How to narrow union types in array with typescript?-CodePudding

I'm trying to put together some data, that's an array of union types, and process it accrodingly. I don't understand why typescript can't narrow this union types down:

interface BaseEvent {
    id: number;
}

interface EventA extends BaseEvent {
    attrA: string;
}

interface EventB extends BaseEvent {
    attrB: string;
}

type Events = (EventA | EventB)[];

const events: Events = [{ id: 1, attrA: "A" }, { id: 2, attrB: "B" }]

events.forEach(event => { // type of event is already: (parameter) event: EventA | EventB
    if (typeof event === "EventB") { // <- it doesn't work 

        const eventId = event.id // can only infer id here
    }
})

I've tried to use intersections suggested by this answer, but it does not seems to work either. What's the right approach here?

TS playground

CodePudding user response：

Since your events have differing properties, you can use in:

if ("attrA" in event) { // event is now EventA

Playground