Common JSON Errors and How to Fix Them

Common JSON Errors

JSON's strict syntax rules mean that even a single misplaced character can break an entire document. Understanding the most common JSON errors will help you debug faster and avoid repeating the same mistakes. Below are the most frequent issues developers encounter, with clear explanations and fixes for each.

1. Trailing Commas

Error: Unexpected token at end of object or array

Trailing commas are permitted in JavaScript objects and arrays but are strictly forbidden in JSON.

Invalid JSON:

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

Fix: Remove the comma after the last item.

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

Arrays with trailing commas:

// Invalid
[1, 2, 3,]

// Valid
[1, 2, 3]

2. Missing or Incorrect Quotes Around Keys

Error: Unexpected identifier or Expected double-quoted property name

JSON requires all object keys to be wrapped in double quotes. Single quotes and unquoted keys are not valid, even though they work in JavaScript.

Invalid JSON:

{
  name: "Alice",
  'occupation': "Engineer"
}

Fix: Always wrap keys in double quotes.

{
  "name": "Alice",
  "occupation": "Engineer"
}

3. Using Single Quotes for String Values

Error: Unexpected identifier or Unterminated string

Just like keys, string values must use double quotes in JSON. Single quotes are not valid.

Invalid JSON:

{ "greeting": 'Hello, world!' }

Fix: Replace single quotes with double quotes.

{ "greeting": "Hello, world!" }

Tip: If your string contains double quotes, escape them with a backslash: "He said, \"Hello\"".

4. Unescaped Special Characters

Error: Unexpected token or Invalid character

Certain characters must be escaped in JSON strings: double quotes ("), backslashes (\), and control characters.

Invalid JSON:

{
  "path": "C:\Users\Documents\file.json",
  "quote": "He said "hello""
}

Fix: Escape backslashes and double quotes properly.

{
  "path": "C:\\Users\\Documents\\file.json",
  "quote": "He said \"hello\""
}

Complete List of JSON Escape Sequences

| Sequence | Represents | |----------|-----------| | \" | Double quote | | \\ | Backslash | | \/ | Forward slash | | \b | Backspace | | \f | Form feed | | \n | Newline | | \r | Carriage return | | \t | Tab | | \uXXXX | Unicode character (e.g., é for e) |

5. Invalid Numeric Formats

Error: Unexpected number or Invalid number

JSON has specific rules for how numbers can be formatted:

• Leading zeros are not allowed (01 is invalid)
• A single leading zero is allowed only for the number 0
• Numbers cannot have trailing dots (10. is invalid)
• Hexadecimal, octal, and binary formats are not supported

Invalid JSON:

{
  "invalid1": 01,
  "invalid2": 10.,
  "invalid3": 0xFF,
  "invalid4": 1,000
}

Fix:

{
  "valid1": 1,
  "valid2": 10.0,
  "valid3": 255,
  "valid4": 1000
}

6. Invalid Values: undefined, NaN, Infinity

Error: Unexpected token for undefined, NaN, Infinity

JavaScript has values that have no JSON equivalent. These will cause parsing errors:

Invalid JSON:

{
  "undefinedVal": undefined,
  "notANumber": NaN,
  "infinity": Infinity
}

Fix: Replace with valid JSON values or use a convention.

{
  "undefinedVal": null,
  "notANumber": null,
  "infinity": 1.7976931348623157e+308
}

7. Extra Commas or Missing Commas

Error: Expected comma or Unexpected token

Commas in JSON must separate every key/value pair and array element, but must not appear after the last item.

Missing comma (invalid):

{
  "name": "Alice"
  "age": 30
}

Fix: Add the missing comma.

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

8. Deeply Nested Structure Issues

Error: Maximum call stack size exceeded or parser timeout

While not a syntax error, deeply nested JSON (10+ levels) can cause performance issues and make debugging extremely difficult.

Problematic structure:

{
  "level1": {
    "level2": {
      "level3": {
        "level4": {
          "level5": {
            "level6": {
              "value": "too deep"
            }
          }
        }
      }
    }
  }
}

Better approach: Flatten the structure or use references.

{
  "users": {
    "alice": { "role": "admin" }
  },
  "permissions": {
    "admin": ["read", "write", "delete"]
  }
}

9. Missing Opening or Closing Braces

Error: Expected ',' or '}' or Unexpected end of JSON input

Unmatched braces and brackets are a common source of JSON errors, especially in large or hand-written JSON files.

Invalid JSON:

{
  "name": "Alice",
  "items": [1, 2, 3
}

Fix: Ensure every { has a matching }, and every [ has a matching ].

{
  "name": "Alice",
  "items": [1, 2, 3]
}

Pro tip: Indent your JSON properly to visually match opening and closing brackets. A JSON formatter can do this automatically.

How to Debug JSON Errors Effectively

1. Use a JSON validator -- Paste your JSON into a free JSON validator to get the exact line and column of the first error.
2. Validate incrementally -- For large files, validate one section at a time to isolate the error.
3. Watch for copy-paste artifacts -- Invisible characters, smart quotes, and non-breaking spaces often sneak in from word processors or web pages.
4. Check character encoding -- Ensure your file is saved as UTF-8. Other encodings can produce invisible errors.
5. Use a linter -- Tools like ESLint with the json plugin catch JSON errors during development.

Remember: JSON parsers typically report only the first syntax error. Fix that one, re-validate, and repeat until the document passes. Our JSON Formatter & Validator tool gives you instant, detailed error messages to speed up this process.