“Unexpected token in JSON”: what it means and how to fix it

Your code called JSON.parse or response.json() and got an error that seems to blame a random character. The message is usually telling you something quite specific.

Read the message first

Older browsers word it as Unexpected token < in JSON at position 0; the meaning is the same. The position is a character offset from the start, counting from 0.

Position 0: you did not get JSON

This is the most common case and the JSON is not the problem. Log the raw text before parsing it:

const res = await fetch(url);
const text = await res.text();
console.log(res.status, res.headers.get("content-type"), text.slice(0, 200));
const data = JSON.parse(text);

Typical culprits: a wrong URL that falls through to your single-page app's index.html, a dev-server proxy that is not set up, an expired session redirecting to a login page, a server error page, or an API that answers errors in plain text. Check res.ok before parsing, and look at the Network tab in developer tools for the real response.

Further in: the JSON itself is broken

JSON is much stricter than JavaScript. These are all invalid:

{ "a": 1, }          // trailing comma
{ 'a': 1 }           // single quotes
{ a: 1 }             // unquoted key
{ "a": 1 } // note   // comments
{ "a": NaN }         // NaN, Infinity and undefined are not JSON
{ "path": "C:\new" }  // a lone backslash must be written \\

Paste the text into the JSON formatter: it uses the browser's own strict parser and shows the line and column of the first problem, which is easier to read than a raw position in a one-line file. If you are not sure what the text even is, What is this? will tell you whether it is JSON, YAML, HTML, Base64 or something else.

Other sneaky causes

• A byte-order mark (an invisible character) at the start of a file saved by some Windows editors.
• Parsing twice: response.json() already returns an object; calling JSON.parse on that object turns it into the text "[object Object]" first, which fails.
• Several JSON objects in one file, one per line (JSON Lines). Parse each line separately.
• Config files with comments such as tsconfig.json are JSONC, not JSON; a plain parser rejects them. If you want comments and no quotes, YAML may suit better, and the YAML to JSON converter moves between the two.

Does the file contain lines like <<<<<<< HEAD or =======? A git merge left conflict markers behind: see git merge conflict markers.

Empty or cut-off body instead of a bad token? See Unexpected end of JSON input.

Questions

Published 3 November 2026.