Type '"test"' cannot be used to index type 'T'

Question

Any idea how this can be achieved?

function getTest<T>(): T["test"] {
    // ...
}

...or dynamically:

function getTest<T, U>(): T[U] {
    // ...
}

Unfortunately, the compiler says Type '“test”' cannot be used to index type 'T'. I'm using TypeScript 2.4.2.

Answer 1

Note that for type inference to work, you'll need to pass a parameter to the function that uses one or more of the generic type parameters.

Otherwise, it will be inferred to {} or any.

---

For the first case:

function getTest<T, U extends {test: T}>(): T {}

For the second case:

function getTest<T, K extends keyof T>(): T[K] {}