How to Debug JSON Errors: Common Issues and Fixes

Reading a JSON SyntaxError

When JSON.parse() fails, the error message tells you exactly where the problem is: "SyntaxError: Unexpected token } in JSON at position 142". This means position 142 in the string contains an unexpected character. Count carefully—positions are 0-indexed.

Paste the failing JSON into the JSON Formatter to get a visual highlighting of the error location.

The 6 Most Common JSON Syntax Errors

1. Trailing comma: A comma after the last item in an array or object. JSON strictly forbids this, though JavaScript allows it. Fix: remove the final comma before the closing bracket.

2. Single-quoted strings: JSON requires double quotes for all strings and keys. Replace every single quote with double quotes.

3. Unquoted keys: {name: "Alice"} is invalid JSON. Every key must be quoted: {"name": "Alice"}.

4. Comments: JSON has no comment syntax. // and / / are not valid. Strip all comments before parsing.

5. NaN and Infinity: These JavaScript values are not valid JSON. Convert them to null or a numeric string before serializing.

6. Escaped characters: Only specific escape sequences are valid in JSON strings: backslash followed by one of ", , /, b, f, n, r, t, or u. Invalid escape sequences cause a parse failure.

Debugging Strategies

Bisect the JSON: If the payload is large, delete half of it and check if it parses. Repeat until you isolate the error. This binary search approach is faster than reading line-by-line.

Use the formatter: The JSON Formatter highlights errors and shows the exact line and character position, which is far more useful than a raw position offset.

Check the source: Was the JSON generated by code? Add logging to print the serialized string before sending. The error is often in the serializer, not the parser.

Validate schema separately: If the syntax is valid but the API rejects it, the issue is schema validation (wrong type, missing required field). Check with your API's documentation.

Handling JSON in APIs

When an API returns malformed JSON (a surprisingly common issue with older services), wrap your JSON.parse() in a try/catch. Log the raw response body before parsing so you can inspect it when errors occur.

For comparing expected vs actual API responses, use the Text Compare tool.

For full API debugging workflows, explore the Developer Tools hub.

Conclusion

JSON errors almost always fall into one of six categories. Train yourself to read the error message, use the JSON Formatter to locate the problem visually, and you will resolve most JSON issues in under a minute.

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