typescripttype-mapping

TypeScript - Obtain type from all properties having a certain type

I have a filter object that comes from an OpenAPI spec; something like:

export interface Filters {
  field1: string[]
  field2: string[]
  field3: boolean
  field4: number
}

I'd like to derive a type from that filters interface by selecting the type of the properties:

Something like:

export type MultivalueFields = Select<Filters, string[]>
// type MultivalueFields = 'field1' | 'field2'

Is there a builtin for this? How can one get the desired outcome?

Solution

  • You can build a utility type such as follows:

    type Values<T> = T[keyof T]
type Select<T, SelectType> = Values<{
  [Prop in keyof T]: T[Prop] extends SelectType ? Prop : never
}>

// typeof A = 'field1'
type A = Select<{field1: string[], field2: string}, string[]>

    All credit goes to @captain-yossarian from Ukraine