Class type interfaces

Class type interfaces define the contract for classes. A class that implements an interface should meet the requirement of the interface:

interface CustomerInterface { 
    id: number; 
    firstName: string; 
    lastName: string; 
    addCustomer(firstName: string, lastName: string); 
    getCustomer(id: number): Customer; 
} 
  
class Customer implements CustomerInterface { 
    id: number; 
    firstName: string; 
    lastName: string; 
    constructor() { } 
    addCustomer(firstName: string, lastName: string) { 
        // code to add customer 
   } 
    getCustomer(id: number): Customer { 
        return this; 
    } 
} 

The class type interface only deals with public members of the class. So, it is not possible to add private members to the interface.