Proxies can be quite useful

fun_with_proxies.js

const vCard = {
    firstName: '',
    lastName: '',
    yearOfBirth: 0
};

// Simple logging:
const vCardLogged = new Proxy(vCard, {
    set: (obj, prop, value) => {
        console.log(`vCard got new value for key "${prop}": "${value}"`);
        obj[prop] = value;
        return true;
    }
});

// Proxies can be stacked:
const vCardValdidatedAndLogged = new Proxy(vCardLogged, {
    set: (obj, prop, value) => {
        // *Very* simple validations
        if (prop === 'yearOfBirth') {
            if (typeof value !== 'number' || value < 1900 || value > (new Date).getFullYear()) {
                throw new Error(`Invalid value provided for "${props}": "${value}"`);
            }
        } else if (typeof value !== 'string') {
            throw new Error(`That doesn't look like name: "${value}"`);
        }

        obj[prop] = value;
        return true;
    },

    // Calculated props
    get: (obj, prop, value) => {
        if (prop === 'age') {
            return (new Date).getFullYear() - obj.yearOfBirth;
        }

        if (prop === 'fullname') {
            return `${obj.firstName} ${obj.lastName}`;
        }

        return obj[prop];
    }
});

const developer = Object.create(vCardValdidatedAndLogged, {});
developer.firstName = 'Jonathan'

try {
    developer.lastName = 42;
} catch (ex) {
    console.error(ex);
}

developer.lastName = 'Weiss';
developer.yearOfBirth = 1984;

console.log(`The developer "${developer.fullname}" is ${developer.age} years old.`);