How to check union arguments of interface types

Question

callbackFn(args: IGovedo | IKrava) {
   // How to check here type of args 
}

In the upper code as you can guess IGovedo and IKrava are interface types. If I use this approach what is the best way to check whether args is IGovedo, IKrava, null, or undefined?

The latest version 1.6 of typescript is used.

Edited: Not really duplicate of older question, but resolved - marked resolving answer.

It is impossible to check an interface at runtime. `instanceOf` will only work on classes — Bruno Grieder, Nov 16 '15 at 09:34
Possible duplicate of [Interface type check with Typescript](http://stackoverflow.com/questions/14425568/interface-type-check-with-typescript) — Bruno Grieder, Nov 16 '15 at 09:35

score 3 · Accepted Answer · answered Nov 16 '15 at 10:32

3

What you need is user-defined type guard functions.

interface IGovedo {
    govedo: string;
}

interface IKrava {
    krava: string;
}

function isGovedo(object: any): object is IGovedo {
    return 'govedo' in object;
}

let foo: IGovedo | IKrava;

if (isGovedo(foo)) {
    // foo has type IGovedo;
} else {
    // foo has type IKrava.
}

answered Nov 16 '15 at 10:32

vilicvane

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Documentation link for reference: https://www.typescriptlang.org/docs/handbook/advanced-types.html – Ryan Q Sep 11 '16 at 08:57

How to check union arguments of interface types

1 Answers1