ES2015 constructor call trick

constructor-call-trick.js

let returnMe;

class HTMLElement {
    constructor() {
        if (returnMe) {
            const temp = returnMe;
            returnMe = null;
            return temp;
        }
        
        // ...
        
        return allocateFromUACode();
    }
}

class XFoo extends HTMLElement {
    constructor() {
        super(); // effectively does this = returnMe
    }
}

function upgradeEl(el) {
    returnMe = el;
    new XFoo();
}

Problem:

class XFoo extends HTMLElement {
    constructor() {
        new XFoo(); // clobbers returnMe
        super();
    }
}