Common JSON Syntax Errors and How to Fix Them

1 Trailing Commas

A trailing comma after the last item in an array or object is the single most common JSON error. JSON doesn't allow it — unlike JavaScript object literals, which do.

{
  "name": "Alice",
  "role": "engineer",  // ← trailing comma
}

{
  "name": "Alice",
  "role": "engineer"
}

This error frequently happens when copying JS object syntax into a .json file, or when adding/removing items from a list. A JSON formatter will catch this immediately with a line number.

2 Unquoted Property Names

All JSON property names must be wrapped in double quotes. Unquoted keys (allowed in JavaScript) cause parsing to fail.

{
  name: "Alice",
  age: 30
}

{
  "name": "Alice",
  "age": 30
}

This often happens when hand-writing JSON or copying console output (which uses JS syntax). The fix is mechanical: quote every key.

3 Single Quotes Instead of Double Quotes

JSON only accepts double quotes (") for strings and keys. Single quotes (') are invalid.

{
  'name': 'Alice',
  'city': 'London'
}

{
  "name": "Alice",
  "city": "London"
}

4 Using JavaScript Values (undefined, functions, etc.)

JSON only supports six value types: strings, numbers, booleans, null, arrays, and objects. Using undefined, NaN, Infinity, or functions will break parsing.

{
  "score": NaN,
  "active": undefined,
  "value": Infinity
}

{
  "score": null,
  "active": false,
  "value": null
}

Numbers in JSON cannot have leading zeros (01 is invalid — use 1) and must be base-10. This error often surfaces when serializing JavaScript objects that contain undefined properties.

5 Adding Comments

JSON does not support comments. Both // and /* */ style comments cause syntax errors.

{
  // User configuration
  "name": "Alice",
  "port": 8080 /* default port */
}

{
  "name": "Alice",
  "port": 8080
}

If you need documentation in your data file, consider moving comments to a separate file or switching to a format that supports them natively, such as JSON5 (which extends JSON with comments) or YAML.

6 Unescaped Special Characters in Strings

Certain characters must be escaped inside JSON string values: backslash (\), double quote ("), newline (\n), tab (\t), and carriage return (\r).

{
  "path": "C:\Users\alice",
  "note": "She said "hi""
}

{
  "path": "C:\\Users\\alice",
  "note": "She said \"hi\""
}

This error is common when JSON contains file paths (Windows backslashes) or user-generated content (quotes in text). When building JSON programmatically, use your language's built-in serializer (JSON.stringify(), json.dumps()) to handle escaping automatically.