JSON Validator

What counts as well-formed JSON

JSON is defined by RFC 8259 and ECMA-404. A JSON text is exactly one value: an object, an array, a string, a number, or one of the literals true, false, and null. The rules that trip people up:

• Double quotes only. Strings and object keys must be wrapped in ". Single quotes are a syntax error.
• No trailing comma. [1, 2, 3,] and {"a": 1,} are invalid. The comma is a separator, not a terminator.
• No comments. There is no // or /* */ in JSON.
• Keys must be quoted strings. {name: "Alice"} is invalid; it must be {"name": "Alice"}.
• Numbers are plain. No leading zeros (0123), no leading +, no leading or trailing decimal point (.5, 5.), no hex. An exponent is fine: 6.022e23.
• No NaN or Infinity. They are valid JavaScript numbers but not valid JSON. Use null or a string.
• One top-level value. Two objects back to back, or trailing text after the value, is invalid. Newline-delimited JSON (NDJSON) is a different format.

The six errors that cause almost every failure

Error	Cause	Fix
Trailing comma	A comma after the last item: {"a":1,} or [1,2,]	Delete the comma before the closing } or ]
Single quotes	Copied from JavaScript or Python: {'a': 'b'}	Replace every ' around strings and keys with "
Missing comma	Two items with only whitespace between them: {"a":1 "b":2}	Add a comma between every key/value pair and every array element
Unquoted key	JavaScript object literal syntax: {name: "Alice"}	Wrap every key in double quotes: {"name": "Alice"}
Unescaped character	A raw newline, tab, or " inside a string	Escape it: \n, \t, \"
Wrong value type	A JavaScript expression as a value: "date": new Date(), "x": undefined	Serialize it first — an ISO string, a number, null

JSON vs JSON5 vs JSONC

Feature	JSON	JSONC	JSON5
Comments // /* */	No	Yes	Yes
Trailing commas	No	Yes	Yes
Single-quoted strings	No	No	Yes
Unquoted keys	No	No	Yes
.5, 5., +1, hex, Infinity, NaN	No	No	Yes
Typical use	APIs, config sent over the wire, data files	tsconfig.json, VS Code settings.json	Hand-edited config where ergonomics matter

JSONC and JSON5 are editor and tooling conveniences. An HTTP API, JSON.parse, and most language standard libraries accept strict JSON only, so anything you send to a service you do not control should validate here with leniency mode off.

Questions