Developer Guide

How to fix common JSON syntax errors

Troubleshooting RFC 8259 compliance & parser exceptions

1. Trailing commas in arrays and objects

Standard JSON strictly forbids trailing commas before closing braces (}) or brackets (]). While modern JavaScript permits them, standard JSON parsers will throw SyntaxError: Unexpected token.

// ❌ Invalid JSON (Trailing commas)
{
  "status": "success",
  "data": [10, 20, 30, ],
}

// ✅ Correct JSON
{
  "status": "success",
  "data": [10, 20, 30]
}

2. Single quotes instead of double quotes

JSON string values and object keys must always use double quotes ("). Single quotes (') or backticks (`) are invalid in JSON.

// ❌ Invalid JSON
{ 'username': 'ada_lovelace' }

// ✅ Correct JSON
{ "username": "ada_lovelace" }

3. Unquoted object keys

In JavaScript object literals, identifiers like name: "Ada" are legal. In JSON, every object key must be wrapped in double quotes.

// ❌ Invalid JSON
{ id: 101, name: "Ada" }

// ✅ Correct JSON
{ "id": 101, "name": "Ada" }

4. Comments in JSON files

Standard JSON does not allow single-line (//) or multi-line (/* ... */) comments. Strip all comments before parsing or use JSONC only in environments that explicitly support it (like VS Code settings).

5. Python booleans and None values

Python outputs True, False, and None. In standard JSON, these must be lowercase: true, false, and null.

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