Can I use arg1 of function to narrow the type of arg2 in the same function?

Question

Here is the example of what I want to do:

const obj = {
  parent1: {
    child1: "child-key1",
    child2: "child-key2",
  },
  parent2: {
    child3: "child-key3",
    child4: "child-key4",
  }
} as const

const func = (
  parentKeyName : keyof typeof obj,
  childKeyName: keyof typeof obj[parentKeyName]
) => {}

// When I fill first arg, I want code completion to suggest me the list of keys for second arg
func("parent2", "child3") 

But the definition of 'func' above gives me an error:

Is there any way to use first arg to narrow the type of second arg?

Answer 1

Using TypeScript Generics:

const obj = {
    parent1: {
        child1: "child-key1",
        child2: "child-key2",
    },
    parent2: {
        child3: "child-key3",
        child4: "child-key4",
    }
} as const

const func = <
    T extends keyof typeof obj,
    K extends keyof typeof obj[T]
>(
    parentKeyName: T,
    childKeyName: K
) => { }

func("parent1", "child1") // ok
func("parent1", "child2") // ok 
func("parent2", "child3") // ok 
func("parent2", "child4") // ok 
func("parent1", "child3") // wrong 
func("parent1", "child4") // wrong 
func("parent2", "child1") // wrong 
func("parent2", "child2") // wrong