Fix JSON Errors: Complete Guide

A JSON parse error happens when JSON.parse() gets a string that isn’t valid JSON. "Unexpected token" means the parser hit a character it didn’t expect (e.g. single quote, comma, or < from HTML). JSON allows only double-quoted strings, no trailing commas, and no comments.

Common cases: parsing API responses (malformed or HTML), user input, localStorage, or files. Also when JSON.stringify drops undefined and you expect the key to be present.

1. Fix syntax: trailing commas, quotes, comments

Remove trailing commas, use double quotes for all strings and keys, and remove comments. Example:

// ❌ Invalid
JSON.parse('{"name": "John",}');
// ✅ Valid
JSON.parse('{"name": "John"}');

2. Unexpected token < (API returns HTML)

The error "Unexpected token <" usually means the server returned HTML (e.g. 404 or login page) instead of JSON. Fix: check Content-Type and only parse when it’s JSON:

const res = await fetch(url);
const ct = res.headers.get('content-type');
if (!ct?.includes('application/json')) {
  const text = await res.text();
  throw new Error('Not JSON: ' + text.slice(0, 100));
}
const data = await res.json();

3. JSON.stringify returns undefined / missing keys

JSON.stringify skips properties with value undefined. To keep keys, use a replacer:

const obj = { a: 1, b: undefined };
JSON.stringify(obj); // '{"a":1}' — b is missing

// To output undefined as null:
JSON.stringify(obj, (k, v) => v === undefined ? null : v);
// '{"a":1,"b":null}'

4. Use try-catch and validate

function parseJSONSafe(str) {
  try {
    return { ok: true, data: JSON.parse(str) };
  } catch (e) {
    return { ok: false, error: e.message };
  }
}

Common Error Examples

JSON.parse('{"x": 1,}');  // ❌
JSON.parse('{"x": 1}');   // ✅

Free tools

Use our JSON Beautifier and JSON Fixer to validate and fix JSON in the browser.