For example I have an array

[
 {
   login: user1,
   parent: ''
 },
 {
   login: user2,
   parent: user1
 },
 {
   login: user3,
   parent: user1
 },
 {
   login: user4,
   parent: user3
 }
]

I need to count how many children user1 has and set him property childrenAmount. Including that one user4 that is user1's grandchild too and deep of this tree can be different.