Generic TypeScript function returning type of object key

Question

I have the following code structure to manage some settings :

interface Settings {}

interface ServerSettings extends Settings {
    host: string;
    port: number;
    valid: boolean;
}

interface SomeOtherSettings extends Settings {
    enabled: boolean;
}

class SettingsProvider {
    constructor(private settings: Settings) {}

    public get<S extends Settings>(
        key: keyof S,
    ): S[typeof key] {
        return (this.settings as unknown as S)[key];
    }
}

const settings: ServerSettings & SomeOtherSettings = {
    host: 'localhost',
    port: 8080,
    valid: true,
    enabled: false,
};
const provider = new SettingsProvider(settings);
const port = provider.get<ServerSettings>('port');

The problem is that the type of port is const port: string | number | boolean (the union of all the possible types of ServerSettings), and not number, as one would expect, given the return type of the function being S[typeof key].

How can I get the right type narrowed ?

One solution could be to pass a second generic to the function like Key extends keyof S but this seems very redundant.

Answer 1

One solution could be to pass a second generic to the function like Key extends keyof S but this seems very redundant.

This sounds like typescript - Providing only some types for generic type - Stack Overflow.

A workaround is to curry the function:

class SettingsProvider {
    constructor(private settings: Settings) {}

    public get<S extends Settings>() {
        return <K extends keyof S>(key: K) => {
            return (this.settings as S)[key];
        }
    }
}

const settings: ServerSettings & SomeOtherSettings = {
    host: 'localhost',
    port: 8080,
    valid: true,
    enabled: false,
};
const provider = new SettingsProvider(settings);

const port = provider.get<ServerSettings>()('port');

const getter = provider.get<ServerSettings>();
const host = getter('host');
const valid = getter('valid');