private-class.js

const getPrivateContainer = () => {
    const privateProperties = new WeakMap();
    const getPrivate = (instance, prop) => {
        if (!privateProperties.has(instance)) {
            privateProperties.set(instance, {});
        }
        return privateProperties.get(instance)[prop];
    };
    const setPrivate = (instance, prop, value) => {
        if (!privateProperties.has(instance)) {
            privateProperties.set(instance, {});
        }
        privateProperties.get(instance)[prop] = value;
    };
    return { getPrivate, setPrivate };
};

const getPrivateClass = clsFactory => clsFactory(getPrivateContainer());

const MyObject = getPrivateClass(({ getPrivate, setPrivate }) => {
    return class {
        constructor(x) {
            setPrivate(this, 'x', x);
        }
        getX() {
            return getPrivate(this, 'x');
        }
    };
});