Switch for specific type in TypeScript

Question

I have an interface Action:

interface Action {} 

And an implementation of this Action SpecificAction:

class SpecificAction implements Action {    payload?: Any } 

Is it possible in TS to construct a switch operator, like this:

let action: Action switch (action) {    case SpecificAction: //it works        console.log(action.payload) // it doesn't  } 

Is it possible in that case to know, that action is already of SpecificAction type?

Answer 1

for the time being it looks like there are a few options, all of them with some drawbacks

• discriminated unions docs stackblitz, but you'll need a dedicated property as discriminator

interface Action {}  class SpecificAction implements Action {   kind: "specific";   payload?: any; }  class ToggleAction implements Action {   kind: "toggle";   toggle: boolean; }  let action: SpecificAction | ToggleAction; switch (action.kind) {   case "specific":     console.log(action.payload) // it works      break;   case "toggle":     console.log(action.toggle) // it works      break;         } 

• User-Defined Type Guards docs stackblitz, but you'll need if statements instead of switch

interface Action {}  class SpecificAction implements Action {   payload?: any; }  class ToggleAction implements Action {   toggle: boolean; }  let isSpecific = (p: any): p is SpecificAction => !!p.payload let isToggle = (p: any): p is ToggleAction => !!p.toggle  let action: Action; if (isSpecific(action)) {   console.log(action.payload) // it works  } else if (isToggle(action)) {   console.log(action.toggle) // it works  } 

• constructor property github stackblitz, but you'll need to cast to desired type for the time being

interface Action { }  class SpecificAction implements Action {   payload?: any; }  class ToggleAction implements Action {   toggle: boolean; }  switch (action.constructor) {   case SpecificAction:     console.log((<SpecificAction>action).payload) // it kinda works      break;   case ToggleAction:     console.log((<ToggleAction>action).toggle) // it kinda works      break;   }