Argument of type is not assignable to parameter of type

Question

The following block of code was ripped out of a much larger app to illustrate what was breaking... If I ignore the error, the code itself executes just fine. Just the type hinting isn't liking it, and not sure why.

I do however believe it's something todo with that Omit type.

I am presented with this error:

Argument of type '{ avatar: string; height: number; width: number; } & Pick<P & IInputProps, Exclude<keyof P, "firstName" | "lastName" | "avatar">>' is not assignable to parameter of type 'WrappedType<P>'.
    Type '{ avatar: string; height: number; width: number; } & Pick<P & IInputProps, Exclude<keyof P, "firstName" | "lastName" | "avatar">>' is not assignable to type 'P'.

And the code is (or a gist):

type Omit<T, K extends keyof T> = Pick<T, Exclude<keyof T, K>>;

interface FunctionComponent<P = {}> {
    (props: P, context?: any): string;
}

// Our component props types
interface IInputProps {
    firstName: string;
    lastName: string;
    avatar: string;
}

interface ISubProps {
    width: number;
    height: number;
}

// The helper type that takes an abstract prop type, adds the downstream ISubProps + a type that's based on our IInputProps
type WrappedType<P extends object = {}> = P &
    ISubProps &
    Omit<IInputProps, 'firstName' | 'lastName'>;

type SubComponent<P> = FunctionComponent<WrappedType<P>>;
type WrappedComponent<P> = FunctionComponent<P & IInputProps>;

function factory<P extends object = {}>(
    Component: SubComponent<P>,
): WrappedComponent<P> {

    // The props here are of type P & IInputProps
    return ({ lastName, firstName, avatar, ...rest }) => {
        const restString = Object.entries(rest)
            .map(([key, value]) => `${key}: ${value}`)
            .join('\n');

        // -- THIS BIT DOESNT WORK
        // Component's types are ISubProps + IInputProps (-firstName, -lastName) + P
        const componentResponse = Component(
            {
                avatar,
                height: 10,
                width: 20,
                ...rest,
            },
        );
        // -- TO HERE

        return `FirstName: ${firstName}\nLastName: ${lastName}\n${restString}\n\n--BEGIN--\n${componentResponse}\n--END--`;
    };
}


// Example impl
const test = factory<{ foo: string }>(props => {
    return `hello: ${props.foo}, you have the avatar of ${
        props.avatar
        } with height ${props.height} and width ${props.width}`;
})({
    firstName: 'firstName',
    lastName: 'lastName',
    avatar: 'avatar',
    foo: 'foo',
});

console.log(test);

Answer 1

The problem is that Typescript is very limited in the math it can do on mapped and conditional types that contain unbound type parameters (such as P in your case).

Although it might seem obvious to us, the compiler can't figure out that if you remove lastName, firstName, avatar from P & { firstName: string; lastName: string; avatar: string; } you get P. As long as the parameter P is in there the compiler will not try to resolve the type of rest instead it will type rest as Pick<P & IInputProps, Exclude<keyof P, "lastName" | "firstName" | "avatar">>

There is no safe way to help the compiler along here, you will just have to use a type assertion to let the compiler know rest will be P

const componentResponse = Component({
    avatar,
    height: 10,
    width: 20,
    ...(rest as P),
});