Json – Typescript `enum` from JSON string

Is there any way to have a TypeScript enum compatible with strings from JSON?

For example:

enum Type { NEW, OLD }

interface Thing { type: Type }

let thing:Thing = JSON.parse('{"type": "NEW"}');

alert(thing.type == Type.NEW); // false

I would like thing.type == Type.NEW to be true. Or more specifically, I wish I could specify the enum values to be defined as strings, not numbers.

I am aware that I can use thing.type.toString() == Type[Type.NEW] but this is cumbersome and seems to make the enum type annotation confusing and misleading, which defeats its purpose. The JSON is technically not supplying a valid enum value, so I shouldn't type the property to the enum.

So what I am currently doing instead is using a string type with static constants:

const Type = { NEW: "NEW", OLD: "OLD" }

interface Thing { type: string }

let thing:Thing = JSON.parse('{"type": "NEW"}');

alert(thing.type == Type.NEW); // true

This gets me the usage I want, but the type annotation string is way too broad and error prone.

I'm a bit surprised that a superset of JavaScript doesn't have string based enums. Am I missing something? Is there a different way this can be done?

Update TS 1.8

Using string literal types is another alternative (thanks @basaret), but to get the desired enum-like usage (above) it requires defining your values twice: once in a string literal type, and once as a value (constant or namespace):

type Type = "NEW" | "OLD";
const Type = {
    NEW: "NEW" as Type,
    OLD: "OLD" as Type
}

interface Thing { type: Type }

let thing:Thing = JSON.parse(`{"type": "NEW"}`);

alert(thing.type === Type.NEW); // true

This works but takes a lot of boilerplate, enough that I don't use it most of the time. For now I'm hoping the proposal for string enums will eventually make the roadmap.

Update TS 2.1

The new keyof type lookup allows for the string literal type to be generated from the keys of a const or namespace, which makes the definition a little less redundant:

namespace Type {
    export const OLD = "OLD";
    export const NEW = "NEW";
}
type Type = keyof typeof Type;

interface Thing { type: Type }

const thing: Thing = JSON.parse('{"type": "NEW"}');
thing.type == Type.NEW // true

Update TS 2.4

TypeScript 2.4 added support for string enums! The above example becomes:

enum Type {
    OLD = "OLD",
    NEW = "NEW"
}

interface Thing { type: Type }
const thing: Thing = JSON.parse('{"type": "NEW"}');
alert(thing.type == Type.NEW) // true

This looks nearly perfect, but there's still some heartache:

• You still have to write the value twice, ie OLD = "OLD", and there's no validation that you don't have a typo, like NEW = "MEW"… this has already bitten me in real code.

• There's some oddities (perhaps bugs?) with how the enum is type checked, its not just a string literal type shorthand, which is what would be truly correct. Some issues I've bumped into:

  enum Color { RED = "RED", BLUE = "BLUE", GREEN = "GREEN" }
  
  type ColorMap = { [P in Color]: number; }
  
  declare const color: Color;
  declare const map: ColorMap;
  map[color] // Error: Element implicitly has an 'any' type because type 'ColorMap' has no index signature.
  
  const red: Color = "RED"; // Type '"RED"' is not assignable to type 'Color'.
  const blue: Color = "BLUE" as "RED" | "BLUE" | "GREEN"; // Error: Type '"RED" | "BLUE" | "GREEN"' is not assignable to type 'Color'.
  

  The equivalent code with enum Color replaced by string literal types work fine…

Yeah, I think I have OCD about this, I just want my perfect JS enums. 🙂