i am trying to find a type from a type using a string as an index type (which does not work). how do i solve that mystery, that LayerPropType will be of type string[] in this example?

is there any way?

type WMSLayerProps = {
  Name?: string
  CRS?: string[]
}

type WMSStyleProps = {
  Name?: string
  Title?: string
}

const OGCWMSLayerProps: (keyof WMSLayerProps)[] = [
  'Name',
  'CRS',
]
const OGCWMSStyleProps: (keyof WMSStyleProps)[] = ['Name', 'Title']

const tagName = "CRS"

// LayerPropType should be of type string[]. but instead i cannot use tagName 
// as an indexType 
type LayerPropType = WMSLayerProps[tagName]
// this is the error message: 

// Type 'tagName' cannot be used as an index type.ts(2538)
// 'tagName' refers to a value, but is being used as a type here. Did you mean 'typeof 
// tagName'?ts(2749)

i am trying to find a type from a type using a string as an index type (which does not work). how do i solve that mystery, that LayerPropType will be of type string[] in this example?

is there any way?

type WMSLayerProps = {
  Name?: string
  CRS?: string[]
}

type WMSStyleProps = {
  Name?: string
  Title?: string
}

const OGCWMSLayerProps: (keyof WMSLayerProps)[] = [
  'Name',
  'CRS',
]
const OGCWMSStyleProps: (keyof WMSStyleProps)[] = ['Name', 'Title']

const tagName = "CRS"

// LayerPropType should be of type string[]. but instead i cannot use tagName 
// as an indexType 
type LayerPropType = WMSLayerProps[tagName]
// this is the error message: 

// Type 'tagName' cannot be used as an index type.ts(2538)
// 'tagName' refers to a value, but is being used as a type here. Did you mean 'typeof 
// tagName'?ts(2749)

• typescript

2 Answers 2

Instead of type LayerPropType = WMSLayerProps[tagName] you should use type LayerPropType = WMSLayerProps[typeof tagName] (note the typeof on the index)

This is because you're declaring a type, so you must use the type to access the property, not its value. In this case both are "the same" because tagName is a constant, so its type is not string but rather "CRS"

Note however that LayerPropType is not of type string[] but rather string[] | undefined because the property CRS of WMSLayerProps is optional

See working playground here

You can't directly access dynamic properties in TypeScript types. You need to use generics to achieve this.

type GetPropertyType<T, K extends keyof T> = T[K];
type LayerPropType = GetPropertyType<WMSLayerProps, typeof tagName>;