Type guarding class in switch statement

Question

I haven't use Typescript in many years and can't remember nor find how to type guard many classes inside a switch statement correctly.

class A {}
class B {}
class C {}

type OneOfThem = A | B | C;

function test(foo: OneOfThem): string {
    switch(/* something using foo */) {
        /* A */:
            return "A";
        /* B */:
            return "B";
        /* C */:
            return "C";

        /* should not need to use "default" as all cases are handled */
    }
}

I found and tried several options like:

• Using foo.constructor
• Using instanceof inside the case statements
• adding an extra member to the three classes to be used in the switch statement

but none of them works (Function lacks ending return statement and return type does not include 'undefined').

Is my memory playing tricks and this is not possible with classes ?

Answer 1

adding an extra member to the three classes to be used in the switch statement

That is indeed probably the simplest solution for your case: it uses Discriminated Union, by adding a discriminant member to each class, with literal types (i.e. not just string or number), so that you (and TypeScript) can tell apart foo is from which class:

class A {
    readonly kind = "a"
    //       ^? "a" string literal, not just `string`
}
class B {
    kind: "b" = "b"
    // ^? "b" string literal, as explicitly typed
}
class C {
    kind = "c" as const
    // ^? "c" string literal, thanks to the const assertion
}

Then simply differentiate based on this kind discriminant property:

function test(foo: OneOfThem): string { // Okay
    switch (foo.kind) {
        case "a":
            foo
            //^? A inferred by TS, based on `foo.kind === "a"` type guard
            return "A";
        case "b":
            foo
            //^? B
            return "B";
        case "c":
            foo
            //^? C
            return "C";
        /* should not need to use "default" as all cases are handled */
    }
}

Playground Link