No way to declare setter/getter only in interfaces

[mindarelus]

export interface ITimer {
get isRunning():boolean;
start():void;
stop():void;
reset():void;
}

Why is this not working? This is the most logical thing anyone would expect. The value of the isRunning property should be changed, only inside the class instances. Otherwise anyone could change it outside, which is not good.

[SimonMeskens]

Use the readonly keyword instead of the get keyword:

export interface ITimer {
    readonly isRunning: boolean;
    start(): void;
    stop(): void;
    reset(): void;
}

[ahejlsberg]

Interfaces make no distinction between regular properties and accessor properties--it's an implementation detail left up to the implementer of the interface. As suggested above, use the readonly keyword to get the effect of a property that has only a get accessor.

[mindarelus]

Thanks.. I guess I'll have to get used to write "readonly" :)

[dungdm93]

@ahejlsberg
using readonly this case is fine. But it make people so confused.
readonly keyword normally indicate un-reassignable property. But in the interface, it act as a getter declaration :(
#11578

Another bad case is implement readonly as computed property:

interface IPerson {
   readonly fullName: string;
}

class Person {
    public firstName: string;
    public lastName: string;

    constructor(firstName: string, lastName: string) {
        this.firstName = firstName;
        this.lastName = lastName;
    }

    public get fullName() {
        return `${this.firstName} ${this.lastName}`;
    }
}

let p: Person = new Person("Bruce", "Wayne");
alert(p.fullName);   // Bruce Wayne

p.firstName = "Clark";
p.lastName = "Kent";
alert(p.fullName);   // Clark Kent

From my aspect, allow getter/setter declared in interface make more sense:

interface IPerson {
   get fullName(): string;
}

[SimonMeskens]

@dungdm93 But it is not reassignable, so it makes sense?

[andredewaard]

I'm trying to get the same working but i get an error:

Argument of type '{ ID: number; firstName: string; lastName: string; }' is not assignable to parameter of type 'IUser | undefined'.
  Type '{ ID: number; firstName: string; lastName: string; }' is not assignable to type 'IUser'.
    Property 'getRole' is missing in type '{ ID: number; firstName: string; lastName: string; }'.

What i try to do:

import { UserRoles } from '@/enums/userRoles';

interface IUser {
  ID?: number | null;
  firstName?: string;
  lastName?: string;
  role?: UserRoles;
  readonly getRole: string;
  readonly getFullName: string;
}

class User implements IUser {

  public ID: number | null = null;
  public firstName: string = '';
  public lastName: string = '';
  public role: UserRoles = UserRoles.GUEST;

  constructor(user?: IUser) {
    if (user) {
      this.ID = user.ID ? user.ID : null;
      this.firstName = user.firstName ? user.firstName : '';
      this.lastName = user.lastName ? user.lastName : '';
      this.role = user.role ? user.role : UserRoles.GUEST;
    }
  }

  public get getRole(): string {
      return UserRoles[this.role];
  }
  public get getFullName(): string {
    return `${this.firstName} ${this.lastName}`;
  }
}

const user: User = new User({ ID: 1, firstName: 'Bruce', lastName: 'Wayne' });

[RyanCavanaugh]

@andre-dw you declared getRole and getFullName as required properties and then didn't provide them.