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10 JSON Validation Mistakes to Avoid

January 15, 2025 7 min read

1. Using Single Quotes

❌ Wrong: {'name': 'John'}
✅ Correct: {"name": "John"}

JSON requires double quotes for both keys and string values.

2. Trailing Commas

❌ Wrong: {"name": "John", "age": 30,}
✅ Correct: {"name": "John", "age": 30}

Trailing commas are not allowed in JSON (unlike JavaScript).

3. Unquoted Keys

❌ Wrong: {name: "John"}
✅ Correct: {"name": "John"}

4. Using undefined

❌ Wrong: {"value": undefined}
✅ Correct: {"value": null}

JSON doesn't support undefined. Use null instead.

5. Comments in JSON

❌ Wrong: {"name": "John" /* comment */}
✅ Correct: {"name": "John"}

Standard JSON doesn't support comments. Use JSON5 if you need comments.

6. Not Validating Before Parsing

// Always use try-catch
try {
  const data = JSON.parse(jsonString);
} catch (error) {
  console.error('Invalid JSON:', error);
}

7. Trusting External Data

Always validate structure and types after parsing:

const data = JSON.parse(jsonString);
if (typeof data.age !== 'number') {
  throw new Error('Invalid data type');
}

8. Ignoring Character Encoding

Always use UTF-8 encoding for JSON. Specify charset in HTTP headers:

Content-Type: application/json; charset=utf-8

9. Not Handling Large Numbers

JavaScript's Number type has limits. Large integers may lose precision:

// Use strings for large numbers
{"id": "9007199254740993"}

10. Forgetting to Escape Strings

Special characters must be escaped:

{"text": "Line 1\\nLine 2\\tTabbed"}

Best Practices

  • Use a linter (ESLint, JSONLint)
  • Validate against JSON Schema
  • Use TypeScript for type safety
  • Test with edge cases
Pro Tip: Use our JSON Validator to catch these mistakes automatically!
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