Max's Dev Blog

Making the switch => object Pattern Work in TypeScript

October 08, 2019

The switch => object pattern is great

I’m a huge fan of the JavaScript pattern where one replaces a switch:

function handle(event) {
  switch (event.kind) {
    case 'goblins_are_coming':
      repelMonsters(event);
    case 'family_is_coming':
      stockFridge(event);
  }
}

with an object:

const eventHandlers = {
  goblins_are_coming: repelMonsters,
  family_is_coming: stockFridge,
};

The second method really shines when we don’t want to specify the kinds of events we are handling in the same file.

import familyEventHandlers from './familyEventHandlers';

export default const eventHandlers = {
  goblins_are_coming: repelMonsters,
  ...familyEventHandlers,
};

The pattern doesn’t work in TypeScript very well

Assume this is written above the rest of the TypeScript examples:

interface GoblinEvent {
  kind: 'goblins_are_coming';
  smell: number;
}

interface FamilyEvent {
  kind: 'family_is_coming';
  notice: number;
}

type OurEvent = GoblinEvent | FamilyEvent;

declare const repelMonsters: (event: GoblinEvent) => void;
declare const stockFridge: (event: FamilyEvent) => void;

TypeScript plays fine with the first JavaScript example.

function handle(event: FamilyEvent | GoblinEvent) {
  switch (event.kind) {
    case 'goblins_are_coming':
      repelMonsters(event);
    case 'family_is_coming':
      stockFridge(event);
  }
}

But can’t make sense of the second one.

const eventHandlers = {
  goblins_are_coming: repelMonsters,
  family_is_coming: stockFridge,
};

declare const e: OurEvent;

eventHandlers[e.kind](e);

This ☝ produces this 👇 error message.

Argument of type 'OurEvent' is not assignable to parameter of type 'GoblinEvent & FamilyEvent'.
  Type 'GoblinEvent' is not assignable to type 'GoblinEvent & FamilyEvent'.
    Property 'notice' is missing in type 'GoblinEvent' but required in type 'FamilyEvent'.ts(2345)

Here’s the great (terrible?) hack to make it work

function handle<P extends OurEvent, T extends P['kind']>(
  payload: P,
  stuff: { [K in T]: (arg0: P) => void }
) {
  stuff[payload.kind](payload);
}

handle(e, eventHandlers);

No type errors! 🎊

… but with a catch.

This works because the inteface is still generic when accessed. To keep this compiling, the object must preseve it’s mapped typings as long as the event is generic.

That is to say, the following produces a type error:

function handleWrapper<P extends OurEvent>(payload: P) {
  handle(payload, eventHandlers);
}

TLDR:

Check this out:

function handle<P extends OurEvent, T extends P['kind']>(
  payload: P,
  stuff: { [K in T]: (arg0: P) => void }
) {
  stuff[payload.kind](payload);
}

Maxwell Burson

Maxwell Burson writes about web developement and TypeScript