Narrow union type with 'in' operator

[jovdb]

Like we can narrow down union types with typeof and instance of, it would be nice if it could also be narrowed with the javascript in operator.

Code

type Message1 = { name: "foo"};
type Message2 = { name: "personAdded", personId: string, fullName: string};
type Message3 = { name: "personSelected", personId?: string };
type AnyMessage = Message1 | Message2 | Message3;

function handleMessage(message: AnyMessage) {

    // message can be: Message1 | Message2 | Message3
    if ("personId" in message) {

        // narrowed to: Message2 | Message3
        switch (message.name) {
            case "personAdded":    console.log(`Person added: ${message.fullName}`); break;
            case "personSelected": console.log("Person selection changed"); break;
            default: exhaustiveFail(message); break; // error: Message1 is not handled
        }
    }    
};

function exhaustiveFail(x: never): never {
    throw new Error(`Unexpected object: ${x}`);
}

When using an exhaustive switch/if, it would allow us to reduce the union type down to only the types that can have the specified property.

Remark
What to do with numbers and symbols?

const a = ["foo", 42];
if (0 in a) {...}
if (Symbol.iterator in a) {...}

[jovdb]

Can be marked as duplicate, found a similar issue: #10485, and #15256

[DanielRosenwasser]

Thanks for digging those up @jovdb.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.