Typescript get T out of Array, where T can be anything

Question

I'll start with example code.

toggle<K extends keyof T, V = T[K] extends any[] ? T[K] : never>(key: K, value: V): ...

I call this function on array-like properties. Since V or T[K] is array-like type (eg. string[]), I want it to be just string. Note that it can be any array type.

Is this possible to get just string type out of V?

interface ITest {
  prop1: number;
  prop2: number[];
}

T is for example ITest When key(K) is prop2, V should be number, else if key is prop1, V should be never

Behaviour:

toggle("prop2", "string") type error

toggle("prop2", [1, 2]) type error

toggle("prop2", 1) valid

Hi, can you put in the question the whole code snippet, as you dont even provide variable `T` here. — Maciej Sikora, Mar 02 '20 at 12:39

score 1 · Answer 1 · answered Mar 02 '20 at 12:42

The simplest way is to use a lookup type with number as the property type:

type ArrayComponent<A extends any[]> = A[number];

You can also do it using a conditional type with an infer declaration:

type ArrayComponent<A extends any[]> = A extends (infer U)[] ? U : never;

Either way, ArrayComponent<V> will be the component type when V is an array type.

Typescript get T out of Array, where T can be anything

1 Answers1