在 Typescript 中使用 `: Interface` 和 `as Interface` 有什么区别？

Question

I'm not sure what these are called so I'm having a bit of trouble searching for an answer. Let's say I have the following interface defined:

interface Person {
  name: string;
  age: number;
}

What's the difference between the following?

const foo: Person = getPerson(data);

and

const foo = getPerson(data) as Person;

Seems like both tell TS that foo is a Person . Is there fundamentally any difference or is it just a matter of using whatever floats my boat?

Answer 1

With

const foo: Person = getPerson(data);

you tell the compiler: I assume that getPerson returns something that implements the Person interface, please check that and scold me if it doesn't.

const foo = getPerson(data) as Person;

is an assertion, with which you assure the compiler that getPerson returns a Person , even if they mean otherwise. Basically, it's a way to shut up the type checker, and, mostly, not a good idea.

Answer 2

const foo: Person = getPerson(data);

Is a type annotation, and means you're saying "I want foo to be of type Person ". However, if getPerson(data) returns something other than a Person , TypeScript will say "No no no, that's not allowed. You want Person but getPerson(data) does not return that, bad!"

---

const foo = getPerson(data) as Person;

as Person is a type assertion, if you wrote this without the as Person , TypeScript would infer foo to be the type of whatever getPerson(data) returns. However, by adding the as Person you are telling TypeScript, explicitly, "Whatever this function returns, even if it's not actually a Person , I am forcing you to type it as a Person ".

Hope that answers it somewhat.