(() => {
  // What `toJSON()` returns is rendered as it stands inside the parentheses,
  // including when it is something with a rendering of its own.
  class Cyclic {
    toJSON() {
      return this;
    }
  }
  class ToSet {
    toJSON() {
      return new Set([1]);
    }
  }
  class ToFunction {
    toJSON() {
      return () => 1;
    }
  }
  class ToSymbol {
    toJSON() {
      return Symbol("d");
    }
  }
  return {
    cyclic: new Cyclic(),
    set: new ToSet(),
    fn: new ToFunction(),
    symbol: new ToSymbol(),
  };
})()

--

cyclic:
/Cyclic(<circle>)
/Cyclic(<circle>)
{
  "/Cyclic": {
    "/circle": 0
  }
}

set:
/ToSet(/Set(...))
/ToSet(/Set(...))
{
  "/ToSet": {
    "/Set": "/..."
  }
}

fn:
/ToFunction((...)=>{...})
/ToFunction((...) => {...})
{
  "/ToFunction": {
    "/function": "<anonymous>(...)"
  }
}

symbol:
/ToSymbol(Symbol("d"))
/ToSymbol(Symbol("d"))
{
  "/ToSymbol": {
    "/uniqueSymbol": "d"
  }
}