TypeScript Mapped Types: Get element type of array

user_2564384 0
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Suppose having a container type with array properties of unknown/generated types T1, T2, etc. (short T*):

interface MultiContainer
{
    Item1: T1[];
    Item2: T2[];
    ...
}

Is it possible to derive the following type using mapped types:

interface SingleContainer
{
    Item1: T1;
    Item2: T2;
    ...
}

I'm looking for some expression like:

type SingleContainer =
    { [ P in keyof MultiContainer ]: MultiContainer[P] }
                                            └─────────── returns T*[] instead of T*

MultiContainer[P]returns the types T*[] but I need an expression that returns T*

Thanks in advance!

mapped-types typescript
2 Answers

From TS 2.8 it's possible to use infer keyword:

type SingleContainer = {
  [P in keyof MultiContainer]: MultiContainer[P] extends (infer T)[] ? T : never
};
user_71341490
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