Map object key to value type in class generic

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I have the following code:

interface Person {
    name: string;
    age: number;
} 

class Store<T, K extends keyof T> {
    constructor(private obj: T) { }

    get(key: K): T[typeof key] {
        return this.obj[key];
    }
}

const person: Person = {
    name: 'John',
    age: 99
};

const store = new Store(person);
const value = store.get('name');

value type shows:

const value = store.get('name');
   // ^^^^^ value: string | number

How to infer the value type of an object given its key as an argument in a class method?

I want value to be properly typed as string in this case. How to achieve this?

Typescript playground

typescript-generics typescript
1 Answers

The get function itself needs to be generic so it can use the type of the argument. This allows it return a specific key from T like T[K].

And the K generic on the class now no longer makes sense. keyof T would be the same thing anywhere in that class, so you can remove it completely.

Put another way Store as a class doesn't think any one key is special, so it doesn't need a generic for that. But the get method needs to get one specific key, so the type system needs to know which specific key, so you need a generic there.

class Store<T> {
    constructor(private obj: T) { }

    get<K extends keyof T>(key: K): T[K] {
        return this.obj[key];
    }
}

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