Invalid numbers in JSON

JSON's number grammar is narrower than JavaScript's. Leading zeros, a leading plus, hex literals and a bare decimal point are all rejected.

Open the JSON formatter

What the error looks like

Different engines word this differently. You may see any of these:

SyntaxError: Unexpected number in JSON at position N
SyntaxError: JSON.parse: unexpected number at line N column N of the JSON data
SyntaxError: Unexpected token 'x', ... is not valid JSON

Why it happens

JSON numbers allow an optional minus, digits, an optional fraction and an optional exponent. Nothing else.
A leading zero such as 007 is invalid, which catches zero-padded identifiers and version numbers.
A leading plus such as +1 is invalid, as is a hexadecimal literal such as 0x1F and a trailing decimal point such as 1. or a leading one such as .5.

Example

Invalid

{
  "id": 007,
  "delta": +1,
  "mask": 0x1F,
  "ratio": .5
}

Valid

{
  "id": 7,
  "delta": 1,
  "mask": 31,
  "ratio": 0.5
}

How this tool reports it

The formatter reports each one with the correction it would make, and converts hexadecimal literals to their decimal value rather than dropping them.

Paste your JSON into the formatter to see the exact line and column, or use the validator if you only need to check it.

Worth knowing

If a leading zero is significant, as it is in a postcode, an account number or a version segment, the value is not a number. Quote it as a string.
JSON puts no limit on the size of a number, but most parsers read it into a double, so integers beyond 2^53 lose precision. Send those as strings. This tool preserves the literal exactly as written so you can see whether that has happened.