10 Most Common JSON Mistakes Developers Make

JSON (JavaScript Object Notation) is the backbone of modern web development, powering APIs, configuration files, and data storage. However, even experienced developers encounter JSON syntax errors that can bring their applications to a halt.

In this comprehensive guide, we'll explore the 10 most common JSON mistakes developers make and show you how to fix them instantly using our free JSON Fixer tool. Each mistake includes real examples, error messages, and step-by-step solutions.

1Trailing Commas Before Closing Braces/Brackets

One of the most common JSON mistakes is adding a comma after the last item in an object or array. While JavaScript allows trailing commas, JSON does not.

{
  "name": "John",
  "age": 30,
  "city": "New York",  ← Trailing comma error
}

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

2Missing Quotes Around Keys

JSON requires all keys to be enclosed in double quotes. Unquoted keys will cause a parse error.

{
  name: "John",        ← Missing quotes
  age: 30,            ← Missing quotes
  city: "New York"
}

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

3Using Single Quotes Instead of Double Quotes

JSON only accepts double quotes for strings. Single quotes will cause a parse error, even though they're valid in JavaScript.

{
  "name": 'John',      ← Single quotes
  "message": 'Hello World'
}

{
  "name": "John",
  "message": "Hello World"
}

4Missing Closing Braces or Brackets

Every opening brace { or bracket [ must have a corresponding closing brace } or bracket ].

{
  "users": [
    {
      "name": "John",
      "age": 30
    }
  ]
  ← Missing closing brace for root object

{
  "users": [
    {
      "name": "John",
      "age": 30
    }
  ]
}

5Unescaped Special Characters in Strings

Special characters like quotes, backslashes, and newlines must be escaped in JSON strings using backslashes.

{
  "message": "He said "Hello""  ← Unescaped quotes
}

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

6Comments in JSON (Not Allowed)

JSON doesn't support comments. Adding // or /* */ will cause parse errors.

{
  // This is a comment  ← Not allowed
  "name": "John",
  /* Another comment */  ← Not allowed
  "age": 30
}

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

7Missing Commas Between Properties

Properties in JSON objects and items in arrays must be separated by commas. Missing commas will cause parse errors.

{
  "name": "John"
  "age": 30      ← Missing comma
  "city": "New York"
}

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

8Using undefined Instead of null

JSON doesn't have an undefined value. Use null instead.

{
  "name": "John",
  "middleName": undefined  ← Not valid in JSON
}

{
  "name": "John",
  "middleName": null
}

9Nested Structure Errors

Complex nested structures can have mismatched braces, brackets, or missing commas that are hard to spot.

{
  "users": [
    {
      "name": "John",
      "address": {
        "city": "New York"
      }  ← Missing comma before closing
    }
  ]
}

{
  "users": [
    {
      "name": "John",
      "address": {
        "city": "New York"
      }
    }
  ]
}

10Extra Commas or Missing Values

Extra commas or missing values after commas will cause parse errors.

{
  "name": "John",
  "age": 30,
  ,  ← Extra comma
  "city": "New York"
}

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

Best Practices to Avoid JSON Mistakes

Conclusion

JSON mistakes are common, but they don't have to slow you down. By understanding these 10 common errors and using our free JSON Fixer tool, you can fix broken JSON in seconds.

Remember: The best way to avoid JSON errors is to validate early and often. Bookmark our JSON Fixerfor instant error detection and repair whenever you encounter malformed JSON.