_ _ proto__ is a property added by the browser to allow you to access the prototype of the object. It's not standard. It can only be used in some browsers. You can modify it like this:

function ClassA(name) {
      this.name = name
    }

    ClassA.prototype.sayName = function () {
      alert(this.name)
    }

    function ClassB(name, age) {
      ClassA.call(this, name)
      this.age = age
    }

    ClassB.prototype.constructor.prototype = ClassA.prototype // constructor.prototype 

    ClassB.prototype.sayAge = function () {
      alert(this.age)
    }

    let instance = new ClassB('', 11)
    instance.sayName(); //
    instance.sayAge();  //11

Object.prototype.__proto__ , it turns out that can be used

function ClassA(name) {
    this.name = name
}

ClassA.prototype.sayName = function () {
    // 
}

function ClassB(name, age) {
    ClassA.call(this, name)
    this.age = age
}

// ClassB.prototype.__proto__ = ClassA.prototype
ClassB.prototype = Object.create(ClassA.prototype)

ClassB.prototype.sayAge = function () {
    // 
}

let instance = new ClassB('', 11)