I am asking how to implement the virtual function in the javascript like C#, let's say
- I have a base class A
- I also have a derived class named B
- The class A has a function named virtualFunction, this is supposed to be a virtual function like in C#, this can be overridden in the derived class B
- The function will be called to execute inside the constructor of the base class, A. If the function is overridden in B, B's one will be invoked otherwise A's one will be invoked
With this example below, I want to get a 'B' alert, not the 'A'.
function A() {
this.virtualFunction();
}
A.prototype.virtualFunction = function() {
alert('A');
};
//-------------------------------------
function B() {}
B.prototype = new A();
B.prototype.virtualFunction = function() {
alert('B');
};
var b = new B();I am asking how to implement the virtual function in the javascript like C#, let's say
- I have a base class A
- I also have a derived class named B
- The class A has a function named virtualFunction, this is supposed to be a virtual function like in C#, this can be overridden in the derived class B
- The function will be called to execute inside the constructor of the base class, A. If the function is overridden in B, B's one will be invoked otherwise A's one will be invoked
With this example below, I want to get a 'B' alert, not the 'A'.
function A() {
this.virtualFunction();
}
A.prototype.virtualFunction = function() {
alert('A');
};
//-------------------------------------
function B() {}
B.prototype = new A();
B.prototype.virtualFunction = function() {
alert('B');
};
var b = new B();If we invoke the function after instanced, like this, then it is okay, but I need "inside the constructor"
var b = new B();
b.virtualFunction();-
2
In the first case you have
alert('A')in both parent and child functions – MysterX Commented Apr 19, 2016 at 13:14 - 4 Even in C#, a base-class constructor doesn't have access to functions in a derived class. While the base constructor is running, the object is not an instance of the derived class yet, so the derived class's overrides haven not been set up yet. It wouldn't (in general) be safe anyway to call methods in the derived class before the derived constructor has run. – Wyzard Commented Apr 19, 2016 at 13:20
-
Don't use new to create prototype objects! Instead, put a super call in
B, and you'll get the expected results. – Bergi Commented Apr 19, 2016 at 13:35 - @Bergi, thanks, I got your idea – khoailang Commented Apr 19, 2016 at 16:00
1 Answer
Reset to default 10So, the first thing to realize is that JavaScript doesn't use the same semantics as C#. Trying to get it to do so will cause you heartache; you're much better off learning how things are done in JS. In fact, most folks don't try to do a lot of inheritance type stuff in JS, but instead favor position (which is actually a best practice in all OO languages I'm familiar with).
That said, there's a few improvements you could make to your code.
function A() {
this.virtualFunction();
}
A.prototype.virtualFunction = function() {
alert('A');
};
//-------------------------------------
function B() {
A.call(this);
}
B.prototype = Object.create(A.prototype);
B.prototype.constructor = B;
B.prototype.virtualFunction = function() {
alert('B');
};
var b = new B();