CODEWITHSUNDEEP
typescript
Ben Carp
Ben Carp

Reputation: 26558

Error when using a generic type with a constraint in Typescript - 2536

I have the following types: 

interface IFilterField <T extends FilterFieldValue > {
    defaultValue: T
    displayStr?: string
}

interface IFilterFields = {
   costMax: IFilterField<number>
   costMin: IFilterField<number>
   .....
}>

When I try to type an object that will link between the filterField and the type of its` value, this works:

export type IFiltersState<FiltersKeys extends keyof IFilterFields> = {
    [K in keyof FiltersKeys]: IFilterFields["costMin"|"costMax"]["defaultValue"]

}

But this fails:

export type IFiltersState<FiltersKeys extends keyof IFilterFields> = {
   [K in keyof FiltersKeys]: IFilterFields[K]["defaultValue"]
}

Even though the constraint is pretty much the same.

Error: 

[ts] Type 'K' cannot be used to index type 'IFilterFields'. [2536]
[ts] Type '"defaultValue"' cannot be used to index type 'IFilterFields[K]'. [2536]

Why is this error occuring?

How can I acheive the purpose of a mapped type, which follows this principles: A new type T2 has some of the keys of type T1, and for each such key typeof T1[key] equals typeof T2[key][SomeFieldName]?

Upvotes: 2

Views: 433

Answers (2)

Ben Carp
Ben Carp

Reputation: 26558

I found my error. Since FiltersKeys is already a union of keys/strings, there is a reduntand keyof in [K in keyof IfilterFields].

Upvotes: 2

Nguyen Phong Thien
Nguyen Phong Thien

Reputation: 3387

Because FiltersKeys is extended from IFilterFields, therefore it could contain different keys. You can use 

export type IFiltersState<FiltersKeys extends keyof IFilterFields> = {
    [K in keyof IFilterFields]: IFilterFields[K]["defaultValue"]
}

Upvotes: 1

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