Common JSON Syntax Errors and How to Fix Them: Unexpected Token, Trailing Commas, and Unquoted Keys

Every developer has encountered the dreaded JavaScript error: SyntaxError: Unexpected token in JSON at position 142. Because JSON is derived from JavaScript object literal syntax, developers often assume it follows the same forgiving rules. It does not.

JSON (defined formally by RFC 8259 and ECMA-404) is an exceptionally strict data interchange format. A single stray comma, an unescaped newline, or single quotation marks will instantly break parsers in Python, Go, Java, and browser environments.

This guide highlights the most frequent JSON syntax mistakes, explains why standard parsers reject them, and demonstrates how to repair them instantly using our private, client-side JSON Formatter and Validator.

Valid JSON vs JavaScript Object Literals: The Key Differences

The core source of confusion is that valid JavaScript syntax is frequently invalid JSON:

The 4 Most Common JSON Syntax Errors and Solutions

1. Trailing Commas in Arrays and Objects

In modern ECMAScript, trailing commas make git diffs cleaner. But in JSON, placing a comma after the final property throws an immediate parse error:

// INVALID JSON
{
  "api_version": 2,
  "status": "active",
}

// VALID JSON
{
  "api_version": 2,
  "status": "active"
}

2. Single Quotes Instead of Double Quotes

Python dictionaries and JavaScript strings allow single quotes. JSON strictly mandates double quotes (ASCII 34) for both property keys and string values:

// INVALID JSON
{ 'user': 'dev_401' }

// VALID JSON
{ "user": "dev_401" }

3. Unescaped Control Characters and Newlines

If your string contains raw line breaks or tabs, the parser will fail with an unexpected token error. String content must use escape sequences such as \n for newlines and \t for tabs:

// INVALID JSON
{
  "description": "First line
Second line"
}

// VALID JSON
{
  "description": "First line\nSecond line"
}

4. Number Formatting Rules (Leading Zeroes and Hexadecimals)

JSON numbers cannot have leading zeroes (e.g. 05 is rejected), cannot start with a decimal point without a leading zero (e.g. .5 must be written as 0.5), and do not support hexadecimal notation (0xFF).

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