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How to Validate JSON Online — Common JSON Errors and How to Fix Them (2026)

DDeepak Singh Chaudhary·March 22, 2026·9 min read

A practical guide to JSON validation: the most common JSON syntax errors, how to spot them, how to fix them, and how to use a free online JSON validator to catch problems instantly before they reach production.

Why JSON Validation Matters

A single misplaced comma or missing bracket in a JSON file can crash an API endpoint, break a configuration file, or throw a parse error at runtime. JSON syntax is strict by design — it was built as an unambiguous data interchange format. This strictness makes JSON validation essential before any JSON reaches a server, configuration system, or production build.

The fastest way to validate JSON is to paste it into a free online validator like the JSON Formatter — it shows exactly which line has an error and what is wrong. This guide also teaches you to spot and fix the 8 most common JSON errors by eye.

The 8 Most Common JSON Errors

1. Trailing Comma

Error: Adding a comma after the last item in an object or array.

Invalid JSON

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

Valid JSON

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

2. Single Quotes Instead of Double Quotes

Error: Using single quotes for strings or keys. JSON requires double quotes.

Invalid JSON

{'name': 'Alice'}

Valid JSON

{"name": "Alice"}

3. Unquoted Keys

Error: Writing object keys without quotes (valid in JavaScript, invalid in JSON).

Invalid JSON

{name: "Alice"}

Valid JSON

{"name": "Alice"}

4. Comments

Error: Adding // or /* */ comments. JSON does not support comments.

Invalid JSON

{"name": "Alice" // user name}

Valid JSON

{"name": "Alice"}

5. undefined, NaN, or Infinity Values

Error: Using JavaScript-specific values. JSON only allows: string, number, boolean, null, array, object.

Invalid JSON

{"value": undefined, "score": NaN}

Valid JSON

{"value": null, "score": 0}

6. Unescaped Special Characters in Strings

Error: Using unescaped double quotes, backslashes, or control characters inside strings.

Invalid JSON

{"msg": "He said "hello""}

Valid JSON

{"msg": "He said "hello""}

7. Missing or Extra Brackets

Error: Unmatched { }, [ ] brackets. Common in long nested structures.

Fix: Use a JSON validator that counts open/close brackets and highlights mismatches. Paste into the JSON Formatter — it shows the exact bracket that is unmatched.

8. Number Format Errors

Error: Leading zeros (01, 02) or using hexadecimal (0xFF) in numbers.

Invalid JSON

{"code": 0xFF, "id": 007}

Valid JSON

{"code": 255, "id": 7}

How to Validate JSON Instantly

1
Open the JSON Formatter & Validator at ddaverse.com/json-formatter
2
Paste your JSON into the left panel
3
Click Format — valid JSON is formatted and syntax-highlighted; invalid JSON shows a red error with the line number and character position
4
Read the error message and fix the issue — common messages: "Unexpected token", "Expected ':'", "Expected ','", "Unexpected end of JSON"
5
Re-paste and format again until the JSON is valid

JSON Error Messages Decoded

Error Message	Likely Cause	Fix
Unexpected token ,	Trailing comma	Remove last comma before } or ]
Unexpected token '	Single quotes used	Replace all ' with "
Expected ':' after key	Unquoted key or missing colon	Quote the key with "
Unexpected end of JSON	Missing closing bracket	Add missing } or ]
Unexpected token u	undefined value used	Replace undefined with null
Unexpected token /	Comment in JSON	Remove all // or /* */ comments

Validate JSON in Code

In production code, always validate JSON before parsing:

// JavaScript — safe JSON parse with error handling

function safeParseJSON(str) {
  try {
    return JSON.parse(str);
  } catch (e) {
    console.error('Invalid JSON:', e.message);
    return null;
  }
}

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Frequently Asked Questions

What is the most common JSON error?

The most common JSON error is a trailing comma — adding a comma after the last item in an array or object. Standard JSON does not allow trailing commas. Example: {"name": "John", "age": 30,} — the comma after 30 is invalid. Remove it to fix the error.

How do I find the line number of a JSON error?

Use an online JSON formatter and validator — paste your JSON and it will highlight the exact line and character position of the error. The JSON Formatter at ddaverse.com/json-formatter shows line-level error highlighting and a clear error message describing what is wrong.

Can I use single quotes in JSON?

No. JSON strictly requires double quotes for all strings and keys. Single quotes are not valid JSON syntax. {'name': 'John'} is invalid JSON. {"name": "John"} is valid. This is one of the most frequent errors when writing JSON by hand or copying from JavaScript code.

What is JSON.parse() and why does it throw errors?

JSON.parse() is a JavaScript function that converts a JSON string into a JavaScript object. It throws a SyntaxError if the input string is not valid JSON — the error message typically includes the invalid character and its position. Always wrap JSON.parse() in a try/catch block in production code to handle malformed JSON gracefully.

What is the difference between JSON and JavaScript object literal?

A JavaScript object literal is flexible: keys can be unquoted, values can be undefined, functions, or Dates, and trailing commas are allowed in modern JS. JSON is a strict data interchange format: all keys must be double-quoted strings, only 6 value types are allowed (string, number, boolean, null, array, object), and trailing commas are forbidden.

How do I validate JSON in VS Code?

VS Code validates JSON files automatically when the file has a .json extension — it underlines errors in red and shows the error message on hover. For JSON inside strings (API responses in code), use the JSON Formatter tool at ddaverse.com/json-formatter — paste the JSON string, click Format, and all validation errors are highlighted immediately.

How to Validate JSON Online — Common JSON Errors and How to Fix Them (2026)

DDeepak Singh Chaudhary·March 22, 2026·9 min read

Why JSON Validation Matters

The 8 Most Common JSON Errors

1. Trailing Comma

Error: Adding a comma after the last item in an object or array.

Invalid JSON

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

Valid JSON

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

2. Single Quotes Instead of Double Quotes

Error: Using single quotes for strings or keys. JSON requires double quotes.

Invalid JSON

{'name': 'Alice'}

Valid JSON

{"name": "Alice"}

3. Unquoted Keys

Error: Writing object keys without quotes (valid in JavaScript, invalid in JSON).

Invalid JSON

{name: "Alice"}

Valid JSON

{"name": "Alice"}

4. Comments

Error: Adding // or /* */ comments. JSON does not support comments.

Invalid JSON

{"name": "Alice" // user name}

Valid JSON

{"name": "Alice"}

5. undefined, NaN, or Infinity Values

Error: Using JavaScript-specific values. JSON only allows: string, number, boolean, null, array, object.

Invalid JSON

{"value": undefined, "score": NaN}

Valid JSON

{"value": null, "score": 0}

6. Unescaped Special Characters in Strings

Error: Using unescaped double quotes, backslashes, or control characters inside strings.

Invalid JSON

{"msg": "He said "hello""}

Valid JSON

{"msg": "He said "hello""}

7. Missing or Extra Brackets

Error: Unmatched { }, [ ] brackets. Common in long nested structures.

Fix: Use a JSON validator that counts open/close brackets and highlights mismatches. Paste into the JSON Formatter — it shows the exact bracket that is unmatched.

8. Number Format Errors

Error: Leading zeros (01, 02) or using hexadecimal (0xFF) in numbers.

Invalid JSON

{"code": 0xFF, "id": 007}

Valid JSON

{"code": 255, "id": 7}

How to Validate JSON Instantly

1
Open the JSON Formatter & Validator at ddaverse.com/json-formatter
2
Paste your JSON into the left panel
3
Click Format — valid JSON is formatted and syntax-highlighted; invalid JSON shows a red error with the line number and character position
4
Read the error message and fix the issue — common messages: "Unexpected token", "Expected ':'", "Expected ','", "Unexpected end of JSON"
5
Re-paste and format again until the JSON is valid

JSON Error Messages Decoded

Error Message	Likely Cause	Fix
Unexpected token ,	Trailing comma	Remove last comma before } or ]
Unexpected token '	Single quotes used	Replace all ' with "
Expected ':' after key	Unquoted key or missing colon	Quote the key with "
Unexpected end of JSON	Missing closing bracket	Add missing } or ]
Unexpected token u	undefined value used	Replace undefined with null
Unexpected token /	Comment in JSON	Remove all // or /* */ comments

Validate JSON in Code

In production code, always validate JSON before parsing:

// JavaScript — safe JSON parse with error handling

function safeParseJSON(str) {
  try {
    return JSON.parse(str);
  } catch (e) {
    console.error('Invalid JSON:', e.message);
    return null;
  }
}

Explore All Free Developer Tools

JSON formatter, validator, converter, diff, tree viewer, JWT decoder, slug generator, cron generator — 13 free developer tools, all in your browser.