Can I use typeof for enforcing certain string

Issue

I have a const value:

const dataConst = 'constType'

And I want to enforce only this value in a certain interface, but this is not possible:

interface ExampleInterface {
data: dataConst
}

I tried this and it seems to work, but I don't know if it's the best practice:

interface ExmapleInterface {
data: typeof dataConst
}

I tried and succeeded, but I want to know if what I did is the best practice.

Solution

I believe there isn't a good or bad way of doing this. Your example is just one of the many ways of achieving what you want.

Perhaps an explanation of why your example works would be useful.

const dataConst = 'constType' assigns an actual value to a dataConst constant. You haven't specified the type explicitly for this const so the type is inferred by the TypeScript compiler. It's smart so it will assign a "string literal" with a value "constType". In contrast, if you were to write something like: let dataConst = 'constType', the type of the dataConst would be a simple string because the compiler knows that the value can change in the future.

You can't use

interface ExampleInterface {
  data: dataConst
}

because this is a type definition and only the "types" can go into it. In this case the dataConst is the actual value. In order to "retrieve" the type of the dataConst constant we can use typeof . Keep in mind this is a different operator from the Javascript one with the same name. It can only be used in the context of Typescript types.

interface ExampleInterface {
  data: typeof dataConst
}

So now with that explanation out of the way is there any other way of doing it? Yes. For example:

declaring a type Value based on value type

const value = 'my-constant-value'
type Value = typeof value

interface Example {
  prop: Value
}

or declaring Value type with a string literal

type Value = 'my-constant-value'
const value: Value = 'my-constant-value'

interface Example {
  prop: Value
}

or perhaps if you don't need an actual const value and just using the types is sufficient

type Value = 'my-constant-value'

interface Example {
  prop: Value
}

or even

interface Example {
  prop: 'my-constant-value'
}

Answered By - Aleksandr Šmailov