Unexpected token , in JSON at position 0: what it means and how to fix it

What the error means

Unexpected token , in JSON at position 0 means the parser expected valid JSON but encountered a character/token that cannot appear there. In practice, it usually means the input is not JSON (or not strict JSON), or it is incomplete.

Most common real-world causes

• There is a leading comma (often from concatenation or a stray delimiter).
• You removed a key/value but left the comma behind.
• A JSON fragment got pasted without surrounding {} or [].

Fast debugging steps

• Search around the reported position for a stray comma.
• Arrays/objects must not start with a comma.
• Validate after each fix to confirm progress.

Code example (javascript)

// Not valid
const bad = ',{"a":1}';

// Fix: remove the comma
const ok = '{"a": 1}';
JSON.parse(ok);

Fix without uploading data

If the JSON contains sensitive data, validate and fix it locally. No Upload Tools runs 100% in your browser.

• JSON Validator to pinpoint the exact syntax error.
• JSON Repair to remove comments/trailing commas when the source is not strict.
• JSON Formatter to pretty-print and inspect structure.
• JSON String Escape if the issue is inside a string (newlines/tabs/quotes).

Workflow: validate -> fix the first error -> validate again -> export/convert.

FAQ

Does the exact token matter? Yes. The token often hints at the root cause: < is usually HTML, u is often undefined, and / often points to comments.

Should I just regex-fix JSON? Avoid blind regex edits. Validate after each change so you know what you fixed and what broke.