Property trap (inspired by ES6 proxies)

create-trap.js

function isObject(test) {
    return Object.prototype.toString.call(test) === '[object Object]';
}

function trap(host, targetName, handler) {
    const prison = {};
    let cell = {};

    Object.defineProperty(prison, targetName, {
        configurable: true,
        enumerable: host.propertyIsEnumerable(targetName),

        get() {
            if (!isObject(cell)) {
                return cell;
            }

            const trap = Object.assign({}, cell);

            setTimeout(() => {
                for (let key in trap) {
                    if (trap.hasOwnProperty(key)) {
                        if (cell[key] !== trap[key]) {
                            cell[key] = handler(host, key, trap[key]);
                        }
                    }
                }
            }, 0);

            return trap;
        },

        set(value) {
            return cell = handler(host, null, value);
        }
    });

    return prison;
}

example.js

const body = trap(document.body, 'style', (host, property, value) => {
    console.log('something changed', property, value);
    // do your magic here
    // host.style[property] = value;
    return value;
});

body.style.opacity = '0';