是这样的，Parent和Child类的原型对象可能有函数，如果按照你这种组合继承的方式，好像子类Child的原型的函数就丢失了：

function Parent() {
    this.parentValue = 'parent'
    this.parentArray = [1, 2, 3]
}

Parent.prototype.sayYes = function () {
    console.warn('parent sayYes')
    console.log(this.parentValue, this.childValue)
}

function Child() {
    Parent.call(this)
    this.childValue = 'child'
}

Child.prototype.sayHello = function () {
    console.warn('child sayHello')
    console.log(this.parentValue, this.childValue)
}

Child.prototype = Object.create(Parent.prototype)
Child.prototype.constructor = Child

var c = new Child()
console.log(c instanceof Child, c instanceof Parent)
console.log(c.constructor)

c.sayYes()
c.sayHello()

结果：

你的组合继承的写法是这样的：

Child.prototype = Object.create(Parent.prototype)
Child.prototype.constructor = Child

如果换成这样会不会好一点？

Child.prototype.__proto__ = Parent.prototype

运行结果好像是正确的：

最后附上完整的测试代码，恳请老师帮忙看看最后这个写法有没有问题：

function Parent() {
    this.parentValue = 'parent'
    this.parentArray = [1, 2, 3]
}

Parent.prototype.sayYes = function () {
    console.warn('parent sayYes')
    console.log(this.parentValue, this.childValue)
}

function Child() {
    Parent.call(this)
    this.childValue = 'child'
}

Child.prototype.sayHello = function () {
    console.warn('child sayHello')
    console.log(this.parentValue, this.childValue)
}


console.warn('Child.prototype.__proto__ === Object.prototype')
console.log(Child.prototype.__proto__ === Object.prototype)
Child.prototype.__proto__ = Parent.prototype
var c1 = new Child()
var c2 = new Child()
console.warn('c1.constructor')
console.log(c1.constructor)
c1.sayHello()
c1.sayYes()
c1.parentArray.push(5)
console.warn('c1, c2')
console.log(c1, c2)
console.warn('c1.constructor === Child, c1.constructor === Parent')
console.log(c1.constructor === Child, c1.constructor === Parent)
console.warn('c1 instanceof Child, c1 instanceof Parent')
console.log(c1 instanceof Child, c1 instanceof Parent)

实验结果：