var data = [ { "rock": "granite", "youngerThan": "schist"},{ "rock": "schist", "youngerThan": "basalt"},{ "rock": "basalt","youngerThan":null },{ "rock": "granite", "youngerThan": "Picrite"}]

data.sort((a,b) =>{

   if(a.youngerThan == b.rock){
         return -1
     }else{ return 0 }

});

data.sort((a,b) =>{
    if(a.youngerThan == null ){
     return -1
    }else if(b.youngerThan == null) { return 1 }

});

data.sort((a,b) =>{
    if(a.rock == b.rock ){
     return -1
    }else {return 1 }

});

data.reverse();

console.log(data);