How can I correctly type the following:

type Events = 'DATA' | 'ABC' | 'DEF'

type PayloadMap = {
    DATA: { x: number }
    ABC: { y: string }
    DEF: null
}

export type Handlers<E extends string, P extends Record<E, any>> = { 
    [event in E]?:  (payload: P[E]) => any
} 

const handlers: Handlers<Events, PayloadMap> = {
    DATA: (payload) => {
        // ERROR: Property 'x' does not exist on type '{ x: number; } | { y: string; } | null'.
        const { x } = payload;
        console.log(x)
        return null
    }
}

I would expect the payload scoped to DATA: to resolve as the single variant { x: number }. How can I achieve this without breaking up the creation of handlers into multiple steps?

CodePudding user response：

Your Handlers type is incorrect, because it would allow the call

handlers.DATA?.({ y: "Hello "});

without error, which is why you're getting an error in DATA:

The correct definition, given what you want, is

export type Handlers<E extends string, P extends Record<E, any>> = { 
    [event in E]?:  (payload: P[event]) => any
}