const config = {};
config.email = 'email@email.com';
config.emailOverwrite = true;
config.overwriteEmails = [
    'e1@mail.com',
    'e2@mail.com'
];

const readOnly = (object) => {
    const handler = {
        get: (target, key, receiver) => {
            if (Object(target[key]) !== target[key]){
                return target[key];
            } else {
                return readOnly(target[key]);
            }
        },
        set: (target, key, value, receiver) => {throw new Error('Read Only')}
    }
    return new Proxy(object, handler);
}

readOnlyObject = readonly(config);

readOnlyObject.overwriteEmails.push('test'); // throws