TypeScript type assertion if my object is empty and hasn't the interface keys

Question

I have an interface:

interface User { 
    name: string,
    age: number
}

function:

function test(user: User): void { 

}

and empty object created with the interface:

const user1 = <User>{};
test(user1);

So I don't understand why my code is compiling, because my empty object hasn't the interface keys. How can I prevent compiling if my object is empty and hasn't the interface keys?

Mu-Tsun Tsai · Accepted Answer · 2019-06-25 07:27:47Z

5

<User>{} is type assertion, which basically means you're telling TypeScript that "Yeah, it is a User alright, don't tell me otherwise". Therefore TypeScript doesn't report any error.

If you want TypeScript to catch your coding error, in this case you should use

const user1: User = {};

instead, and you'll get the error you're asking for. For more information, see this page.

edited Jun 25, 2019 at 7:27

answered Jun 25, 2019 at 7:15

Mu-Tsun Tsai

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2 Comments

HDallakyan Over a year ago

Yes, but if I will write const user1 = <User>{z: 4}; I will get an error, so why if I use an empty object it doesn't show me error?

Mu-Tsun Tsai Over a year ago

This is because in this case the neither types User and {z: number} overlap the other one. Overlapping is necessary for using type assertion.

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