Common JSON Errors and How to Fix Them

You might have noticed that JavaScript and many modern programming languages permit trailing commas, but JSON specification is stricter. A trailing comma after the final property or array element will cause parsing to fail.

{
  "name": "John",
  "email": "john@example.com",
  "age": 28,
}

{
  "name": "John",
  "email": "john@example.com",
  "age": 28
}

The same rule applies to arrays:

[
  "apple",
  "banana",
  "orange",
]

[
  "apple",
  "banana",
  "orange"
]

This confusion often arises because JavaScript uses both single and double quotes interchangeably. However, JSON is stricter and only accepts double quotes to delimit strings and keys.

{
  'title': 'Welcome',
  'description': 'This is an article',
  'author': 'Jane Doe'
}

{
  "title": "Welcome",
  "description": "This is an article",
  "author": "Jane Doe"
}

JavaScript's object literal syntax allows you to omit quotes for simple keys, but JSON does not. Every key must be wrapped in double quotes.

{
  name: "Alice",
  email: "alice@example.com",
  isActive: true
}

{
  "name": "Alice",
  "email": "alice@example.com",
  "isActive": true
}

When you have multiple properties or array elements, each must be separated by a comma. Omitting these separators is a common typo that breaks the entire document.

{
  "id": 1
  "name": "Product A"
  "price": 29.99
}

{
  "id": 1,
  "name": "Product A",
  "price": 29.99
}

The same applies to array elements:

[
  { "id": 1, "name": "Item 1" }
  { "id": 2, "name": "Item 2" }
  { "id": 3, "name": "Item 3" }
]

[
  { "id": 1, "name": "Item 1" },
  { "id": 2, "name": "Item 2" },
  { "id": 3, "name": "Item 3" }
]

While JSON5 (an extended specification) does support comments, strict JSON does not. If you need configuration with comments, consider using YAML, TOML, or JSON5 instead. Alternatively, document your JSON structure separately.

{
  // This is the user ID
  "id": 42,
  /* User's full name */
  "name": "Bob Smith"
}

{
  "id": 42,
  "name": "Bob Smith",
  "_comment": "This is the user record for Bob Smith"
}

If you need to preserve documentation, consider keeping comments in a separate markdown file, or use a configuration format that supports comments like YAML or TOML.

In JavaScript, undefined is a valid value, but JSON has no representation for it. When you encounter an undefined value, use null instead, which is JSON's explicit way to represent "no value."

{
  "name": "Charlie",
  "nickname": undefined,
  "age": null
}

{
  "name": "Charlie",
  "nickname": null,
  "age": null
}

Another approach is to simply omit the property if it has no value:

{
  "name": "Charlie",
  "age": null
}

JSON numbers should not have leading zeros. If you're formatting numbers like IDs or codes with leading zeros, store them as strings instead.

{
  "zipcode": 01234,
  "employeeId": 007,
  "productCode": 0456
}

{
  "zipcode": "01234",
  "employeeId": "007",
  "productCode": "0456"
}

Alternatively, if these are genuinely numeric values without leading zeros:

{
  "zipcode": 1234,
  "employeeId": 7,
  "productCode": 456
}

Special characters need to be properly escaped using backslash notation. This includes quotes, newlines, tabs, and other control characters.

{
  "quote": "She said "Hello"",
  "path": "C:\Users\Documents",
  "multiline": "Line 1
Line 2"
}

{
  "quote": "She said \"Hello\"",
  "path": "C:\\Users\\Documents",
  "multiline": "Line 1\nLine 2"
}

Common escape sequences in JSON:

• \" — Double quote
• \\ — Backslash
• \n — Newline
• \t — Tab
• \r — Carriage return
• \b — Backspace
• \f — Form feed
• \uXXXX — Unicode character (where XXXX is the hex code)