Typescript pattern matching with subtype of string literal type

Question

export interface Action{
    type: string;
}

    export interface LoadTodos {
      type: "LOAD_TODOS_ACTION"
    }

    export interface AddTodo {
      type: "ADD_TODO_ACTION",
      todo: Todo
    }

export type KnownAction = LoadTodos | LoadTodosSuccess | AddTodo;

function isSomeType<T extends KnownAction>(x: any): x is T {
    return x && typeof (x.type) === "LOAD_TODOS_ACTION";
}


let knownAction: actions.KnownAction = { type: "LOAD_TODOS_ACTION" };


if (isSomeType(knownAction)) {
    let loadTodosAction = knownAction; // load Todos type
}

I'm expecting the behaviour above. I want to do this in a generic way so I don't want to repeat if, else statement in isSomeType function.Basically I want to convert string literal type into appropriate type based on 'type' property. My attempt was to do this:

function isSomeType<T extends Action>(x: any): x is T {
    type p1 = T['type'];
    return x && typeof (x.type) === p1;
}

But the message says: 'p1' only refers to a type but is used as a variable here.

Is there a way to do this?

MistyK · Accepted Answer · 2016-12-29 17:48:00Z

1

Surprisingly the answer is: do nothing.

function isSomeType<T extends KnownAction>(x: T):T {
    return x;
}

Basically typescript will interfere type automatically whenever function with the above signature is called.

EDIT:

 let knownAction: actions.KnownAction = { type: "LOAD_TODOS_ACTION" }; // KnownAction type
 let knownAction2 = isSomeType(knownAction) // LoadTodosAction type

edited Dec 29, 2016 at 17:48

answered Dec 29, 2016 at 17:15

MistyK

6,2825 gold badges48 silver badges81 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

Paarth Over a year ago

isn't this always trivially true?

MistyK Over a year ago

I wouldn't expect this behavior. I have .NET background and if you pass an object of type KnownAction you'll get KnownAction in return. Here you get an exact subtype.

Paarth Over a year ago

So here's my confusion. Originally you were writing something that looked like a user defined type guard. I didn't realize that this was no longer a type guard at first. What you have here genuinely does nothing.

MistyK Over a year ago

I am trying to convert string literal type into appropriate type without specyfiyng it explicitly. See edit.

Paarth Over a year ago

this feels equivalent to let knownAction2 = knownAction

Paarth · Accepted Answer · 2016-12-29 17:12:42Z

0

Not a definitive answer, but I don't believe you can.

FWIW the recommended approach would probably be a switch on the discriminating member.

As for your code, you may be losing track of where type information is lost. The typeof operator will not be meaningful here. typeof is a runtime operation and even if you write type: "LOAD_TODOS_ACTION" in your code at runtime typeof x.type will return string which I expect is not helpful to you. A string literal type means that the only acceptable assignable value is that string, not that a new type is introduced.

So in order to achieve what you're looking for we'd need to be able to get an interface member's compile time type information. As far as I know, there is no way to do this. When there is one, this is possible, but until then I don't believe it is.

answered Dec 29, 2016 at 17:12

Paarth

10.5k4 gold badges30 silver badges37 bronze badges

1 Comment

MistyK Over a year ago

It's not possible to do it this way because type aliases are not available in the runtime.

Collectives™ on Stack Overflow

Typescript pattern matching with subtype of string literal type

2 Answers 2

5 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

5 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related