Generate a JSON instead of an image for the legend with OpenLayers

I’m not clear on how I can pass the params to getLegendUrl. My aim is to obtain the JSON and for this scope I use the code below:

  var graphicUrl = wmsSource.getLegendUrl(
    resolution,
    'params': 'application/json',
  );
  console.log(graphicUrl);

In the console I see only the error below concerned to 'params': 'application/json',:

Uncaught SyntaxError: missing ) after argument list

var graphicUrl = wmsSource.getLegendUrl( resolution,
     {'REQUEST': '...',
      'LAYER': '...'
     }
);