switch(true){
    case marks > 90:
        console.log('AA');
        break;
    case 80 < marks <= 90:
        console.log('AB');
        break;
    case 70 < marks <= 80:
        console.log('BB');
        break;
    case 60 < marks <= 70:
        console.log('BC');
        break;
    case 50 < marks <= 60:
        console.log('CC');
        break;
    case 40 < marks <= 50:
        console.log('CD');
        break;
    case 30 < marks <= 40:
        console.log('DD');
        break;
    case marks <= 30:
        console.log('FF');
        break;    
}