Access Object property in Typescript

Question

I have function written in typescript where i am passing the object and key as an input parameter and in function return i am expecting the object with key value pair:

const article = {
    id: 1,
    title: "TypeScript Explained",
    active: true,
    getSlug: () => 'typescript-explained',
};

type Pick<T, U extends keyof T> = { [K in U]: T[K] };

function pick<T,U extends keyof T>(obj:T,key:U):Pick<T,U>{
    return  {[key]: obj[key]}
}

pick(article,'title')

But on calling the function i am getting the error like Type '{ [x: string]: T[U]; }' is not assignable to type 'Pick<T, U>'.

Can someone help?

Please format the question so we can read it properly

Lyokolux
– Lyokolux

2023-01-31 16:12:38 +00:00
Commented Jan 31, 2023 at 16:12 — Lyokolux
– Lyokolux, Commented Jan 31, 2023 at 16:12

vairaden · Accepted Answer · 2023-01-31 16:38:54Z

1

Typescript infers that type of {[key]: obj[key]} is { [x: string]: T[U]; }.

Thats why it panics when you try to return it as Pick<T,U>

You can either let it infer return type:

 function pick<T, U extends keyof T>(obj: T, key: U) {
    return { [key]: obj[key] };
  }

then function returns { [x: string]: T[U]; }

or explicitly set type of {[key]: obj[key]} with as:

 function pick<T, U extends keyof T>(obj: T, key: U): Pick<T, U> {
    return { [key]: obj[key] } as Pick<T, U>;
  }

then function returns value of type

Pick<{
    id: number;
    title: string;
    active: boolean;
    getSlug: () => string;
}, "title">

answered Jan 31, 2023 at 16:38

vairaden

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4 Comments

Vinayak Vohra Over a year ago

what about function pick<T, U extends keyof T>(obj: T, key: U): Partial<T> { return { [key]: obj[key] } as T; } ?

vairaden Over a year ago

@VinayakVohra Result type is: Partial<{ id: number; title: string; active: boolean; getSlug: () => string; }> since T is typeof obj

ram ahuja Over a year ago

Yes, that seems to be working. Thank you. But when i already defined key as Union of keys(with keyof operator) from the object then why its treating it as 'string' rather than a object key.

vairaden Over a year ago

@ramahuja Second argument of pick() has typeof "id" | "title" | ... <all keys of first argument as strings> so typeof key will be string since you can pick any of available strings I think

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