Monaco Editor add javascript display inside json property

I have a json file that contain at some places a “eval” property that contain javascript code. My goal is to be able to have it display like it’s was a javascript file.

This is an example file :

{
  "fn": "zcl:attr",
  "ep": 1,
  "cl": "0x0000",
  "at": "0x000A",
  "eval": "Item.val = Attr.val"
}

Here the code Item.val = Attr.val is javascript

I tried some suggestion made by copilot but did not get a good result;

    monaco.languages.setMonarchTokensProvider('json', {
      tokenizer: {
        root: [
          [/"eval"s*:s*"(.*)"/, {
            token: 'source.js',
            nextEmbedded: 'javascript',
            log: 'eval detected',
          }],
          [/"[^"]*"/, 'string'],
          [/bd+b/, 'number'],
          [/[{}[],]/, 'delimiter'],
          [/:/, 'delimiter.colon'],
          [/s+/, 'white'],
        ],
      },
    })

I also did some researchs on existing code but found nothing that I was able to make it work