了解TypeScript中的特定继承类型

Question

Could somebody please explain what does this kind of inheritance in TypeScript mean? There is any chance to create alias for everything what we have after extends keyword?

class MyClass extends (FooClass as { new(): BarClass })

Answer 1

It's important to keep in mind that classes in TypeScript exist both in 'value' space (the realm of objects that exist at run time) and in 'type' space (the realm of concepts that only exist at compile time). Consider how this gets compiled into ES5:

class MyClass extends (FooClass as { new(): BarClass }) { }

Becomes:

var MyClass = /** @class */ (function (_super) {
    __extends(MyClass, _super);
    function MyClass() {
        return _super !== null && _super.apply(this, arguments) || this;
    }
    return MyClass;
}(FooClass));

Note that FooClass is passed in as part of the type definition idiom as a reference to the _super class, but BarClass isn't referenced at all (hint: because it's in 'type' space). So the FooClass here references the value of FooClass , but assumes the value of FooClass is of type { new(): BarClass } .

In other words the type that's used here for the purpose of type checking is just { new(): BarClass } but at run-time, MyClass will actually extend FooClass . I suppose how much difference this distinction really makes depends on the exact definitions of FooClass and BarClass .