# SyntaxError: Unexpected token < in JSON at position 0

**Code:** `SyntaxError: JSON` · **Category:** JavaScript · **Seen on:** JavaScript, PHP, React

`JSON.parse()` / `response.json()` received something that isn’t JSON — very often an HTML error page.

## Meaning

The `<` at position 0 is the start of `<!DOCTYPE html>` or `<html>`: the server returned an HTML page (404, 500, login redirect, SPA fallback) where your code expected JSON. Newer browsers say “Unexpected token '<', \"<!DOCTYPE \"... is not valid JSON”.

## Common causes

- Wrong API URL — the request hits the frontend’s index.html fallback
- Server error returned an HTML error page (500, 502, 404)
- Authentication redirect to a login page
- Dev server not proxying `/api` to the backend
- PHP warnings/notices printed before the JSON output
- Empty response body (“Unexpected end of JSON input”)

## Quick fix

1. Log `await res.text()` instead of `res.json()` to see what actually came back
2. Check `res.ok` and `Content-Type` before parsing
3. Verify the full request URL in the Network tab
4. Turn off `display_errors` in production PHP so warnings don’t corrupt JSON

## Detailed fix by platform

### JavaScript

1. Parse defensively:

```javascript
const res = await fetch('/api/items');
const type = res.headers.get('content-type') || '';
if (!res.ok || !type.includes('application/json')) {
  throw new Error(`Expected JSON, got ${res.status} ${type}: ${(await res.text()).slice(0, 200)}`);
}
const data = await res.json();
```


### PHP

1. Send `header('Content-Type: application/json')`, echo only `json_encode(...)`, and make sure no whitespace/BOM or warnings are printed before it.

### React

1. Vite: add `server: { proxy: { '/api': 'http://localhost:8000' } }` so API calls don’t hit index.html.

## How to diagnose

1. **Raw body** — What does res.text() contain?
2. **Status** — Is it a 4xx/5xx/3xx?
3. **URL** — Is the path/host correct?
4. **Server output** — Any warnings printed before JSON?

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Source: [WhatIsMyError](https://whatismyerror.com/errors/javascript/syntaxerror-unexpected-token-json)