TroubleshootingJanuary 10, 2025•7 min read

Common JSON Validation Errors and How to Fix Them

A comprehensive guide to the most common JSON syntax errors and step-by-step solutions to resolve them quickly.

JSON validation errors can be frustrating, especially when you're trying to debug API responses or configuration files. The good news is that most JSON errors fall into predictable categories. This guide covers the most common validation errors you'll encounter and provides clear solutions for each one.

1. Missing or Extra Commas

Comma-related errors are among the most frequent JSON validation issues. JSON requires commas between elements but prohibits trailing commas.

❌ Error: Missing comma between elements

{
  "name": "John Doe"
  "age": 30,
  "city": "New York"
}

Error: Expected ',' or '}' after property value

✅ Fix: Add missing comma

{
  "name": "John Doe",
  "age": 30,
  "city": "New York"
}

❌ Error: Trailing comma

{
  "name": "John Doe",
  "age": 30,
  "city": "New York",
}

Error: Trailing comma in object

✅ Fix: Remove trailing comma

{
  "name": "John Doe",
  "age": 30,
  "city": "New York"
}

2. Unmatched Brackets and Braces

Every opening bracket [ must have a closing bracket ], and every opening brace { must have a closing brace }.

❌ Error: Missing closing brace

{
  "users": [
    {
      "id": 1,
      "name": "John"
    },
    {
      "id": 2,
      "name": "Jane"
    }
  ]
  // Missing closing brace for the main object

Error: Unexpected end of JSON input

✅ Fix: Add missing closing brace

{
  "users": [
    {
      "id": 1,
      "name": "John"
    },
    {
      "id": 2,
      "name": "Jane"
    }
  ]
}

💡 Pro Tip

Use proper indentation to visually track opening and closing brackets. Most code editors can highlight matching brackets when you hover over them.

3. Incorrect String Quotes

JSON strings must use double quotes ("). Single quotes (') are not valid in JSON.

❌ Error: Single quotes used

{
  'name': 'John Doe',
  'age': 30,
  'isActive': true
}

Error: Unexpected token ' in JSON

✅ Fix: Use double quotes

{
  "name": "John Doe",
  "age": 30,
  "isActive": true
}

❌ Error: Unescaped quotes in string

{
  "message": "He said "Hello" to me"
}

Error: Unexpected token H in JSON

✅ Fix: Escape internal quotes

{
  "message": "He said \"Hello\" to me"
}

4. Invalid Property Names

All property names (keys) in JSON must be strings enclosed in double quotes.

❌ Error: Unquoted property names

{
  name: "John Doe",
  age: 30,
  email: "john@example.com"
}

Error: Unexpected token n in JSON

✅ Fix: Quote all property names

{
  "name": "John Doe",
  "age": 30,
  "email": "john@example.com"
}

5. Invalid Value Types

JSON only supports specific value types: strings, numbers, booleans, null, objects, and arrays. JavaScript-specific values like undefined or functions are not valid.

❌ Error: Using undefined

{
  "name": "John Doe",
  "middleName": undefined,
  "age": 30
}

Error: Unexpected token u in JSON

✅ Fix: Use null or omit the property

{
  "name": "John Doe",
  "middleName": null,
  "age": 30
}

❌ Error: Including functions

{
  "name": "John Doe",
  "greet": function() {
    return "Hello!";
  }
}

Error: Unexpected token f in JSON

✅ Fix: Remove functions or represent as strings

{
  "name": "John Doe",
  "greetingMessage": "Hello!"
}

6. Number Format Errors

JSON has specific rules for number formatting. Leading zeros, trailing decimals, and certain notations are not allowed.

❌ Error: Leading zeros

{
  "price": 029.99,
  "quantity": 007
}

✅ Fix: Remove leading zeros

{
  "price": 29.99,
  "quantity": 7
}

❌ Error: Trailing decimal point

{
  "price": 29.,
  "discount": 5.
}

✅ Fix: Complete the decimal or use integers

{
  "price": 29.0,
  "discount": 5
}

7. Control Characters and Encoding Issues

Invisible control characters can cause validation errors that are hard to spot visually.

⚠️ Common control character issues:

BOM (Byte Order Mark): Invisible character at the beginning of files
Non-breaking spaces: Look like regular spaces but aren't
Tab vs. Space mixing: Can cause parsing issues
Line ending inconsistencies: Different systems use different line endings

Solution: Use a proper text editor that can show invisible characters, or copy-paste your JSON into TidyJSON to identify and fix these issues.

8. Debugging Tips and Tools

1. Use Line Numbers

Most JSON validation tools provide line numbers where errors occur. Use these to quickly locate problems in large files.

2. Validate Incrementally

When dealing with large JSON files, comment out sections and validate smaller parts to isolate the problematic area.

3. Use Formatting Tools

Properly formatted JSON makes errors easier to spot. Use tools like TidyJSON to format your JSON and highlight syntax issues.

Try TidyJSON →

4. Browser Developer Tools

Most browser developer consoles can parse JSON. Try JSON.parse(yourString) in the console for quick validation.

Quick Reference: Error Messages

Error Message

Most Likely Cause

"Unexpected end of JSON input"

Missing closing bracket or brace

"Unexpected token ' in JSON"

Using single quotes instead of double quotes

"Expected ',' or '}'"

Missing comma between object properties

"Trailing comma in object"

Extra comma after the last property

"Unexpected token u in JSON"

Using undefined (use null instead)

Conclusion

Most JSON validation errors are straightforward to fix once you know what to look for. The key is understanding JSON's strict syntax rules and using the right tools to identify and resolve issues quickly.

Remember that JSON is less forgiving than JavaScript object literals – it requires double quotes, forbids trailing commas, and doesn't support JavaScript-specific values. When in doubt, use a JSON validator to catch errors before they cause problems in your applications.

💡 Final Tip

Keep TidyJSON bookmarked for quick JSON validation and formatting. It catches syntax errors in real-time and helps you maintain clean, valid JSON in all your projects.