如何在元素不存在的情况下添加样式属性，或者使用Javascript将给定的样式添加到预先存在的样式属性？

var element = document.getElementById('myElement');
if (element) {
    element.classList.add('newStyle');
}

var virtualElement = document.createElement('div');
virtualElement.style.color = 'red';
virtualElement.style.fontWeight = 'bold';

var existingElement = document.getElementById('existingElement');
if (existingElement) {
    existingElement.style.color = virtualElement.style.color;
    existingElement.style.fontWeight = virtualElement.style.fontWeight;
}