7 Common JSON Errors and How to Fix Them

1. “Unexpected Token” at Position 0

This is the most googled JSON error. The message SyntaxError: Unexpected token < in JSON at position 0 almost always means you are not receiving JSON at all. Instead, your API or server is returning HTML — often a 404 page or a redirect.

Why it happens

• closeThe API endpoint URL is wrong (typo, missing path segment).
• closeThe server returns an HTML error page instead of JSON.
• closeA proxy or CDN is intercepting the request.
• closeYou called response.json() on a non-JSON response.

How to fix it

fetch("/api/data")
  .then(res => res.json()) // Server returned HTML!
  .catch(err => console.error(err));

fetch("/api/data")
  .then(res => {
    const contentType = res.headers.get("content-type");
    if (!contentType?.includes("application/json")) {
      throw new Error(`Expected JSON, got ${contentType}`);
    }
    return res.json();
  })
  .then(data => console.log(data))
  .catch(err => console.error(err));

Quick fix: Log the raw response text with res.text() first. If you see HTML (starts with <!DOCTYPE or <html), the problem is your URL or server config — not your JSON.

2. Trailing Comma After the Last Item

JavaScript allows trailing commas in objects and arrays. JSON does not. This is one of the most frequent mistakes when hand-editing JSON.

{
  "name": "Alice",
  "age": 30,
}

{
  "name": "Alice",
  "age": 30
}

The fix is simple: remove the comma after the last property or array element. Tools like our JSON Formatter will highlight the exact line where the trailing comma appears.

3. Single Quotes Instead of Double Quotes

JSON requires double quotes around all strings and property keys. Single quotes are valid in JavaScript and Python, but they will cause a parse error in JSON.

{
  'name': 'Alice',
  'city': 'New York'
}

{
  "name": "Alice",
  "city": "New York"
}

Tip: If you are converting a Python dictionary to JSON, use json.dumps() instead of str(). The str() function will produce single-quoted output.

4. Unquoted Property Keys

In JavaScript, object keys do not need quotes. In JSON, every key must be a double-quoted string.

{
  name: "Alice",
  age: 30
}

{
  "name": "Alice",
  "age": 30
}

5. Missing Comma Between Items

Forgetting a comma between properties or array elements produces an “unexpected token” error at the position of the next key or value.

{
  "name": "Alice"
  "age": 30
}

{
  "name": "Alice",
  "age": 30
}

This error is easy to miss in large files. Our JSON Formatterpinpoints the exact line number so you don't have to hunt for it manually.

If you need to compare two versions of a JSON file to find what changed, use our Text Compare tool to highlight differences side by side.

6. Comments in JSON

JSON does not support comments. No // single-line, no /* */ block comments. Adding them will break parsing immediately.

{
  // This is a user object
  "name": "Alice",
  "age": 30 /* years old */
}

{
  "name": "Alice",
  "age": 30
}

Alternative: If you need comments in config files, consider using JSONC (JSON with Comments — used by VS Code), JSON5, or YAML. You can convert between YAML and JSON with our YAML to JSON converter.

7. Unescaped Special Characters in Strings

Certain characters inside JSON strings must be escaped with a backslash. The most common offenders:

{
  "path": "C:\Users\file.txt",
  "message": "She said "hello""
}

{
  "path": "C:\\Users\\file.txt",
  "message": "She said \"hello\""
}