Typescript typing property getter creator

Question

My issue is similar to the one described here and here (and probably other places too). I feel like it's a simple Lookup Type issue (documented here). But my use case is slightly different and I cannot get it to work.

I have a function that takes an object of Type and returns a "getter" function for this object. Here's a simple playground.

type User = {
    name: string,
    age: number,
};

const makeGetter = <Type, Key extends keyof Type>(obj: Type) => (key: Key) => obj[key];

const user: User = {
    age: 55,
    name: 'Brian',
};

const getter = makeGetter(user);
const u = getter('age');

However, the selected property is always a union of all possible property types.

What am I missing?

Shaegi · Accepted Answer · 2021-04-26 08:16:53Z

1

The problem is that your generic will be created on creation of the makeGetter function. At this time Keyisn't resolved to a specific key yet. Move the generic to the curried function.

const makeGetter = <Type extends Object>(obj: Type) => <K extends keyof Type>(key: K) => obj[key]

Playground

answered Apr 26, 2021 at 8:16

Shaegi

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1 Comment

bmakan Over a year ago

After meddling with it for a while longer, I reached the same conclusion as well. This way it indeed works.

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