Typescript: Why Partial Utility Type doesn't work on Indexed Access Types

Question

When using Partial on indexed access type, it doesn't set the properties to optional:

e.g.

class Klass {
    prop = {
        a: true,
        b: 42, 
    };

    func() {
        let val: Partial<typeof this["prop"]>;
        val = {a: false}; // error: Type '{ a: false; }' is not assignable to type 'Partial<this["prop"]>'.
        val = {a: false, b: 13}; // ok
    }
}

(see in typescript playground)

my use case is something like:

class Base {
    setState(s: Partial<typeof this["state"]>){}
}

class Subclass extends Base {
    state = {
        a: true,
        b: 42,  
    };

    func() {
        this.setState({a: false});
    }
}

why is this not working and how can i get it to work?

Answer 1

You can use the name of the class and access prop without using this:

let val: Partial<Klass["prop"]>

Playground