A constructor method is Not required inside of a class.

Sometimes you make classes without a constructor when using inheritance. In this case we don't want any object to be an instance of shape. But we don't want to rewrite the function toString every time when we make a new class for a shape. So Rectangle and Circle 'extend' the Shape class, this way they have access to the attributes and functions of the parent class. I hope that this answers your question :)

class Shape {
    toString () {
        return `Shape`
    }
}
class Rectangle extends Shape {
    constructor (width, height) {
      ...
    }
    toString () {
        return "Rectangle > " + super.toString()
    }
}
class Circle extends Shape {
    constructor (radius) {
      ...
    }
    toString () {
        return "Circle > " + super.toString()
    }
}

A class might contain a number of data fields and/or methods, but nothing requiring any specific initialization. In that case, no explicit constructor is needed. But every instance still gets all the methods and fields.