Class type interfaces define the contract for classes. A class that implements an interface should meet the requirements 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): void { 
        // code to add customer 
    } 
    getCustomer(id: number): Customer { 
        // code to return customer where the id match with id parameter 
    } 
}  

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