How to save one of JSON keys, using the method filter()?

Question

I have implemented such a code. I only want to throw one key into the array under the name ("args"). Unfortunately my code returns in line 91 "TypeError: Cannot read properties of undefined (reading 'length')". How to fix it?

All is into async function :)

var objmapPolygon = {}, objmapBsc = {};

    let jsonValueBsc = JSON.parse(fs.readFileSync('eventsBsc.json'));

    let jsonValuePolygon = JSON.parse(fs.readFileSync('eventsPolygon.json'));


    var mapped = jsonValuePolygon.map( 
      function (v) { for (var l in v) {this[l] = v[l];} }, 
      objmapPolygon
    );


     console.log(objmapPolygon)

     var mapps = jsonValueBsc.map( 
      function (v) { for (var l in v) {this[l] = v[l];} }, 
      objmapBsc
    );
    
    console.log(objmapBsc)

    const resultPolygon = mapps.filter(mapp => mapp.length < 7);

    console.log(resultPolygon);