard against undefined before sending, and send `JSON.stringify(value)` rather than a template literal.
Its sibling is the body `"[object Object]"`, which means an object was concatenated into a string instead of serialised.
What each parser reports
Every message below was captured by running this exact input through the parser named. 16 of 21 parsers rejected it; 5 accepted it.
| Parser | Exact message |
|---|---|
| CPython json.loads CPython 3.12.3 | Expecting value: line 1 column 1 (char 0)what this message means → |
| encoding/json Unmarshal Go go1.22.2 | invalid character 'u' 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 | Unrecognized token 'undefined': was expecting (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 identifier "undefined"what this message means → |
| json-bigint Node.js 22.22.2 | Unexpected 'u' |
| JSON5 Node.js 22.22.2 | JSON5: invalid character 'u' at 1:1what this message means → |
| JSON::PP Perl 5.038002 | malformed JSON string, neither array, object, number, string or atom, at character offset 0 (before "undefined")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 JSON value: line 1 column 1 (char 0)what this message means → |
| Ruby JSON.parse Ruby 3.2.3 | unexpected token at 'undefined' |
| serde_json::from_str Rust (serde_json 1.x) | expected value at line 1 column 1what this message means → |
| SpiderMonkey JSON.parse SpiderMonkey 115 | JSON.parse: unexpected character at line 1 column 1 of the JSON datawhat this message means → |
| System.Text.Json Deserialize .NET 8.0.30 | 'u' is an invalid start of a value. Path: $ | LineNumber: 0 | BytePositionInLine: 0.what this message means → |
| System.Text.Json JsonDocument .NET 8.0.30 | 'u' is an invalid start of a value. LineNumber: 0 | BytePositionInLine: 0.what this message means → |
| ujson CPython 3.12.3 | Expected object or value |
| V8 JSON.parse Node.js 22.22.2 | "undefined" 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 | A value must be an object, array, number, string, or one of the three literal names, which the section states must be lowercase. No other literal names are allowed. |
| Ecma International | A value is an object, array, number, string, true, false or null. |
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Bare unquoted word as a JSON document
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Whole JSON document wrapped in quotes
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NDJSON fed to a whole-document JSON parser
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