Angular deep copy of a JSON object updated the modifed version

General
1

I have a JSON object like below,

export class MyComponent {
   @Input() cityinfoJSON; // this JSON comes as following from parent component as below
        let cityinfoJSON =
        {
            "id": 0,
            "name": "",
            "description": "",    
            "address": {
                "city": "NY",
                "zipcodes": [],
                "countyList": {
                "countyName": '',
                "countyzip": "claim_type",
                "countyPlaceVerified": "N"
                },
                "sort": []
            }
        }
   
 let OriginalJSON
    ngOninit() {
    
  // I am creating an unmodified version of it using 
   this.OriginalJSON = JSON.parse(JSON.stringify(cityinfoJSON));

   }
 }

When cityinfoJSON’s countyPlaceVerified is modified to ‘Y’, I don’t expect OriginalJSON’S countyPlaceVerified to change. It is also getting changed to Y’. This shouldn’t happen, as I am deep copying it. How can we achieve as OriginalJSON remains unchanged.