function updateObject(oldObject, newObject) {
    const [newKey, newValue] = Object.entries(newObject)[0];
    if (newKey in oldObject) {
        oldObject[newKey] += newValue;
    } else {
        oldObject[newKey] = newValue;
    }
    return oldObject;
}


const oldObject = { 1: 10, 2: 20, 3: 30 };
const newObject = { 2: 15 };

const updatedObject = updateObject(oldObject, newObject);
console.log(updatedObject);