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Late binding JS: Dive into Classes

To understand how the insides of classes bound by inheritance relate to each other, we need to understand the concept of late binding.

Remember the HTMLElement base class from the last lesson. this is actively used to refer to properties within it:

// Base class for all tags. It knows how to work with attributes.
class HTMLElement {
  constructor(attributes = {}) {
    this.attributes = attributes;
  }

  getAttribute(key) {
    return this.attributes[key];
  }
}

Suppose we create an HTMLAnchorElement class object (which inherits HTMLElement). Then what class object would be this inside the methods of the parent class? The correct answer is HTMLAnchorElement, which is the class whose object we're creating right now. Take a look at this example:

class A {
  constructor() {
    this.name = 'From A';
  }

  getName() {
    console.log(this.constructor);
    return this.name;
  }
}

class B extends A {}

const b = new B();
console.log(b.getName());
// [class B extends A]
// => From A

this feature is called late binding. It means that at the point the class is defined, the type of this is unknown. The current object can be an object of any class that inherits from the current one. It looks as though all the code inside the base class were copied and transferred to each successor class. For late binding, it doesn't matter how deep the inheritance hierarchy is. this will always be an object of the class that's constructed in the code.

Late binding is an important element in the workings of inheritance. Without it, class interaction would become much more complicated and limited. Each object would have to know for sure which class the properties and methods in the inheritance chain belong to. You would need to use special syntax to access them.


Recommended materials

  1. Late binding (Wiki)

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