Can comments be used in JSON?

No.

JSON is data-only. If you include a comment, then it must be data too.

You could have a designated data element called "_comment" (or something) that should be ignored by apps that use the JSON data.

You would probably be better having the comment in the processes that generates/receives the JSON, as they are supposed to know what the JSON data will be in advance, or at least the structure of it.

But if you decided to:

{
   "_comment": "comment text goes here...",
   "glossary": {
      "title": "example glossary",
      "GlossDiv": {
         "title": "S",
         "GlossList": {
            "GlossEntry": {
               "ID": "SGML",
               "SortAs": "SGML",
               "GlossTerm": "Standard Generalized Markup Language",
               "Acronym": "SGML",
               "Abbrev": "ISO 8879:1986",
               "GlossDef": {
                  "para": "A meta-markup language, used to create markup languages such as DocBook.",
                  "GlossSeeAlso": ["GML", "XML"]
               },
               "GlossSee": "markup"
            }
         }
      }
   }
}

No, comments of the form //… or /*…*/ are not allowed in JSON. This answer is based on:

• https://www.json.org
• RFC 4627: The application/json Media Type for JavaScript Object Notation (JSON)
• RFC 8259 The JavaScript Object Notation (JSON) Data Interchange Format (supercedes RFCs 4627, 7158, 7159)