largest-hulls.js

var fc = // ... a featurecollection of polygons and multipolygons

var polys = turf.featurecollection([])

fc.features.forEach(function(poly){
  if (poly.geometry.type === 'Polygon') {
    polys.features.push(poly)
  } else if (poly.geometry.type === 'MultiPolygon') {
    // start max area at -Infinity
    var max = {
      feature: null,
      area: -Infinity
    };

    poly.geometry.coordinates.forEach(function(rings){
      var hull = turf.polygon(rings)
      var area = turf.area(hull);
      if(area >= max.area) {
        max = {
          feature: hull,
          area: area
        };
      }
    })

    // push the largest hull to the polys featurcollection
    polys.features.push(max.feature)
  } else throw new Error('unknown geometry type detected')
})

console.log(JSON.stringify(polys))