Call an overridden method in base class constructor in typescript

Question

When I'm calling an overridden method from the base class constructor, I cannot get a value of a sub class property correctly.

Example:

class A
{
    constructor()
    {
        this.MyvirtualMethod();
    }

    protected MyvirtualMethod(): void
    {

    }
}

class B extends A
{
    private testString: string = "Test String";

    public MyvirtualMethod(): void
    {
        alert(this.testString); // This becomes undefined
    }
}

I would like to know how to correctly override functions in typescript.

Answer 1

The order of execution is:

1. A's constructor
2. B's constructor

The assignment occurs in B's constructor after A's constructor—_super—has been called:

function B() {
    _super.apply(this, arguments);   // MyvirtualMethod called in here
    this.testString = "Test String"; // testString assigned here
}

So the following happens:

var b = new B();     // undefined
b.MyvirtualMethod(); // "Test String"

You will need to change your code to deal with this. For example, by calling this.MyvirtualMethod() in B's constructor, by creating a factory method to create the object and then execute the function, or by passing the string into A's constructor and working that out somehow... there's lots of possibilities.

Answer 2

The key is calling the parent's method using super.methodName();

class A {
    // A protected method
    protected doStuff()
    {
        alert("Called from A");
    }

    // Expose the protected method as a public function
    public callDoStuff()
    {
        this.doStuff();
    }
}

class B extends A {

    // Override the protected method
    protected doStuff()
    {
        // If we want we can still explicitly call the initial method
        super.doStuff();
        alert("Called from B");
    }
}

var a = new A();
a.callDoStuff(); // Will only alert "Called from A"

var b = new B()
b.callDoStuff(); // Will alert "Called from A" then "Called from B"

Try it here

Answer 3

If you want a super class to call a function from a subclass, the cleanest way is to define an abstract pattern, in this manner you explicitly know the method exists somewhere and must be overridden by a subclass.

This is as an example, normally you do not call a sub method within the constructor as the sub instance is not initialized yet… (reason why you have an "undefined" in your question's example)

abstract class A {
    // The abstract method the subclass will have to call
    protected abstract doStuff():void;

    constructor(){
     alert("Super class A constructed, calling now 'doStuff'")
     this.doStuff();
    }
}

class B extends A{

    // Define here the abstract method
    protected doStuff()
    {
        alert("Submethod called");
    }
}

var b = new B();

Test it Here

And if like @Max you really want to avoid implementing the abstract method everywhere, just get rid of it. I don't recommend this approach because you might forget you are overriding the method.

abstract class A {
    constructor() {
        alert("Super class A constructed, calling now 'doStuff'")
        this.doStuff();
    }

    // The fallback method the subclass will call if not overridden
    protected doStuff(): void {
        alert("Default doStuff");
    };
}

class B extends A {
    // Override doStuff()
    protected doStuff() {
        alert("Submethod called");
    }
}

class C extends A {
    // No doStuff() overriding, fallback on A.doStuff()
}

var b = new B();
var c = new C();

Try it Here

Answer 4

below is an generic example

    //base class
    class A {
        
        // The virtual method
        protected virtualStuff1?():void;
    
        public Stuff2(){
            //Calling overridden child method by parent if implemented
            this.virtualStuff1 && this.virtualStuff1();
            alert("Baseclass Stuff2");
        }
    }
    
    //class B implementing virtual method
    class B extends A{
        
        // overriding virtual method
        public virtualStuff1()
        {
            alert("Class B virtualStuff1");
        }
    }
    
    //Class C not implementing virtual method
    class C extends A{
     
    }
    
    var b1 = new B();
    var c1= new C();
    b1.Stuff2();
    b1.virtualStuff1();
    c1.Stuff2();