Extending Array prototype with a generic function

Question

I am trying to add a simple generic function to the Array prototype but TypeScript gives me an error about typings that I don't get.

interface Array<T> {
    pluck<T, TKey extends keyof T>(this: T[], key: TKey): T[TKey][];
}

Array.prototype.pluck = function pluck<T, TKey extends keyof T>(this: T[], key: TKey): T[TKey][] {
    return this.map(item => item[key]);
}

Here is a Codesandbox: https://codesandbox.io/s/typescript-playground-export-3oww1?fontsize=14&hidenavigation=1&theme=dark

Thanks for your help :)

Just omit the typings for your function implementation, you already typed it via the interface declaration — cyr_x
– cyr_x, Commented Mar 23, 2020 at 9:40
Please put the relevant code in this question, so it can be of use to people in the future when they visit here. — VLAZ
– VLAZ, Commented Mar 23, 2020 at 9:40
Dont do that, just create your new Array which will extend original one. In general you should not override any prototypes, and for sure not the core ones. — Maciej Sikora
– Maciej Sikora, Commented Mar 23, 2020 at 10:15

Dmitriy · Accepted Answer · 2020-03-23 14:19:12Z

1

When you refer to a generic parameter in the interface or class definition, you should not create one in the method. Right now TypeScript is correctly pointing out that T coming from Array<T> is not the same as T coming from pluck<T. So if you remove one in the interface definition, everything is fine:

interface Array<T> {
    pluck<TKey extends keyof T>(this: T[], key: TKey): T[TKey][];
}

answered Mar 23, 2020 at 14:19

Dmitriy

2,8321 gold badge19 silver badges15 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Extending Array prototype with a generic function

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related