Can JSON have comments?

No. Comments are not part of the JSON specification (RFC 8259). A JSON parser will reject any file that contains // or /* */ comments. If you need comments in a config file, consider JSONC (JSON with Comments, used by VS Code) or YAML, which supports # comments natively.

Is undefined a valid JSON value?

No. JSON only supports these value types: string, number, boolean (true/false), null, object, and array. undefined is a JavaScript concept that does not exist in JSON. When you JSON.stringify() an object with undefined values in JavaScript, those keys are silently omitted from the output.

What is the difference between JSON validation and JSON Schema validation?

JSON validation checks syntax — is this text a well-formed JSON document? It verifies brackets, quotes, commas, and value types. JSON Schema validation goes further — it checks structure and constraints: is "age" a number between 0 and 150? Is "email" present and a string? You define the rules in a schema document.

Are trailing commas allowed in JSON?

No. Trailing commas after the last item in an object or array are a syntax error in JSON. This is one of the most common mistakes, especially coming from JavaScript where trailing commas are allowed. Example of invalid JSON: {"name": "Jane", "age": 30,} — the comma after 30 is illegal.

How to Validate JSON

What makes JSON valid?

A JSON document is valid when it follows the rules of the JSON specification (RFC 8259). There are two levels of correctness:

Syntax validity — the text is well-formed JSON: correct brackets, quoted keys, no trailing commas, no comments.
Schema validity — the data has the expected structure: required fields exist, values have the right type, numbers are within allowed ranges.

A JSON parser validates syntax. JSON Schema validates structure. Both matter — but you need syntax to be valid first.

Common JSON errors

These are the mistakes that most often break JSON documents:

✕ Common mistake Trailing comma

Invalid

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

Valid

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

The comma after the last property is illegal in JSON. JavaScript allows it — JSON does not.

✕ Common mistake Single quotes instead of double quotes

Invalid

{'name': 'Jane'}

Valid

{"name": "Jane"}

JSON requires double quotes for both keys and string values. Single quotes are a JavaScript convenience, not valid JSON.

✕ Common mistake Unquoted keys

Invalid

{name: "Jane", age: 30}

Valid

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

All object keys must be quoted strings. This is the most common mistake for developers coming from JavaScript object literals.

✕ Common mistake Comments

Invalid

{
  "timeout": 30 // seconds
}

Valid

{
  "timeout": 30
}

JSON has no comment syntax. Remove comments before parsing, or switch to JSONC / YAML for config files that need documentation.

✕ Common mistake undefined, NaN, Infinity

Invalid

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

Valid

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

JSON only allows: string, number, boolean, null, object, array. undefined, NaN, and Infinity are JavaScript values with no JSON equivalent.

JSON syntax rules — quick reference

RuleValidInvalid

String quotes"hello"'hello'

Object keys"key": 1key: 1

Trailing comma[1, 2, 3][1, 2, 3,]

Null value"x": null"x": undefined

Booleantrue / falseTrue / False

Number3.14.14 or 01

Root value{} or []bare string

Validate JSON in code

Every major language has a built-in JSON parser. Wrap the parse call in a try/catch to validate:

JavaScript / Node.js

javascripttry { const data = JSON.parse(str) console.log('Valid JSON', data) } catch (e) { console.error('Invalid JSON:', e.message) }

Python

pythonimport json try: data = json.loads(s) print('Valid JSON', data) except json.JSONDecodeError as e: print('Invalid JSON:', e)

PHP

php$data = json_decode($str, true); if (json_last_error() !== JSON_ERROR_NONE) { echo 'Invalid JSON: ' . json_last_error_msg(); }

Go further with JSON Schema

Syntax validation only checks that the JSON is parseable. JSON Schema lets you validate the content — required fields, value types, formats, and ranges.

example JSON Schema{ "$schema": "https://json-schema.org/draft/2020-12/schema", "type": "object", "required": ["name", "age", "email"], "properties": { "name": { "type": "string", "minLength": 1 }, "age": { "type": "integer", "minimum": 0, "maximum": 150 }, "email": { "type": "string", "format": "email" } } }

With this schema, {"name": "Jane", "age": -5, "email": "jane@example.com"} would fail validation because age is below the minimum of 0 — even though it's perfectly valid JSON syntax.

Use the JSON Schema Generator tool to automatically create a schema from any valid JSON — then refine it to add your constraints.

What makes JSON valid?

Common JSON errors

JSON syntax rules — quick reference

Validate JSON in code

JavaScript / Node.js

Python

PHP

Go further with JSON Schema

Frequently asked questions