Infer exact object property type when returning it via function-CodePudding

Given the following state:

  type DataType = {
        a: string,
        b: number,
        c: boolean
  }

  const testObject: DataType = {
        a: 'Dummy',
        b: 1,
        c: true
  }

  type A = typeof testObject['a'] // string
  type B = typeof testObject['b'] // number
  type C = typeof testObject['c'] // boolean

It is clear that I can get the type of each property of the DataType object by using either typeof testObject['a'] or DataType['a'].

However, when I try to do the same via accessor inside a function, I cannot get the exact type of the property I am accessing. Example below:

  function testAccess<T>(o: T, p: keyof T): T[typeof p] {
        return o[p]
  }

  const a = testAccess(testObject, 'a') // string | number | boolean

Am I missing something, and is there any way to narrow down the type to the exact key that we are sending?

TS playground link: https://www.typescriptlang.org/play?jsx=0#code/C4TwD...

CodePudding user response：

The trick is to include the key type as a generic parameter:

function testAccess<T, P extends keyof T>(o: T, p: P): T[P] {
      return o[p]
}

const a = testAccess(testObject, 'a') // string

Playground link