How type assertions "as" work in typescript

Question

Explain how as works please:

I have a simple interface.

interface myModel {
    name: string;
    id: number;
}

and there is a function that returns an object { } as myModel;

function getMyModel(): myModel{
    return {
        name: 'kek',
        id: 8,
        data: 'pew',
        names: '321'
    } as myModel
}

Why, if I add properties that are not in myModel , the compiler does not throw an error and logs an object with properties that are not in the myModel interface?

console.log(getMyModel());
//[LOG]: {
//  "name": "kek",
//  "id": 8,
//  "data": "pew",
//  "names": "321"
//} 

How does as work?

And why if I write like this:

let test: myModel = {
         name: 'kek',
         id: 8
         data: 'pew',
         names: '321'
}

then on the line with data: 'pew', the compiler will highlight it as an error?

Answer 1

as completely overrides type checking.

When you use it, you are telling the compiler that the object is of type myModel and it will treat it as such.

It is a way to tell the compiler that you, the programmer, know what type the object it better than the compiler does.

---

In your particular example the as statement is very clearly a lie.

It makes more sense when you have something like:

 const myHttpResponse = await fetch(url_to_user_json);
 const myUserObject = (await myHttpResponse.json()) as User;

...since TypeScript only knows that the json() method returns something that can be parsed from JSON (so a string, or a number, or any kind of basic object, etc, etc).

---

I generally prefer to avoid as entirely in favour of performing a runtime check with a custom type guard function so if something goes wrong (e.g. the API design changing and returning JSON in a different shape) then I can catch it early and with a clearer error message).