JSON Parse Error 'Unexpected token'? 5 Common Causes and How to Fix Them

SyntaxError: Unexpected token is probably the JSON parse error that frontend and backend developers encounter most. The browser throws Unexpected token , in JSON at position 42, Node.js throws Unexpected token } in JSON at position 128 — you can see the position, but often can't find the problem without a JSON Formatter that highlights the exact syntax error.

JSON's syntax is actually very strict, and the causes of parse failure basically come down to a handful. This article rounds up the 5 most common JSON parse errors, each with a wrong example and a fix. Once your JSON parses, you can take validation further with a JSON Schema Validator.

Cause 1: Trailing Comma

This is the most frequent error. JavaScript object literals allow trailing commas, but JSON does not.

// Wrong: an extra comma after the last element
{
  "name": "ToolVault",
  "version": "2.0",
  "features": ["formatter", "validator",],
}

The error is usually Unexpected token } in JSON or Unexpected token ] in JSON, because after the comma the parser expects another value but encounters a closing bracket.

// Correct: remove the trailing comma
{
  "name": "ToolVault",
  "version": "2.0",
  "features": ["formatter", "validator"]
}

Common scenario: When copying an object literal from JavaScript code to use as JSON, it's easy to bring along trailing commas.

Cause 2: Using Single Quotes

The JSON standard only allows double quotes. Single quotes throw Unexpected token ' in JSON directly.

// Wrong: used single quotes
{
  'name': 'ToolVault',
  'description': 'A developer tool'
}

// Correct: use double quotes throughout
{
  "name": "ToolVault",
  "description": "A developer tool"
}

Common scenario: When converting from Python dict output or YAML, it's easy to retain the single-quote habit.

Cause 3: Unescaped Special Characters

Certain characters in JSON strings must be escaped, or the parser will think the string ended prematurely.

// Wrong: the string contains unescaped double quotes and a raw newline
{
  "message": "He said "hello" to me",
  "path": "C:\new\test"
}

First problem: the double quotes around "hello" aren't escaped, so the parser thinks the string ends after He said . Second problem: \n and \t get interpreted as newline and tab characters.

// Correct: escape the special characters
{
  "message": "He said \"hello\" to me",
  "path": "C:\\new\\test"
}

Characters that need escaping: " → \", \ → \\, newline → \n, tab → \t, carriage return → \r.

Common scenario: Concatenating JSON strings without escaping variable values, or dropping a Windows path straight into JSON.

Cause 4: BOM Header (Byte Order Mark)

This is a sneaky one. A UTF-8 file may start with a BOM character (\uFEFF) — invisible to the eye, but the JSON parser throws Unexpected token in JSON at position 0.

// Simulating a JSON string with a BOM
const jsonWithBOM = "\uFEFF{\"name\": \"test\"}";

JSON.parse(jsonWithBOM);
// SyntaxError: Unexpected token  in JSON at position 0

// Fix: strip the BOM
const clean = jsonWithBOM.replace(/^\uFEFF/, "");
JSON.parse(clean); // { name: "test" }

Common scenario: UTF-8 files saved by Windows Notepad include a BOM by default. If your JSON comes from a file read (fs.readFile, a local fetch), a position-0 error is almost certainly a BOM.

// Safe way to read a JSON file in Node.js
const fs = require("fs");
const raw = fs.readFileSync("config.json", "utf-8");
const data = JSON.parse(raw.replace(/^\uFEFF/, ""));

Cause 5: Comments in JSON

The JSON standard does not support any form of comments. Both // and /* */ cause parsing to fail.

// Wrong: contains comments
{
  // This is the server address
  "host": "api.example.com",
  "port": 8080 /* default port */
}

The error is usually Unexpected token / in JSON at position X.

// Correct: remove all comments
{
  "host": "api.example.com",
  "port": 8080
}

Common scenario: Parsing a .jsonc (VS Code config format) or a config file with comments directly as JSON. If you genuinely need comments, consider JSON5 or YAML.

// If you must handle JSON with comments, strip them first (simple cases)
const stripComments = (str) =>
  str.replace(/\/\/.*$/gm, "").replace(/\/\*[\s\S]*?\*\//g, "");

JSON.parse(stripComments(jsonWithComments));

Quick Localization and Fix

When you hit an Unexpected token error, troubleshoot in this order:

1. Look at the position number in the error and locate the specific character.
2. Check whether the area around that position has any of the 5 issues above.
3. If position is 0, check for a BOM first.
4. If the JSON was generated by code, check the escaping logic in the concatenation.

For long JSON, manual inspection is painfully slow. Paste the content into the JSON Formatter — it validates syntax in real time, locally in your browser, pinpoints the exact error location, and auto-formats valid JSON. All processing happens locally, so you don't need to worry about API data or config leaking to a third-party server.

Position math that trips people up

Error offsets count characters in the string the parser received — not "visual columns" after your editor wrapped lines, and not bytes if you mixed encodings. A UTF-8 multi-byte character still advances the JSON character index by one code unit in JavaScript strings. If the reported position looks "early," check whether a proxy prepended a BOM or a logging wrapper prepended a status line before the JSON body. When the token is literally <, stop debugging JSON syntax: you are almost certainly looking at an HTML error page (see the companion article linked below).

FAQ

What does Unexpected token in JSON mean?

The parser hit a character that is illegal at that position — often a trailing comma, a single quote, an unescaped control character, a BOM at position 0, or a comment (/ ).

How do I fix "Unexpected token is not valid JSON"?

1. Jump to the reported position.
2. Check the five causes above.
3. If the token is '<', the body is HTML (wrong URL / auth / proxy) — see 5 reasons your API returned HTML.
4. Paste into the JSON Formatter to highlight the first error instantly.

Why does JavaScript allow trailing commas but JSON.parse does not?

Object / array literals in JS are a different grammar from JSON. Always JSON.stringify when emitting JSON, or strip trailing commas before parse.

Can comments exist in JSON?

Not in standard JSON. Use JSONC/JSON5/YAML if you need comments, or strip them before JSON.parse.

Provided by ToolVault. If the offending token is '<' (HTML instead of JSON), see 5 reasons your API returned HTML. Related tools: JSON Formatter, JSON Diff, JSON Schema Validator. Related reading: JSON formatting guide. See the homepage for more developer tools.

Implementer's note

Building the JSON formatter taught us the top need is not formatting but error localization: an Unexpected token without line/column says nothing. We wrap the parser with position extraction, converting token offsets into line/column and highlighting them on the source — so the debugging path in this article completes inside the tool itself.