"undefined" is not valid JSON
This message comes from V8 JSON.parse on Node.js 22.22.2. In this registry it is produced by 1 distinct mistake. Positions in the real message vary with your input; the causes do not.
Causes
What produces this message
The literal string 'undefined' as a JSON document
Input
undefinedExact message: "undefined" is not valid JSON
Fix: Guard against undefined before sending, and send `JSON.stringify(value)` rather than a template literal.
Same bug, other languages
What other parsers say about the same input
If a colleague reports a different message for what looks like the same file, this is why.
| Parser | Message for the same inputs |
|---|---|
| CPython json.loads | Expecting value: line L column C (char N) |
| JSON5 | JSON5: invalid character 'u' at 1:1 |
| JSON::PP | malformed JSON string, neither array, object, number, string or atom, at character offset N (before "…") |
| Jackson ObjectMapper | Unrecognized token 'undefined': was expecting (JSON String, Number, Array, Object or token 'null', 'true' or 'false') |
| JavaScriptCore JSON.parse | JSON Parse error: Unexpected identifier "undefined" |
| Ruby JSON.parse | unexpected token at '…' |
| SpiderMonkey JSON.parse | JSON.parse: unexpected character at line L column C of the JSON data |
| System.Text.Json Deserialize | 'u' is an invalid start of a value. Path: $ | LineNumber: L | BytePositionInLine: C. |
| System.Text.Json JsonDocument | 'u' is an invalid start of a value. LineNumber: L | BytePositionInLine: C. |
| encoding/json Unmarshal | invalid character 'u' looking for beginning of value |
| json-bigint | Unexpected 'u' |
| json_decode | Syntax error |
| orjson | unexpected character, expected a JSON value: line L column C (char N) |
| serde_json::from_str | expected value at line L column C |
| ujson | Expected object or value |
Sources
Standards this registry is checked against
- RFC 8259 (STD 90) — The JavaScript Object Notation (JSON) Data Interchange Format, T. Bray, Ed., 2017. The IETF Internet Standard for JSON. Obsoletes RFC 7159 and RFC 4627.
- ECMA-404, 2nd edition — The JSON Data Interchange Syntax, Ecma International, 2017. Ecma’s grammar for JSON. A normative reference of RFC 8259; the two define the same syntax.
- The Unicode Standard — The Unicode Standard, Core Specification, The Unicode Consortium, current. Normatively referenced by RFC 8259 for the definition of a character.
- IEEE 754-2019 — IEEE Standard for Floating-Point Arithmetic, IEEE, 2019. Referenced by RFC 8259 §6 as the basis for its interoperability guidance on numbers.
Only standards bodies and peer-reviewed venues are cited. Error strings on this site are observed by executing each parser; the standards above define what the parser is reacting to.