JS get length of an object

var person={
    "first_name":"Harry",
    "last_name":"Potter",
    "age":14
};
var personSize = Object.keys(person).length; //gets number of properties (3) 

//length of this object is 4
let object = {
  'A':{
    '10':['a','b','c']
  }, 
  'B':{
    '20':['a','b','c']
  }, 
  'C':{
    '30':['a','b','c']
  }, 
  'D':{
    '40':['a','b','c']
  },   
}

//Get the keys of the object (A,B,C,D) and print how many there are
Object.keys().length
> prints: 4