How to write a rule to prevent the removal of a node from the database

Question

I am trying to write rules to protect the database. But I am confused by writing a rule that will prevent the removal of any node from the database. I read about newData.exists, but when I tried to run it in the simulator, the deletion was done! How can a node be deleted by setting its value to zero, so I tried to simulate the value of node equal to null, and it was successful, which is undesirable.

Suppose I have this node:

root{
Number of Users:20
}

And I wrote these rules:

"Number of Users":{
".read":true,
 ".write":"auth!==null && newData.exists()"
 }

I'm making any mistake, please correct me.

Answer 1

, node:

".write": "!data.exists()"

, , node:

".write": "newData.exists()"

: