How can I patch data to Check-boxes Array in Angular 4/5

I am using angular reactive form. how can i patch or set data to array of check-boxes.I have array like this

 departmentList = [
        {id: 1, name: 'Dermatolgy'},
        {id: 2, name: 'Surgery'},
        {id: 3, name: 'Dental'},
        {id: 4, name: 'SkinCareLaser'},
   ];

Let Suppose i want to set true or checked Surger .(on base of DB data).

I am using following code for adding data on registration user.

    <div *ngFor="let item of departmentList">
      <input type="checkbox" formControlName="departmentControl"
       name=
       value=""
      (change)="selectDepartment($event,item)">
      
     <span></span>
   </div>

Class.TS

selectedDepartment: any = [];
selectDepartment(event: any, item: any) {
    console.log(event.checked);

if (event.target.checked) {

        this.selectedDepartment.push(item.id);
    }
    else {
        let updateItem = this.selectedDepartment.find(this.findIndexToUpdate, item.id);

        let index = this.selectedDepartment.indexOf(updateItem);

        this.selectedDepartment.splice(index, 1);
    }
     console.log(this.selectedDepartment);

}

that's working fine.

Question : how can i patch or set data to array on Update Function on base of another arrays data that is fetched from server .