CODEWITHSUNDEEP
typescript
Peter
Peter

Reputation: 505

Accessing properties in union types

Please consider the simple union type example below:

interface Alarm {
    alarmText: string,
    quali: number
}

interface Car {
    speed: number
}

type unionT = Alarm | Car;


var alarm: Alarm = {
    alarmText: "ALARM!!!",
    quali: 42
};

var bar: unionT = alarm;

bar.alarmText // ERROR

When the type unionT consist of different interfaces like Alarm and Car I cannot access the alarmText property anymore. The way I understand it is that the compiler cannot infer that I am referring to the alarm object. Only non-disjoint properties can be accessed in union types. Right?

If this is correct, how can I ever get my original alarm object back once it is declared in a variable of the union type? I first though that I could do a type guard like

if (typeof bar === 'Alarm') {
    bar.alarmText
}

But typeof bar is just 'object' and thus the gaurd does not make any sense.

Anyone?

Upvotes: 3

Views: 482

Answers (1)

Fenton
Fenton

Reputation: 250882

You can write a custom type guard to achieve this:

function isAlarm(a: Alarm | Car): a is Alarm {
    // Some check to see if this is an Alarm
    return (<Object>a).hasOwnProperty('alarmText');
}


if (isAlarm(bar)) {
    bar.alarmText;
}

Upvotes: 1

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