CODEWITHSUNDEEP
typescript
Hivaga
Hivaga

Reputation: 4496

Extract array values as Typescript types

Basically is there a way to extract array values as a string types? Here is the pseudo code:

class A
{
    public keys = ['name', 'age']
}

type ArrayKeys = P in A['keys'] // 'name' or 'age'

How to narrow ArrayKeys to be with possible values of A['keys'].

Upvotes: 2

Views: 2023

Answers (2)

Madara's Ghost
Madara's Ghost

Reputation: 174957

You will need to assure TypeScript that keys will not change by adding an as const cast (otherwise, TypeScript will just assume it's a string array), note that this will make TypeScript enforce this. You will not be able to change the array.

After that, it's fairly simple:

class A
{
    public keys = ['name', 'age'] as const
}

type ArrayKeys = A['keys'][number];

On the playground.

The [number] part is the "accessing" of the array's value. (Array<T>)[number] will always be T.

Upvotes: 10

T.J. Crowder
T.J. Crowder

Reputation: 1074148

I don't think you can literally do that, since of course types are a static compile-time feature and arrays are a dynamic runtime feature. ...unless you add const as Madara Uchiha shows us.

But you could use a string enum instead of an array, and then get its values to fill in the array.

enum ArrayKeys {
    name, age
}
class A {
    public keys = Object.keys(ArrayKeys).filter(key => isNaN(Number(key)));
}

On the playground.

This also has the advantage of providing ArrayKeys.name and ArrayKeys.age as enum values.

Upvotes: 2

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