TypeScript: How to get nested functions to union their types through a chain-CodePudding

What I'm trying to do is create a series of nested functions which can be chained together, and the final callback's arguments will be a union of the parents. Example of my failed attempt so far:

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export type SomeHandler<T> = (args: T) => void;

type WithFooArgs = { foo: string }
const withFoo = <T,>(handler: SomeHandler<T & WithFooArgs>) => (args: T & WithFooArgs) => handler(args);

type WithBarArgs = { bar: string }
const withBar = <T,>(handler: SomeHandler<T & WithBarArgs>) => (args: T & WithBarArgs) => handler(args);

type WithZooArgs = { zoo: string }
const withZoo = <T,>(handler: SomeHandler<T & WithZooArgs>) => (args: T & WithZooArgs) => handler(args);


export default withFoo(withBar(withZoo((args) => {
    // I want args to be type `WithFooArgs & WithBarArgs & WithZooArgs`
})));

The goal is to have a bunch of these which I can chain together in different ways.

CodePudding user response：

You are trying to change the generics of withZoo(..) based upon the expressions around it which is not possible. You could create a single generic that takes in these middleware-like callbacks then use the data from those callbacks to type a single callback function like the following:

export type SomeHandler<T> = (args: T) => void;

type WithFooArgs = { foo: string }
const withFoo = <T,>(handler: SomeHandler<T & WithFooArgs>) => (args: T & WithFooArgs) => handler(args);

type WithBarArgs = { bar: string }
const withBar = <T,>(handler: SomeHandler<T & WithBarArgs>) => (args: T & WithBarArgs) => handler(args);

type WithZooArgs = { zoo: string }
const withZoo = <T,>(handler: SomeHandler<T & WithZooArgs>) => (args: T & WithZooArgs) => handler(args);

// https://stackoverflow.com/a/50375286/10873797
type UnionToIntersection<U> = 
  (U extends any ? (k: U)=>void : never) extends ((k: infer I)=>void) ? I : never;

function createHandler<Handler extends SomeHandler<any>>(handlers: Handler[], cb: (args: UnionToIntersection<Parameters<Parameters<Handler>[0]>[0]>) => void) {
    return (args: any) => {
        return cb(handlers.reduce((acc, cur) => cur(acc), args));
    };
}

export default createHandler([withFoo, withBar, withZoo], (args) => {
    console.log(args); // WithFooArgs & WithBarArgs & WithZooArgs
});

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