Valid vs Invalid JSON

Valid JSON follows a strict specification (RFC 8259). Even one character out of place causes the entire document to fail parsing. Understanding the difference between valid and invalid JSON — and why — saves hours of debugging.

A valid JSON document must satisfy all of these:

• The root value is an object {}, array [], string, number, boolean, or null
• All object keys are strings in double quotes
• Strings use double quotes — not single quotes, not backticks
• No trailing commas after the last item in an object or array
• true, false, and null are lowercase
• No comments of any kind
• No JavaScript-only values like undefined, NaN, or Infinity

{"a": 1, "b": [2, 3]}

That is valid. Now compare these common mistakes:

Trailing comma — parse error in every standard parser:

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

Single quotes — valid JavaScript, invalid JSON:

{'name': 'Alice'}

Unquoted key — valid JavaScript object literal, invalid JSON:

{name: "Alice"}

undefined is not a JSON value — use null or omit the property:

{"result": undefined}

NaN and Infinity are not valid — JSON has no representation for these:

{"ratio": NaN, "limit": Infinity}

Comments are not allowed — even // breaks JSON parsers:

{
  "port": 3000
  // default port
}

JSON's strictness is intentional. A simpler grammar means parsers in any language can be small, fast, and interoperable. JSON5, JSONC, and YAML relax these rules — but they are not JSON and cannot be parsed by JSON.parse().

1. Try JSON.parse(yourString) in a browser console — it throws a SyntaxError with a line number if invalid
2. Paste into an online validator
3. Look at the error message — it usually says "Unexpected token" and points to the problem character

JSON can be syntactically valid but semantically wrong. For example, this is valid JSON:

{
  "age": -999,
  "email": "not-an-email",
  "status": "launched"
}

It parses without error, but the values may break your application. That is where JSON Schema validation comes in — see JSON Schema Basics.

Paste suspect JSON into the JSON Validator for an instant validity check with precise error locations. For deeper debugging — including identifying where data violates expected shapes — try the JSON Debugger.