Fix "Unexpected token" and other JSON parse errors

"Unexpected token < in JSON at position 0" is one of the most searched error messages in web development. The message looks cryptic, but the character it names and the position it reports almost always point straight at the cause. This guide decodes the common JSON parse errors across browsers, Node.js and Python, and shows how to fix each one.

How to read a parse error

A parse error tells you two things:

1. The unexpected character, which is what the parser found where it expected something else.
2. The position, the character offset (counting from 0) or the line and column where it stopped.

The real mistake is often just before that position. A trailing comma, for instance, is reported at the closing brace that follows it.

Different engines word the same problem differently:

Engine	Example message
Chrome / Node.js	Unexpected token '<', "<!DOCTYPE html>" is not valid JSON
Firefox	JSON.parse: unexpected character at line 1 column 1 of the JSON data
Safari	JSON Parse error: Unrecognized token '<'
Python	json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

"Unexpected token <" at position 0

The response is HTML, not JSON. You asked for JSON but received a web page, usually one of these:

• A 404 or 500 error page, because the URL is wrong or the server crashed.
• A login page, because your session expired or a token is missing.
• Your app's own index.html, because a single-page app's fallback route answered an API URL that doesn't exist.
• A proxy or CDN error page.

Fix: check response.ok and the Content-Type header before parsing, and log the first part of the body when it isn't JSON:

javascript
const res = await fetch('/api/orders');
const type = res.headers.get('content-type') || '';
if (!res.ok || !type.includes('application/json')) {
  const body = await res.text();
  throw new Error(`Expected JSON, got ${res.status}: ${body.slice(0, 200)}`);
}
const data = await res.json();

"Unexpected end of JSON input" / "Expecting value: line 1 column 1"

The input is empty or truncated. Common causes:

• The server returned 204 No Content, or an empty body with status 200.
• You called response.json() on a response to a HEAD request.
• The response was cut off by a timeout, or the file was only partly written.
• In Python, you read the file once and then tried to read it again, so the second read returned an empty string.

Fix: check for an empty body before parsing, and treat it as "no data" rather than an error if the API allows it.

"Unexpected token u in JSON at position 0" / "undefined" is not valid JSON

Older browsers and Node.js versions use the first wording; current versions use the second. Either way, you're parsing the string "undefined". That usually means JSON.parse(localStorage.getItem('key')) ran when the key had never been set, or a variable was undefined and got converted to a string. Check for a missing value first.

"Unexpected token o in JSON at position 1" / "[object Object]" is not valid JSON

You're parsing something that's already an object. JSON.parse({}) converts the object to the string "[object Object]", which isn't JSON. Older engines report the o at position 1; current ones quote the whole string. Remove the extra JSON.parse, which often follows response.json() or an axios response, since axios parses JSON for you.

Errors in the middle of the document

These point to a syntax mistake inside otherwise valid JSON:

Message mentions	Likely cause	Fix
} or ] after a comma	Trailing comma	Remove the last comma
' (single quote)	Single-quoted strings or keys	Use double quotes
/	A comment	Remove // and /* */ comments
A letter such as N, I or u	NaN, Infinity or undefined	Use null or remove the key
A letter at the start of a key	Unquoted key {a: 1}	Quote the key
Bad control character	A raw line break or tab in a string	Escape it as \n or \t

Each case is explained with examples in Validate JSON (common errors & fixes).

"Unexpected non-whitespace character after JSON"

The input contains more than one JSON value, such as two objects one after another, or a valid object followed by extra text. If each line is a separate record, the data is NDJSON and must be parsed line by line.

A quick debugging routine

1. Print the raw text, not the parsed value, and look at the first 200 characters.
2. Check the status code and Content-Type of the response.
3. Paste the text into the JSON Formatter. It highlights the line and column where the syntax breaks.
4. Look one token before the reported position.

For language-specific details, see Parse and stringify JSON in JavaScript and Work with JSON in Python.