const x = {
    foo: 29
};

const y1 = { foo: 42 };
const y2 = {};
const y3 = { foo: null };
const y4 = { foo: undefined };

console .log(Object.assign({}, x, y1));
console .log(Object.assign({}, x, y2));
console .log(Object.assign({}, x, y3));
console .log(Object.assign({}, x, y4));

console.log('-------------- curve ball ---------')
console .log(Object.assign({}, x, JSON.parse(JSON.stringify(y3))));
console .log(Object.assign({}, x, JSON.parse(JSON.stringify(y4))));