Useful error checking for promise chaining

is-instance.ts

type Constructor<T> = new (...args: any[]) => T;
type Then<T, R> = (value: T) => R;

export function ifInstance<T, R>(instance: Constructor<T>, then: Then<T, R>) {
    return (value: unknown) => {
        if (value instanceof instance) {
            return then(value);
        }

        throw value;
    };
}

export function ifError<R>(then: Then<Error, R>) {
    return ifInstance(Error, then);
}

usage.ts

// Usage

declare function something(): Promise<void>;

class MyError extends Error {
    constructor(public readonly code: number) {
        super('Something went wrong!');
    }
}

// Catch all possible results for whatever could
// be thrown in this promise chain. 
const result = await something()
    .catch(ifInstance(MyError, err => err.code))
    .catch(ifError(err => err.message))
    .catch(() => null);