Fix "Unexpected token" errors in JSON

What the error message tells you

Modern V8 (Chrome / Node) reports errors like:

SyntaxError: Unexpected token } in JSON at position 42 (line 4 column 1)

Three pieces matter:

• The token character (} here) — the character that broke the parser, not necessarily the character that's wrong. The actual mistake is usually just before this position.
• The position — a zero-indexed character offset into the input.
• Line and column — what humans want. Older Node versions and Safari only give you the position; you have to walk the input yourself to translate.

Our linter does the translation for you and shows the exact line and column for any error.

The seven most common causes

1. Trailing comma

By far the most frequent cause. JSON does not allow a trailing comma before } or ]. Many programmers reach for JSON after writing JavaScript object literals (which do allow it) and get caught.

{
  "name": "Eli",
  "items": [1, 2, 3,]   ← trailing comma after 3
}

Fix: remove the comma. If you can't control the producer, run the input through a JSON5 parser instead.

2. Single quotes instead of double

JSON strings must use double quotes. Single quotes are valid JS but invalid JSON.

{ 'name': 'Eli' }

Fix: replace all single quotes with double. If your data contains legitimate apostrophes, escape them: "can't" or leave them inside double-quoted strings unescaped.

3. Unquoted keys

JS lets you write { name: "Eli" }. JSON requires every key to be a quoted string.

{ name: "Eli" }     ← keys must be in double quotes

4. Comments

JSON has no comment syntax. Both // and /* */ will trip JSON.parse. If you need comments, use JSONC (used by VS Code's settings) or JSON5 — but most APIs reject both.

5. The response wasn't JSON at all

If you see Unexpected token < in JSON at position 0, the response started with < — almost always an HTML error page from a misbehaving API. Check status code and Content-Type before parsing:

const res = await fetch(url);
if (!res.ok) throw new Error(`HTTP ${res.status}`);
if (!res.headers.get("content-type")?.includes("application/json")) {
  throw new Error("Expected JSON, got " + res.headers.get("content-type"));
}
return res.json();

Unexpected token U at position 0 usually means the input was the literal string undefined. Look for an upstream function that returned undefined instead of a JSON payload (e.g. localStorage.getItem for a missing key).

6. BOM or hidden whitespace

UTF-8 byte-order marks () at the start of a file will trip JSON.parse. Files saved by some Windows tools include a BOM by default. Strip it before parsing:

JSON.parse(text.replace(/^/, ""))

7. Mismatched / missing brackets

A missing } or ] often surfaces with the token reported as the very next character (or end of input) — far from where the actual mistake is. The linter helps by highlighting the exact line where the parser gave up so you can scan upward.

Three rules that prevent most of these

1. Don't hand-write JSON for anything more than a one-line config. Use JSON.stringify(value, null, 2) in your producer.
2. Lint at the boundary. Validate JSON immediately after fetching, reading from disk, or receiving from a queue — before passing it deeper into your code.
3. Use a typed schema (Zod, valibot, JSON Schema) once the shape matters. Catches the cases JSON.parse can't, like a missing required field or wrong type.

Reference

Strict JSON is defined by RFC 8259 (the current standard, replacing 7159 / 4627). Common relaxed variants:

• JSONC — JSON with comments. Used by VS Code's settings.json and tsconfig.json. No trailing commas.
• JSON5 — comments, trailing commas, single quotes, unquoted keys. Closer to JS object literal syntax.
• NDJSON / JSON Lines — one JSON value per line, no outer array. Great for streaming. Each line is parsed individually.

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