JSON.parse() throws a SyntaxError when the text you give it is not valid JSON. Check the parser’s reported location and the characters just before it, then look for common causes such as single quotes, trailing commas, invalid numbers, unfinished strings, or extra text. If you are parsing a network response, inspect its status and raw body first: it may not contain JSON at all.
What the error means
JSON.parse(text) reads JSON text and returns the corresponding JavaScript value. It throws a SyntaxError if the input does not conform to JSON grammar. JSON is similar to JavaScript object-literal syntax, but it is stricter: names and strings must use double quotes, trailing commas are forbidden, and only JSON’s defined value forms are allowed. The standard says a JSON parser “MUST accept all texts that conform to the JSON grammar.” (RFC 8259, section 9)
Valid JSON text contains one complete value: an object, array, number, string, true, false, or null. It cannot be a JavaScript expression or a fragment that needs surrounding code to make sense. See MDN’s JSON.parse() reference.
Read the error location, but check just before it
Browser and runtime messages vary. They may report a line and column, a character position, an unexpected token, or unexpected end of input. Treat that location as a clue, not a guaranteed pointer to the original mistake: a missing quote or comma earlier in the text can make a later character look invalid. Inspect the reported character and the punctuation immediately before it. MDN lists common message forms in its JSON.parse bad-parsing error reference.
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For example, an “unexpected end” message often means that the input ended before a string, object, array, escape, or number was complete. A complaint about a character after a value can indicate extra non-whitespace text following otherwise complete JSON.
Fix the common JSON syntax errors
Use double quotes for names and strings
JSON strings and object property names require double quotes. This JavaScript-like text is not JSON:
{'name': 'Ada'}Write it as:
{"name": "Ada"}Remove trailing commas
The final array item or object member cannot be followed by a comma:
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[1, 2] // valid JSON
{"name": "Ada"} // valid JSON[1, 2,] and {"name": "Ada",} are invalid JSON.
Use valid JSON numbers
Numbers cannot have leading zeros except for zero itself, and a decimal point or exponent must be followed by digits. For example, 01 and 1. are invalid; use 1 or 1.0 where appropriate. An exponent such as 1e is incomplete; provide its digits. NaN and Infinity are not JSON numbers.
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The accepted literal values are lowercase true, false, and null. Values such as True, undefined, NaN, and Infinity are not valid JSON values.
Finish strings, structures, and escapes
Close every quoted string, array, and object. Inside strings, escape quotation marks, reverse solidus characters (backslashes), and control characters according to JSON’s string rules. A raw line break inside a quoted JSON string is invalid. Check that each opening delimiter has a matching closing delimiter and that commas and colons separate members and values correctly. The normative rules are in RFC 8259.
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Remove extra text after the JSON value
A JSON document is not a JSON value followed by a log message, warning, or second value. If parsing points to unexpected content at the end, check whether the producer appended text after the complete JSON. Whitespace after a JSON value is allowed; arbitrary text is not.
Check whether the input is actually JSON
When the input comes from fetch(), inspect the HTTP status and actual response body in the browser’s Network panel, or temporarily log the raw text before parsing. A server can return an HTML error page, plain text, an empty body, or a JSON response with unwanted text appended. These are different from a JSON punctuation error: editing the expected data structure will not fix a response that was never JSON.
Response.json() reads the response body and parses it as JSON; it rejects with a SyntaxError if the body cannot be parsed. See MDN’s Response.json() reference. During diagnosis, inspect status, headers, and body rather than assuming that a successful request necessarily returned JSON.
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Use one parsing step
If you already called response.json(), the result is a JavaScript value, not the original JSON text. Passing that value to JSON.parse() again is usually a logic error. Check what the variable contains and its type before parsing it a second time.
Debug safely and prevent recurrence
Keep the original text available while diagnosing, and fix the producer or source data where possible. This pattern logs the parse error and input without hiding the failure:
try {
const value = JSON.parse(rawText);
// Use value after parsing succeeds.
} catch (error) {
console.error("Could not parse JSON", error);
console.error("Raw input:", rawText);
}If the data begins as a JavaScript value, serialize it instead of manually assembling braces, quotes, and commas:
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const payload = { name: "Ada", active: true };
const rawText = JSON.stringify(payload);
const copy = JSON.parse(rawText);JSON.stringify() produces JSON text from a JavaScript value; it does not make arbitrary text from an external source trustworthy. Do not use eval() as a workaround: it executes JavaScript rather than validating input as JSON. Refer to MDN’s parsing and serialization guidance and the JSON standard.
Common troubleshooting cases
| Symptom | What to inspect | Fix |
|---|---|---|
| Unexpected end of JSON input | Whether the text is empty or ends inside a string, object, array, escape, or exponent | Obtain the complete response or finish the JSON value at its source. |
| Unexpected token near a quote or property name | Single quotes, an unquoted property name, or a missing quote earlier in the text | Use double quotes for JSON names and strings; repair the earlier delimiter if the reported position is downstream. |
| Error near a comma or closing bracket | A trailing comma or missing separator | Remove the final comma or insert the missing comma between valid members. |
| Error on a number or literal | Leading zeros, incomplete fraction/exponent, capitalization, or JavaScript-only values | Use JSON number grammar and lowercase true, false, or null. |
| HTML or plain text appears in the raw body | Response status, content type, endpoint, and server error output | Handle the non-JSON response appropriately or correct the request/server behavior before parsing. |
| Parsing fails only after another parse call | Whether the value is already an object, array, or other parsed value | Remove the redundant JSON.parse(); parse only raw JSON text. |
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Does JSON allow comments?
No. JSON text has no comment syntax; comments must not be included in the value passed to a JSON parser.
Can a JSON document contain a top-level string, number, or null?
Yes. JSON text may contain any single complete JSON value, including a string, number, boolean, or null, not only an object or array.
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