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TroubleshootingMay 12, 2026·5 min read

Common JSON Errors and How to Fix Them

A comprehensive guide to the most common JSON syntax errors, why they happen, and how to quickly fix them using our tools.

If you've worked with JSON, you've likely encountered the dreaded "Unexpected token" or "Invalid JSON" error. These errors can be frustrating, especially when the problem isn't immediately obvious. Let's look at the most common JSON errors and how to fix them.

1. Trailing Commas

One of the most common errors. JSON does not allow a comma after the last element in an array or object, unlike JavaScript.

// Invalid - trailing comma in array
{
  "colors": ["red", "green", "blue",]
}

// Invalid - trailing comma in object
{
  "name": "John",
  "age": 30,
}

// Valid - no trailing commas
{
  "colors": ["red", "green", "blue"]
}

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

2. Single Quotes Instead of Double Quotes

JSON requires double quotes for strings. Single quotes are not valid, even though they work in JavaScript.

// Invalid - single quotes
{
  'name': 'John',
  'email': '[email protected]'
}

// Valid - double quotes
{
  "name": "John",
  "email": "[email protected]"
}

3. Unquoted Property Names

In JSON, all property names (keys) must be enclosed in double quotes. JavaScript allows unquoted keys, but JSON does not.

// Invalid - unquoted keys
{
  name: "John",
  age: 30
}

// Valid - quoted keys
{
  "name": "John",
  "age": 30
}

4. Comments

JSON does not support comments. If you need to include comments, consider using a dedicated field or switching to a format like JSON5 or JSONC.

// Invalid - comments not allowed
{
  // User information
  "name": "John",
  /* This is the user's age */
  "age": 30
}

// Valid - use a description field instead
{
  "_comment": "User information",
  "name": "John",
  "age": 30
}

5. Missing or Extra Brackets/Braces

Every opening bracket or brace must have a corresponding closing one. This error is common when manually editing JSON.

// Invalid - missing closing brace
{
  "user": {
    "name": "John"
}

// Invalid - extra bracket
{
  "items": ["a", "b", "c"]]
}

6. Incorrect Boolean or Null Values

JSON boolean and null values must be lowercase. Other forms like True, TRUE, False, FALSE, None, or NULL are invalid.

// Invalid - wrong casing
{
  "isActive": True,
  "isAdmin": FALSE,
  "middleName": None
}

// Valid - lowercase
{
  "isActive": true,
  "isAdmin": false,
  "middleName": null
}

7. Missing Commas Between Elements

Each element in an array or property in an object must be separated by a comma.

// Invalid - missing comma
{
  "name": "John"
  "age": 30
}

// Invalid - missing comma in array
["apple" "banana" "cherry"]

8. Invalid Escape Sequences

Special characters in strings must be properly escaped. Common escape sequences include \" for quotes, \\ for backslash, and \n for newline.

// Invalid - unescaped quotes
{
  "message": "He said "Hello""
}

// Valid - properly escaped
{
  "message": "He said \"Hello\""
}

9. Leading Zeros in Numbers

JSON does not allow leading zeros in numbers (except for decimal numbers less than 1).

// Invalid - leading zeros
{
  "code": 007,
  "zipCode": 01234
}

// Valid - no leading zeros (use strings if needed)
{
  "code": 7,
  "zipCode": "01234"
}

10. Undefined Values

JavaScript's undefined is not valid in JSON. Use null instead, or omit the property.

// Invalid
{
  "value": undefined
}

// Valid
{
  "value": null
}

Debugging Tips

Use our JSON Formatter to validate and find errors
Check the error message for line numbers and character positions
Look for the common issues listed above
Use our JSON Fixer to automatically repair simple errors
When copying from code, ensure you're not including JavaScript-specific syntax

Conclusion

Most JSON errors are simple syntax mistakes that are easy to fix once you know what to look for. Bookmark this page for quick reference, and use our free tools to validate and repair your JSON data.

Fundamentals