TypeScript: Generic type to enforce values from const assertion [duplicate]

Question

How can I write a generic type which can be used for all const assertions to enforce their respective values?

Code

const GENDER = {
  male: 'm',
  female: 'f',
} as const;

type GenderType = typeof GENDER[keyof typeof GENDER];

// enforces userGender to be either 'm' of 'f'
let userGender: GenderType;

userGender = 'f'; // valid
userGender = 'm'; // valid
userGender = 'o'; // invalid

How can I generalise it such that it can be used with all const assertions?

// generalise the boilerplate of
// typeof SOMETHING[keyof typeof SOMETHING]

type GenderType = ValueType<GENDER>;
type FooType = ValueType<FOO>;
type BarType = ValueType<BAR>;
type BazType = ValueType<BAZ>;

You can make a type alias that works like ValueType<typeof FOO> but no ValueType<FOO>, because FOO is a value but generics take types and not values. — jcalz
– jcalz, Commented Jul 25, 2022 at 16:53
So what do you want to see as an answer here? If type ValueType<T> = T[keyof T] and then ValueType<typeof GENDER> is okay, great. If you insist on ValueType<GENDER> then the answer here is just "no, this can't be done" and I can write up an explanation for why. Let me know which way to go with this. — jcalz
– jcalz, Commented Jul 25, 2022 at 17:35

Vinay Sharma · Accepted Answer · 2022-07-25 18:42:02Z

1

Solution

Use a generic type that accepts the type of the object literal.

// ===== const assertion ===== //

const GENDER = {
  male: 'm',
  female: 'f',
} as const;

// ===== generic solution ===== //

type ValueOf<T> = T[keyof T]

// ====== generic usage ====== //

type GenderType = ValueOf<typeof GENDER>;

// enforces userGender to be either 'm' or 'f'
let userGender: GenderType;

userGender = 'f'; // valid
userGender = 'm'; // valid
userGender = 'o'; // invalid

// === more usage examples === //

type FooType = ValueOf<typeof FOO>;
type BarType = ValueOf<typeof BAR>;
type BazType = ValueOf<typeof BAZ>;

answered Jul 25, 2022 at 18:42

Vinay Sharma

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