Looking at json spec, I'm not sure whether a
Per ECMA-404, The JSON Data Interchange Standard (pdf), which is linked from the JSON.org page you linked:
A JSON text is a sequence of tokens formed from Unicode code points that conforms to the JSON value grammar.
A JSON value can be an object, array, number, string, true, false, or null.
As such, the value
123 is valid JSON, representing an integer.