TypeScript Mapped Types: Get element type of array

Question

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!

Frank Modica · Accepted Answer · 2017-06-30 14:50:04Z

4

I believe this does what you need:

type SingleContainer = {[P in keyof MultiContainer]: MultiContainer[P][0]}

answered Jun 30, 2017 at 14:50

Frank Modica

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Comments

Andrew · Accepted Answer · 2020-11-03 17:58:04Z

2

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

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

answered Nov 3, 2020 at 17:58

Andrew

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1 Comment

wensveen Over a year ago

Quite genius! This allows you to construct a Flat<T> type: type Flat<T> = { [P in keyof T]: T[P] extends (infer A)[] ? A : T[P]; }. Usage:

interface MixedContainer { Item1: T1; Item2: T2[] }; type FlatContainer = Flat<MixedContainer>; // { Item1: T1; Item2: T2 }

Throw in some string template typing and you can even go from { Items: T1[] } to { Item: T1 }.

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