Просто используйте Object.keys()
вот так:
var obj ={
car:{
maruti: {wheels: 4, mileage:15},
dezire: {wheels: 4, mileage:19}
},
bike:{
suzuki:{wheels: 2, mileage:45},
honda:{wheels: 2, mileage:85}
}
}
Object.keys(obj).forEach(key => obj[key] = [obj[key]]);
console.log(obj);
И для сортировки автомобилей по пробегу:
var obj ={
car:{
maruti: {wheels: 4, mileage:15},
dezire: {wheels: 4, mileage:19}
},
bike:{
suzuki:{wheels: 2, mileage:45},
honda:{wheels: 2, mileage:85}
}
}
Object.keys(obj).forEach(key => obj[key] = [obj[key]]);
Object.keys(obj).forEach(key => obj[key].sort((a, b) => a.mileage - b.mileage));
console.log(obj);