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

Question

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]?

Ben Carp · Accepted Answer · 2019-01-30 14:36:05Z

2

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

answered Jan 30, 2019 at 14:36

Ben Carp

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1 Comment

Ben Carp Over a year ago

Should I delete this question altogether?

Nguyen Phong Thien · Accepted Answer · 2019-01-30 13:08:12Z

1

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"]
}

answered Jan 30, 2019 at 13:08

Nguyen Phong Thien

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