What causes an "unexpected token" in JSON?

The most common causes: single quotes instead of double quotes, unquoted property keys, JavaScript-specific values (undefined, NaN, Infinity) that are not valid JSON, comments (//, /* */), trailing commas, or a server returning HTML instead of JSON.

Why does my JSON have an unexpected token at position 0?

"Unexpected token at position 0" usually means the string does not start with a valid JSON character. This often happens when a server returns an HTML error page (starting with <) instead of JSON. Check that your request actually returned JSON by logging the raw response.

How do I fix "Unexpected token u in JSON at position 0"?

"Unexpected token u" usually means the string being parsed is the JavaScript keyword undefined — i.e. your variable was never set. Check that the string is not undefined before passing it to JSON.parse(). The "u" is the first character of the word "undefined".

Does FixTools show the exact position of the unexpected token?

Yes. FixTools reports the line number and column number of the unexpected token. For deeply nested or long JSON, this makes it much faster to find the problem than scanning manually.

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Fix JSON "Unexpected Token" Error

Q: How do I fix "Unexpected token u in JSON at position 0"?

"Unexpected token u" usually means the string being parsed is the JavaScript keyword undefined — i.e. your variable was never set. Check that the string is not undefined before passing it to JSON.parse(). The "u" is the first character of the word "undefined".

Q: Does FixTools show the exact position of the unexpected token?

Yes. FixTools reports the line number and column number of the unexpected token. For deeply nested or long JSON, this makes it much faster to find the problem than scanning manually.

"Unexpected token" is one of the most frustrating JSON error messages — it tells you something is wrong but not always exactly what. The error means the JSON parser encountered a character it did not expect at that position. Common causes: a single quote instead of a double quote, a JavaScript undefined or NaN value that serialised into the JSON, a comment left in the JSON, or an unquoted key. Paste your JSON into FixTools and it will tell you exactly where the unexpected token is and what character is causing the problem.

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Identifies the unexpected token and its position

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Paste the JSON that threw the unexpected token error and click Validate.

How It Works

Step-by-step guide to fix json "unexpected token" error:

1
Paste the JSON with the unexpected token error
Copy your JSON and paste it into the FixTools editor.
2
Click Validate
FixTools identifies the unexpected token, reports its line and column, and describes what was found versus what was expected.
3
Identify the cause
Common causes at the reported position: a single quote (replace with double quote), an unquoted key (add double quotes), the word undefined or NaN (replace with null), or a // comment (remove it).
4
Fix and revalidate
Correct the character at the reported position and click Validate again to confirm.