private-props.js

const MAP = (typeof self.WeakMap !== 'undefined') ? new WeakMap() : false;
const PROP = '__private__';

function internal(object) {
    if (MAP) {
        if (!MAP.has(object)) {
            MAP.set(object, {});
        }
        return MAP.get(object);
    }
    if (!this.hasOwnProperty(internal.fallbackKey)) {
        this[internal.fallbackKey] = {};
    }
    return this[internal.fallbackKey];
}

internal.fallbackKey = PROP;

internal.destroy = function(object) {
    if (MAP) {
        if (MAP.has(object)) {
            MAP.delete(object);
        }
    } else {
        delete this[PROP];
    }
};

Demo

class Demo {
    constructor(id) {
        internal(this).id = id;
    }
    get fullId() {
        return `full-${internal(this).id}`
    }
}

let demo = new Demo(2);

console.log(demo.id); // undefined
console.log(demo.fillId); // "full-2"