Is there a method to transform array values into strings? Here is what I came up with:
class A
{
public keys = ['name', 'age']
}
type ArrayKeys = P in A['keys'] // 'name' or 'age'
Any suggestions on how to limit ArrayKeys to only being within the values of A['keys']?
To ensure TypeScript that the keys will remain unchanged, you can use an as const cast. This way, TypeScript will treat it as a fixed array type instead of just assuming it's a string array, providing you with enforced immutability. Once this is done, implementing it is straightforward:
class A
{
public keys = ['name', 'age'] as const
}
type ArrayKeys = A['keys'][number];
Explore further on the playground.
The usage of [number] indicates accessing the value within the array. In generic terms, (Array<T>)[number] always returns T.
It may not be possible to accomplish that directly, as types are a static compile-time feature while arrays are a dynamic runtime feature. ...except if you introduce the use of const as demonstrated by Madara Uchiha.
Alternatively, you could opt for a string enum in place of an array and then extract its values to fill the array.
enum ArrayKeys {
name, age
}
class A {
public keys = Object.keys(ArrayKeys).filter(key => isNaN(Number(key)));
}
This approach also offers the benefit of utilizing ArrayKeys.name and ArrayKeys.age as enum values.
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