Conditional Types

conditional_types.ts

// From https://artsy.github.io/blog/2018/11/21/conditional-types-in-typescript/
// Includes extra exercises.

type Action =
  | {
      type: "INIT"
    }
  | {
      type: "SYNC"
    }
  | {
      type: "LOG_IN"
      emailAddress: string
    }
  | {
      type: "LOG_IN_SUCCESS"
      accessToken: string
    }

type ActionType = Action["type"]

type OnlyWithoutType<T> = T extends any ?
  {} extends Omit<T, 'type'> ? T : never
  : never;

type OnlyWithType<T> = T extends any ?
  {} extends Omit<T, 'type'> ? never : T
  : never;

type EmptyActions = OnlyWithoutType<Action>;
type EmptyActionTypes = EmptyActions['type'];
type CompexActions = OnlyWithType<Action>;
type ComplexActionTypes = CompexActions['type'];
  
//type X = Omit<
declare function dispatch<T extends EmptyActionTypes>(type: T) : void;

declare function dispatch<T extends ComplexActionTypes>(
    type: T,
    args: ExtractActionParameters<Action, T>
): void

type ExcludeTypeKey<K> = K extends "type" ? never : K

type ExcludeTypeField<A> = { [K in ExcludeTypeKey<keyof A>]: A[K] }

type OnlyTypeKey<K> = K extends "type" ? K : never;

type ExtractActionParameters<A, T> = A extends { type: T }
    ? ExcludeTypeField<A>
  : never
  
// All clear! :)
dispatch("LOG_IN_SUCCESS", {
    accessToken: "038fh239h923908h"
})

dispatch("LOG_IN_SUCCESS", {
    // Type Error! :)
    badKey: "038fh239h923908h"
})

// Type Error! :)
dispatch("BAD_TYPE", {
    accessToken: "038fh239h923908h"
})

// Works!
dispatch("INIT")

// Type Error! :)
dispatch("INIT", {})