how can I restrict the keys of an object to be the values of an object?

Question

I have an object defined with string keys and number values:

class MyService {
  private static readonly MAPPING: { [ index: string ]: number } = {
    a: 16,
    b: 32,
  };
}

And I would like to have another object that only allows the values (numbers) defined in that mapping as keys



class MyService {
  private static readonly MAPPING: { [ index: string ]: number } = {
    a: 16,
    b: 32,
  } as const;
  public static states: { [ index in ?!? (Object.values(MyService.MAPPING)) ?!? ]: boolean } = {}; // <-- clearly this is wrong
}

So that: MyService.states[16] = true would be valid, but MyService.states[17] = true would not.

How can I accomplish this?

Andrietta · Accepted Answer · 2022-02-01 23:09:08Z

1

Since you are not allowed to define a type within a class, you will have to define the type using the constant object on the root level of the file and use it in your class.

const MAPPING = {
  a: 16,
  b: 32,
} as const;

type MAPPING_VALUE = typeof MAPPING[keyof typeof MAPPING];

class MyService {
  public static readonly MAPPING: { [index: string]: number } = MAPPING;
  public static states: {
    [index in MAPPING_VALUE]: boolean;
  } = {
    16: true,
    32: false,
  };
}

edited Feb 1, 2022 at 23:09

answered Feb 1, 2022 at 22:28

Andrietta

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4 Comments

Anton Hirov Over a year ago

For initialization, we can restrict access by union types, but for value access, we can't do that. In other words MyService.states[44] = true; will be also valid due to dynamic property search. For dot notation, we need a valid identifier, like this $16.

Andrietta Over a year ago

i've just tried it and it seems it's not like you described MyService.states[16] = true is valid according to ts, but typescript MyService.states[44]= true isn't.

patrick Over a year ago

Interesting.. Thank you. I still am getting used to the idea of having things like that defined outside of classes, that has always felt "wrong" to do.

Andrietta Over a year ago

in case you use a type that doesn't belong to your MAPPING values you will see (eg: 17):

Element implicitly has an 'any' type because expression of type '17' can't be used to index type '{ 16: boolean; 32: boolean; }'.   Property '17' does not exist on type '{ 16: boolean; 32: boolean; }'.

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