TypeScript - Obtain type from all properties having a certain type-CodePudding

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?

CodePudding user response：

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

CodePudding user response：

There are several version of types that do this. The most modern one is usig the as clause in mapped types:


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

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

Playground Link

You can also use a similar type to pick properties of a specific type:


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

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

Playground Link