How to Fix "Unexpected Token" in JSON: A Complete Debug Guide

If you've ever called JSON.parse() and seen SyntaxError: Unexpected token < in JSON at position 0, you're in the right place. This is the single most searched JSON error on the internet — and it has a handful of well-known root causes, all of which are fixable in minutes.

What Does "Unexpected Token" Actually Mean?

The JSON parser reads your string one character at a time. When it encounters a character it didn't expect at that position — an angle bracket <, a single quote ', a letter, or even a space in the wrong place — it throws a SyntaxError and stops immediately. The position number in the error (e.g. at position 0) tells you exactly where parsing broke.

The 5 Most Common Causes

1. The API returned HTML instead of JSON

This is the #1 cause of Unexpected token < at position 0. Your server returned an HTML error page (404, 500, or a login redirect) instead of the JSON your code expected. The very first character of HTML is < — which is not valid JSON.

const res = await fetch('/api/data');
// Always check status BEFORE parsing:
if (!res.ok) throw new Error(`HTTP ${res.status}`);
const data = await res.json(); // now safe

2. Single quotes instead of double quotes

JSON requires double quotes (") for both keys and string values. Single quotes are valid JavaScript but are not valid JSON.

{ 'name': 'Alice' }   // ❌ single quotes

{ "name": "Alice" }   // ✅ double quotes

3. Trailing commas

JavaScript objects and arrays allow trailing commas, but JSON strictly forbids them. Hand-edited config files are the most common offender.

{
  "host": "localhost",
  "port": 3000,   // ❌ trailing comma
}

4. Comments in JSON

Unlike JavaScript or YAML, JSON has no comment syntax. Any // or /* */ comment will cause the parser to fail immediately.

5. Byte Order Mark (BOM) in the file

Files saved with a UTF-8 BOM start with invisible bytes that look like garbage to the JSON parser. This is common with files created in Windows Notepad. The fix is to re-save the file as UTF-8 without BOM.

How to Fix It Fast

• Paste your JSON into our JSON Validator — it pinpoints the exact line and character causing the error.
• Check the Network tab in DevTools if the JSON comes from an API call.
• Log the raw string before parsing: console.log(typeof raw, raw.slice(0, 50)) to confirm you're actually getting JSON.
• Use optional chaining and try/catch around JSON.parse() to handle errors gracefully in production.

function safeParseJSON(str) {
  try {
    return { data: JSON.parse(str), error: null };
  } catch (err) {
    console.error('JSON parse failed:', err.message, '\nInput:', str.slice(0, 100));
    return { data: null, error: err.message };
  }
}