What is the process for declaring a member variable as an extended type in TypeScript?

Is it possible to declare a "member variable" as an "extension object" rather than a static type (without relying on an interface)?

Imagine something like this pseudocode:

class Foo {
    bar -> extends Rectangle;
    
    constructor(barInstance:IRectangle){
       this.bar = barInstance;

       this.bar.getArea(); //<-- code completion works because of interface IRectangle

       // no type error
       this.bar.someCustomFunction = function() {
       }
    }

}

as opposed to

class Foo {
    bar: IRectangle;
    //or
    bar:Rectangle;
}

This approach allows for adding properties that are not defined in the base class or interface without triggering type errors, while still benefiting from code completion provided by the base class. A bit of a cheeky take on strict typing, right? Lazy but effective.

Answer №1

Understanding Intersection Types

Definition of IRectangle interface:
    The IRectangle interface has a single method called getArea, which returns a number.

Explanation of Foo class:
    The Foo class contains a property named bar, which is an intersection type combining the IRectangle interface and an object with any keys/values.
    In the constructor, an instance of IRectangle is passed in as an argument and assigned to the bar property.
    The code then calls the getArea method on the bar property successfully because it adheres to the IRectangle interface requirements.
    Additional functionality is added to the bar property without causing any type errors by defining a new function.

Overall, intersection types allow for creating complex types by combining multiple interfaces or objects together.

Answer №2

Think about using a restricted generic type parameter.

interface Parent {
  value: number;
}

interface Offspring extends Parent {
  attribute: string;
}

class Example<P extends Parent> {
  data: P
}

var example = new Example<Offspring>();
example.data.attribute; // now validated by the compiler

Answer №3

It seems like you're asking about implementing interfaces and classes in TypeScript. Here is an example that demonstrates this concept:

interface IShape {
    calculateArea(): void;
}

class Shape implements IShape {
    calculateArea(): void {}
    customFunction(): void {}
}

class Bar<S extends IShape> {
    baz: S;

    constructor(bazInstance: S){
        this.baz = bazInstance;
        this.baz.calculateArea();

        // no type error
        if (this.baz instanceof Shape) {
            (this.baz as any as Shape).customFunction = function() {}
        }
    }
}

You can also view and interact with the code here.

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