[Typescript] Extend Functionality of a TypeScript Class with Decorators

In this lesson, you will learn how to extend a class's functionality through typescript decorators. Decorators provide a way to add both annotations and a meta-programming syntax for class declarations and members.

class Lesson {
    lessonNumber: number = 0;
}

const lesson = new Lesson();
console.log(lesson.lessonNumber); // 0

Decorator:

function classDecorator<T extends { new (...args: any[]): {}}>(target: T) {
    return class extends target {
        lessonNumber = 3;

        toString() {
            return `${target.name}::${JSON.stringify(this, null, 2)}`
        }
    }
}

@classDecorator
class Lesson {
    lessonNumber: number = 0;
}

const lesson = new Lesson();
console.log(lesson.lessonNumber); // 3
console.log(lesson.toString()) 
/**
 * "Lesson::{
        "lessonNumber": 3
    }"  
*/