How to get Type of Array in Typescript generics

Question

I have a method like this:

public select(fieldName: keyof TType)

Where TType could be an array type. In case of an array type

Answer 1

You will need a little helper to extract the boxed type:

type Unboxed =
    T extends (infer U)[]
        ? U
        : T;

Then your method can look like this:

interface User {
    id: symbol;
    name: string;
}

class Foo {
    select(fieldName: keyof Unboxed) {
        console.log(fieldName) // "id" | "name"
    }
}

As to your extra question: yes, it's possible, but it may feel a bit strange.

class Foo {
    select(fieldName: keyof Unboxed) {
        console.log(fieldName)
    }
}

new Foo()
  .select('addEventListener')

Type parameters are meant to describe the arguments living inside the method or the generic type of the class. So perhaps you wanted to do the following:

class Foo {
    select(fieldName: keyof Unboxed) {
        console.log(fieldName)
    }
}

new Foo()
  .select('addEventListener')