无法使用 TypeScript 推断数组类型

Question

This is my code:

interface ItemA{
  x:number
  y:string
}

interface ItemB{
  z:boolean
}

interface Data{
  a:ItemA[]
  b:ItemB[]
  bool:boolean
}

type ItemType=keyof Data&('a'|'b')

function f<K extends ItemType>(data:Data,key:K):void{
  type T=Data[K] extends (infer TT)[]?TT:never
  const values:T[]=data[key]

  values.forEach(
    (t:T):void=>{
      console.log(t)
    }
  )
}

I'd like T to be ItemA when key is 'a' and ItemB when it's 'b' . In this way, values could be of type ItemA[] or ItemB[] . How could I achieve this?

I'm getting this TypeScript error:

20:9 Type 'ItemA[] | ItemB[]' is not assignable to type 'T[]'.
  Type 'ItemA[]' is not assignable to type 'T[]'.
    Type 'ItemA' is not assignable to type 'T'.

Answer 1

Like this?

interface ItemA{
  x:number
  y:string
}

interface ItemB{
  z:boolean
}

interface Data{
  a: ItemA[]
  b: ItemB[]
  bool: boolean
}

type ItemType = 'a' | 'b';

function f(data: Data, key: ItemType): void
{
  type T = Data[typeof key]
  const values: T = data[key]
  values.forEach(
    (t: ItemA | ItemB) => {
      console.log(t)
    }
  )
}

Playground