DOMRect??

domrect.js

const privates = new WeakMap();

class DOMRect {
    constructor(left, top, width, height) {
        privates.set(this, {
            left: +left,
            top: +top,
            width: +width,
            height: +height
        });
    }

    get left() { return privates.get(this).left; }
    get top() { return privates.get(this).top; }
    get width() { return privates.get(this).width; }
    get height() { return privates.get(this).height; }

    get right() { return this.left + this.width; }
    get bottom() { return this.top + this.height; }
}

class DOMRectImmutable extends DOMRect {}

// DOES NOT WORK; SEE BELOW
class DOMRectMutable extends DOMRect {
    set left(value) { privates.get(this).left = +value; }
    set top(value) { privates.get(this).top = +value; }
    set width(value) { privates.get(this).width = +value; }
    set height(value) { privates.get(this).height = +value; }
}


// THE PROBLEM:

const r = new DOMRectImmutable(1, 2, 3, 4);

Object.getOwnPropertyDescriptor(DOMRectMutable.prototype, 'left').set.call(r, 10);

console.log(r.left); // 10! uh-oh

// Solution: add branding checks

const mutableDomRects = new WeakSet();

function assertIsBranded(obj) {
    if (!mutableDomRects.has(obj)) {
        throw new TypeError('You can only apply DOMRectMutable2 methods to DOMRectMutable2 instances!');
    }
}

class DOMRectMutable2 extends DOMRect {
    [Symbol.create]() {
        const newDomRect = {};
        mutableDomRects.add(newDomRect);
        return newDomRect;
    }

    set left(value) {
        assertIsBranded(this);
        privates.get(this).left = +value;
    }

    set top(value) {
        assertIsBranded(this);
        privates.get(this).top = +value;
    }

    set width(value) {
        assertIsBranded(this);
        privates.get(this).width = +value;
    }

    set height(value) {
        assertIsBranded(this);
        privates.get(this).height = +value;
    }
}

// Now:

Object.getOwnPropertyDescriptor(DOMRectMutable2.prototype, 'left').set.call(r, 10);
// Throws TypeError.