打字稿：通用类型使用Extends关键字而不是带有接口的Implements

Question

From what I understand, an object can implement an interface and an object can extend a class.

However, when using generic type, one must use extends to say that the object must implements the interface.

Ex:

interface ITest {
    a:number;
}

function myF1<U extends ITest>(a:U){}    //valid
function myF2<U implements ITest>(a:U){} //error

Is there a reason for this inconsistency? The use of implements makes more sense to me.

Answer 1

an object can implement an interface and an object can extend a class

A class can implement an interface, and a class can extend a class. An interface can also extend an interface (that is, a type can extend a type), which is consistent with classes extending other classes. And that's what's happening here: when you write U extends ITest , you are saying that the generic type U extends the type ITest .

Furthermore, in TypeScript, when you define a class:

class Foo {}

...you are actually specifying two things: first, you are saying that you want some code (the standard constructor function), and second, you are specifying a type called Foo , which you can use as if it were an interface, because that's pretty much what it is. (In a certain sense, a class is just an interface plus some code.)