index.ts

type Omit<T, K extends keyof T> = Pick<T, Exclude<keyof T, K>>
type Override<T, K extends keyof T, V> = Omit<T, K> & { [N in K]: V }

type AnyStateConfig = {
    type: "initial" | "state" | "final"
    states: {[K: string]: AnyStateConfig}
}

type EmptyStateConfig = {
    type: "state"
    states: {}
}


class State<C extends AnyStateConfig = EmptyStateConfig>{

    constructor(
        public readonly config: C
    ){

    }

    type<StateType extends AnyStateConfig["type"]>(type: StateType): State<Override<C, "type", StateType>>{
        return new State({ ...(this.config as any), type})
    }
}