Map未返回正确的数据类型

 chew = '1235 51'.split(' ');
     item_id = chew[0];
     slot_go = chew[1];
     chew.map(function(v){
            return Number(v);
        }); 
console.log(chew);
 chew = '1235 51'.split(' ');
     item_id = ch