Why is assigning a value to super in the constructor in the ES6 standard? It is assigning a value to an instance of the class

class A {
  constructor() {
    this.x = 1;
  }
}
class B extends A {
  constructor() {
    super();
    this.x = 2;
    super.x = 3;
    console.log(super.x); // undefined
    console.log(this.x); // 3
  }
}
let b = new B();

Why is assigning a value to super in the constructor in the ES6 standard? It is assigning a value to an instance of the class.
Why is super.x undefined while this.x is 3