协变与逆变

必看 协变与逆变 | 深入理解 TypeScript

class Animal {
    a !: string
}
class Dog extends Animal {
    b !: string
}
class Greyhound extends Dog {
    c !: string
}
class GermanShepherd extends Dog {
    d !: string
}

function f (arg1: (arg1: Dog) => Dog): string {
    return ''
}

// 1、Greyhound → Greyhound
function f1(farg1: Greyhound): Greyhound {
    return {
        a: '',
        b: '',
        c: ''
    }
}
// 2、Greyhound → Animal
function f2(farg1: Greyhound): Animal {
    return {
        a: '',
    }
}
// 3、Animal → Animal
function f3(farg1: Animal): Animal {
    return {
        a: '',
    }
}
// 4、Animal → Greyhound
function f4(farg1: Animal): Greyhound {
    return {
        a: '',
        b: '',
        c: ''
    }
}

f(f4)

TypeScript: Playground