Typescript: constants in an interface

Question

How do I place a constant in an Interface in typescript. Like in java it is:

interface OlympicMedal {
  static final String GOLD = \"Gold\";
  static final S         


        

Answer 1

There is a workaround for having constants in a interface: define both the module and the interface with the same name.

In the following, the interface declaration will merge with the module, so that OlympicMedal becomes a value, namespace, and type. This might be what you want.

module OlympicMedal {
    export const GOLD = "Gold";
    export const SILVER = "Silver";
}

interface OlympicMedal /* extends What_you_need */ {
    myMethod(input: any): any;
}

This works with Typescript 2.x

Answer 2

Just use the value in the interface in place of the type, see below

export interface TypeX {
    "pk": "fixed"
}

let x1 : TypeX = {
    "pk":"fixed" // this is ok
}

let x2 : TypeX = {
    "pk":"something else" // error TS2322: Type '"something else"' is not assignable to type '"fixed"'.
}

Answer 3

This seems to work:

class Foo {
  static readonly FOO="bar"
}

export function foo(): string {
  return Foo.FOO
}

You can have private constants as well like this. It seems interfaces can't have static members though.

Answer 4

You cannot declare values in an interface.

You can declare values in a module:

module OlympicMedal {
    export var GOLD = "Gold";
    export var SILVER = "Silver";
}

In an upcoming release of TypeScript, you will be able to use const:

module OlympicMedal {
    export const GOLD = "Gold";
    export const SILVER = "Silver";
}

OlympicMedal.GOLD = 'Bronze'; // Error

Answer 5

A recommended way to establish constants in an interface, as shown here and is similar to another answer here, is to do:

export class Constants {
  public static readonly API_ENDPOINT = 'http://127.0.0.1:6666/api/';
  public static readonly COLORS = {
    GOLD: 'Gold',
    SILVER: 'Silver',
    BRONZE: 'Bronze'
  };
}

This is the preferred way to define constants without being hacky, or disabling settings in the TSLinter (because module and namespace will through many warnings via the linter).