How to Fix Common JSON Errors: Unexpected Token, Trailing Comma & More

Why JSON parsing is so strict

Unlike JavaScript object literals, JSON has no tolerance for shortcuts. There are no trailing commas, no unquoted keys, no single-quoted strings, and no comments. JSON.parse() throws the instant it hits something outside the spec, and the error message tells you roughly where — but not always why. The fixes below cover the errors you'll run into most.

1. Trailing comma after the last item

The most common mistake of all. A comma left after the final property or array item is invalid JSON, even though it's fine in JavaScript.

// Invalid — trailing comma after "active": true
{
  "name": "Ada",
  "active": true,
}

// Valid
{
  "name": "Ada",
  "active": true
}

2. Unquoted or single-quoted keys

JSON keys must be wrapped in double quotes — not single quotes, and not left bare like a JavaScript identifier.

// Invalid
{ name: "Ada", 'role': "developer" }

// Valid
{ "name": "Ada", "role": "developer" }

3. Unclosed brackets or braces

A missing closing } or ] usually produces "Unexpected end of JSON input" — the parser ran out of text before every bracket it opened was closed. Count your opening and closing brackets, especially in deeply nested objects.

// Invalid — missing closing brace for "address"
{
  "name": "Ada",
  "address": { "city": "London"
}

// Valid
{
  "name": "Ada",
  "address": { "city": "London" }
}

4. "Unexpected token" at a specific character

This is the parser's way of saying "I found a character here that doesn't belong." The most frequent causes are a stray comma, a missing comma between two properties, or a value that isn't valid JSON (like undefined or a bare word).

// Invalid — missing comma between properties
{
  "name": "Ada"
  "role": "developer"
}

// Invalid — undefined is not valid JSON
{ "middleName": undefined }

// Valid
{
  "name": "Ada",
  "role": "developer",
  "middleName": null
}

5. Comments

JSON has no comment syntax at all — neither // nor /* */ is allowed, and including one will fail parsing every time, even if every other character is valid.

// Invalid — JSON doesn't support comments
{
  // this is the user's name
  "name": "Ada"
}

The fastest way to find the mistake

Rather than scanning line by line, paste the JSON into a formatter that surfaces the exact parse error. Our JSON Formatter & Validator highlights exactly where parsing failed as soon as you paste, so you can jump straight to the offending character instead of guessing.