Change `this` reference to parent object in JS class constructor

Question

I have a class constructor with a nested object structure. Each level has a property x.

this.x > this.a.x > this.a.b.x

I want this.a.b.x to point to the value of this.a.x, but it instead points to this.x.

class Example {
    constructor() {
        this.x = 0;
        this.a = {
            x: this.x + 5,
            b: {
                x: this.x // I want this to equal 5
            }
        }
    }
}

const ex = new Example();
console.log(ex.x) // 0
console.log(ex.a.x) // 5
console.log(ex.a.b.x) // 0 <-- I want this to be 5

How can I change the this reference in this.a.b to point to this.a rather than this??